A cold, arid planet whose destiny remains an enigma

Earth and Mars seen from space. The diameter of Mars is half that of the Earth and its surface is entirely solid (© NASA).

Mars is the fourth planet in the solar system. It follows an elliptical orbit around the Sun with an average distance (or “semi-major axis”) of 228 million kilometres, taking two Earth years to complete one rotation. Although Mars years are therefore twice as long as Earth years, a day lasts more or less the same time as ours.

Like Earth, Mars has cyclical seasons since its axis of rotation is very similar to that of our own planet. When the two celestial bodies are compared, other similarities such as the existence of ice caps at the North and South poles stand out; these can be easily observed through even a small telescope.

However, the similarities stop there, and when seen from close up, significant differences become apparent. The first, and the most marked, has to be the size. With a mean diameter of 6,770 kilometres, the Red Planet is half the size of Earth. Its size means that Mars has a mass one tenth of that of Earth, and a gravity field one third as strong. This gravitational difference complicates things when it comes to developing a seismometer designed to operate on Mars, and testing it on Earth.

Mars photographed by the Hubble space telescope (© NASA, J. Bell and M. Wolff).

Seen from space, the two planets reveal another major difference: their colours. Earth is a magnificent blue and green globe dotted with brown and white, while Mars displays hues of red and ochre. In contrast with Earth, 70% of which is covered by oceans, Mars is a desert on a planetary scale; a celestial body whose rocky, rusty surface stretches as far as the eye can see under a pallid sky.

Phobos and Deimos, the two companions of Mars

While Earth has the fortune to be joined on its journey around the Sun by a massive body, the Moon, Mars has just two small companions: Phobos and Deimos.

Although the general consensus today is that the Moon is made up of terrestrial material ejected into space after a gigantic collision with a giant protoplanet billions of years ago, the origin of the Red Planet’s two moons is still a matter of debate. One explanation favours a collision similar to that which created the Earth-Moon system, while another scenario assumes that Phobos and Deimos are two small asteroids captured by Mars’ gravitational attraction. These asteroids then fell into a quasi-circular orbit aligned with the planet’s equator.

Phobos is 27 kilometres long pole to pole, and sufficiently close to Mars to periodically raise its surface by a tiny amount through gravitational attraction. This tidal phenomenon would be more spectacular if the Red Planet had oceans. The SEIS seismometer flown on the InSight probe will attempt to measure the attractive effect of Phobos and will use the data collected to probe Mars’ interior down to the core.

In a few tens of millions of years, the effect of tidal forces will cause Phobos to break up into a multitude of fragments of all sizes. These may join up to form a disc, developing into a beautiful ring around the Red Planet.

The great dichotomy

The topographical study of Mars—made possible through the analysis of images obtained from orbit and, more recently, through laser altimetry measurements—shows that the northern and southern hemispheres of the planet are very different. This is one of the planet’s most remarkable features.

To the South are highly cratered, and hence very old, highlands. Easily 4 billion years old, the exposed rocks of the southern hemisphere are far more ancient and much better preserved than the oldest rocks to be found on Earth. In the absence of plate tectonics (a mechanism that irreversibly alters rocky materials), Mars’ southern hemisphere is a veritable open air museum.

The great Martian dichotomy: Mars is literally split into two distinctly different hemispheres: to the South are ancient cratered highlands, while to the North are much younger lowlands (© NASA).

The situation is completely different in the North: we find instead low plains whose much younger surface is only occasionally marked by impact craters, smaller in both size and number.

Planetary scientists refer to this asymmetry between North and South (which has no equivalent on Earth) as the great Martian dichotomy.

To this day, its origin remains a burning question. Two main hypotheses have been put forward. The first involves a gigantic impact with a protoplanet during the first eons of the solar system. The collision would have literally crushed the northern hemisphere, making the crust thinner and opening up many fractures through which lava would have poured, thereby making the surface younger, i.e. covering the older ground and obliterating any impact craters.

The second hypothesis does not involve an external phenomenon, such as a massive impact, but rather some internal phenomenon specific to Mars itself. It is possible that Mars’ mantle may have been heterogeneous and that, under the northern hemisphere, a gigantic plume of molten material or some great convective activity may have occurred, leading to a sinking of the crust and to vast lakes of lava being spewed out onto the surface. Located specifically under the northern hemisphere, the origin of such a cataclysmic event is unknown. By landing close to the boundary between the ancient highlands of the South and the young lowlands of the North, the InSight probe may have a chance of raising even ever so slightly the shroud of mystery surrounding the Martian dichotomy.

A frozen world surrounded by a thin, unbreathable atmosphere

The Martian atmosphere (seen here from the side) is very thin and unbreathable (© NASA).

Yet more advanced investigations, thanks in particular to space missions but also to the analysis of Martian meteorites that have fallen to Earth, continue to reveal the extra-terrestrial nature of the Red Planet.

In contrast to Earth, this world no longer has a magnetic field and it is very probable that the disappearance of this protective shield (without which life on Earth would not be possible) has driven the almost complete disappearance of the Martian atmosphere.

Mars is now covered by just a very thin layer of unbreathable air, consisting almost entirely of carbon dioxide (CO2). Such a thin atmosphere is unable to retain the heat from the Sun, which explains why the Martian surface is completely frozen. The average temperature is -53°C, and the contrast between day and night is immense. While mid-summer temperatures at the equator can reach 0°C at midday, night-time temperatures can easily plunge to -70°C. These enormous variations in temperature make seismic measurements very difficult given that, ideally, seismometers should operate in environments where the temperature is as stable as possible.

Water ice in an impact crater (© ESA/DLR/FU Berlin G. Neukum).

Very low temperatures, coupled with a very low atmospheric pressure, mean that water cannot exist on Mars in its liquid state. This molecule, which has played a central role in Earth’s history both from a geological (plate tectonics) and biological point of view, can exist on the surface of Mars only as water vapour in the air or in the form of ice.

While the polar caps harbour large quantities of water ice—as does the ground at high latitudes—it is highly probable that considerable amounts have escaped into space, most likely due to the disappearance of the magnetic field and because of the small planet’s consequently weak field of gravity.

Traces of liquid water flows

However, a study of the Martian relief at ancient sites shows the two types of formation that seem to have been created by the flow of liquid water. The first, somewhat like river beds, bears a striking resemblance to the traces of water courses that meander along the bottom of our terrestrial valleys. These valley networks may well have been carved out by rainwater runoff.

The second type of flow trace is even more astonishing: immense ravines that often originate in chaotic terrains, stretching for huge distances over the Martian surface and that seem to have collapsed in on themselves, leaving behind an impassable jumble of rocks of all sizes, hurled against one another.

Crater and flow channels in the Hephaestus Fossae region (ESA/DLR/FU Berlin G. Neukum).

Everything in these regions seems to indicate that an incredible amount of water was released, leading to landslides and the collapse of surrounding areas. A cataclysmic flood carrying chunks of rock as big as an office block alongside sharp-edged icebergs apparently surged over the surface, scraping, scratching and tearing at the rocky stratum and destroying everything in its path. Like ugly scars, the canyons and valleys that can still be seen on Mars merely hint at the violence unleashed by these Martian tsunamis, which literally scoured entire expanses of the planet.

Hence it must be concluded that a very long time ago, and in contrast to what we see today, liquid water appears to have played a very important role in forming the Martian relief. Something drastic occurred in Mars’ history that put an end to these phenomena.

A world where the atmosphere can freeze

Aesthetically pleasing formations caused by sublimation of frozen carbon dioxide (dry ice) at the southern polar cap during the spring thaw (© NASA/JPL/University of Arizona).

Another consequence of the glacial temperatures on Mars highlights the importance of a molecule which, on Earth, exists only as a gas: carbon dioxide.

This molecule is not only the main component of the Red Planet’s atmosphere, but it can also solidify to form dry ice, i.e. frozen carbon dioxide. In winter, when temperatures can fall to -120°C, a significant part of the atmosphere solidifies at the polar caps. The encrustation of dry ice that forms gives rise to strange and bewitching features totally unknown on Earth. Their disturbing beauty has fascinated us from orbit, and we can only dream of what they would look like from the planet itself.

Under certain conditions on Mars, carbon dioxide can also liquify and give rise to flows that behave unlike anything that we are used to seeing with water on Earth. Some scientists believe that carbon dioxide has played a major role on Mars. It is this, rather than liquid water, that could be involved in the formation of certain flow patterns seen from orbit.

A dune of black sand photographed by the Curiosity rover inside the Gale impact crater (© NASA/JPL-Caltech/MSSS).

Surface activity

If we take a look at surface activity, we quickly discover that the primary phenomenon on Mars that is still active is none other than the wind, which blows all the way around the globe, raising a cloud of very fine dust. Oxidised and with magnetic properties, this covers the surface like a reddish shroud, giving the planet its characteristic colour.

In contrast with the Earth’s crust, whose fragments have been in a process of ceaseless separation and collision for billions of years through tectonic motion, permanently altering the shape and division of the continents, Mars appears to be a dead world, its geology frozen in time for all eternity.

Giant volcanoes and canyons

The Olympus Mons volcano is 25 kilometres high, and is big enough to cover the whole of metropolitan France (© NASA).

Despite decades of observation, no eruption or effusion of lava has ever been detected live on the surface of Mars. However, the planet has no lack of volcanic apertures. Standing 25 kilometres tall, the Olympus Mons volcano is one of the highest in the solar system.

The Martian surface also bears the scars where immense chunks of the crust have been torn out; the most striking examples are the rifts in the gigantic canyon of Valles Marineris. Four thousand kilometres long, this system of chasms extends over the equator and gouges the crust down to the astonishing depth of 10 km.

However, despite this evidence of a violent geological past, nothing today seems to be on the move, a situation that makes it more than ever necessary to set up a seismometer on the surface. Seismic measurements will take the planet’s pulse for the first time, and find out whether there is still movement deep within.

Mars’ decline and its failure to maintain any geological activity over billions of years is due in part to the absence of plate tectonics. It is this planetary mechanism on Earth (in which water appears to play a central role) that is responsible for the dynamic geology of our world. Plate tectonics are directly responsible for raising up mountain ranges, uncontrolled volcanic activity (whether at the bottom of the ocean or in the open air) or terrible earthquakes. On Mars, it appears that this mechanism never started, or just stopped very early on.

The young Mars

An avalanche close to the northern polar cap (© NASA/JPL/University of Arizona).

While the scientific community continues to debate the appearance of the primordial Red Planet billions of years ago, it seems increasingly probable that the young Mars was a cousin of the Earth.

Formed in a similar part of the solar system, these two celestial bodies took shape through accreting the same materials, including immense quantities of water.

The heat coming from the planetary interior fed the volcanoes, out of which lava flows and improbable quantities of gas emerged, eventually forming a thick atmosphere.

At the centre of the planet, molten metals formed a metallic core soon set in motion by convection currents that generated a protective magnetic field. This shield of invisible lines of force surrounded the planet, blocking out the erosive effect of the solar wind and protecting the young atmosphere. Temperatures at the surface were conducive to the condensation of liquid water. The consequent runoff soon formed lakes (e.g. inside impact craters), seas and perhaps even oceans.

Map of Mars’ crustal magnetic anomalies: a residual magnetic field testifying to the action of a dynamo at the very start of the planet’s history, exists in the southern hemisphere. Its activity clearly decreases in the northern regions  (© Connerney, J.E.P. et al., (2005) Proc. Natl. Acad. Sci. USA, 102, N°42, 14970-14975).

Thanks to the many probes currently in orbit and the rovers surveying the planet’s surface, our understanding continues to advance by leaps and bounds. Relics of the ancient magnetic field have been discovered by the magnetometer on board Mars Global Surveyor. The instrument’s observations have revealed that among the oldest rocks on the planet in the southern hemisphere, certain iron-rich minerals have preserved fossil remnants of the magnetic field that surrounded Mars millions of years ago at the very start of its existence. In the northern hemisphere, where the rocks are younger, the magnetisation of the crust is much less developed, a situation consistent with the early disappearance of this internal magnetic field.

Taking advantage of the development of sophisticated infrared spectrometers in the last few years, planetary scientists have started to detect mineral concentrations among the oldest areas of the planet that can only have formed in the presence of water. Grey haematite was first detected from orbit, and then directly on the ground as a result of one of the rover’s investigations. Sulphate-bearing rocks, laid down following the percolation of sulphur-laden acidulated water, were then revealed. Even more interesting, clay deposits formed by sedimentation in slightly alkaline lakes, and older than the sulphate beds, have been detected at several locations on the surface. After landing in the Gale impact crater in August 2012, Curiosity very quickly realised that it was travelling over the bed of a former lake!

Blankets of fog lying in the chasms of Valles Marineris (© ESA/DLR/FU Berlin G. Neukum).

History of a planet’s agony

The history of Mars seems to have unfolded in three stages. Until it was about 3.9 billion years old, the planet was most likely a hot and humid world with a global protective magnetic field, a thick atmosphere, vigorous volcanic activity and a water cycle in which rain fed rivers emptying into seas. Known as the Noachian, this idyllic picture-postcard period was in particular characterised by thick clay deposits. Alas, it was not to last.

For reasons unknown, the magnetic field surrounding the planet failed, leaving the planet to the mercy of the solar wind and its erosive effects. At the beginning, the ever-increasing volcanic activity injected huge quantities of gas into the atmosphere, regenerating the air that was vanishing into space, to be lost forever. Since these volcanic gases were very heavily laden with sulphur, the planet started to become more acidic. Clay was replaced by sulphate deposits, which are characteristic of what is known as the Hesperian.

With the loss of its magnetic field, Mars was set upon an irreversible path leading it straight to its geological death. As time passed, its internal heat began to fall, slowing down and eventually halting volcanic activity. Since it was no longer being replaced, the atmosphere became so thin that it resulted in an abrupt fall in temperature, and the planet ended up frozen.

Some 3.5 billion years after the Red Planet’s formation, the Amazonian period began. This is when Mars started to resemble what it still looks like today: a planetary desert whose reddish rusty surface was battered by winds. Propelling fields of sand dunes, raising dust storms and causing clouds of water ice or frozen carbon dioxide to wander in the thin cold air, the action of the wind became the only phenomenon capable of affecting the surface of Mars.

Black trails left on the ground by dust devils (© NASA/JPL/University of Arizona).

Nevertheless, the planet periodically underwent major climatic changes. In contrast to Earth, Mars cannot rely on the presence of a massive natural satellite like the Moon to stabilise its axis of rotation.

Like a wildly gyrating top, the planet’s axis of rotation sometimes tilts violently, pointing one of its poles towards the Sun. These erratic changes in inclination, coupled with a highly elliptical orbit that can sometimes elongate or shorten in a chaotic manner, triggered dramatic changes to the climate during which the polar ice caps started to melt, while sheets of ice advanced towards what is now the equator.

Scientists have uncovered many signs of these glacial epochs that dramatically marked the planet. The last one is thought to have occurred just 5 million years ago.

There is almost no doubt that the origin of this tormented destiny lies within the depths of the planet. Its small size, which ultimately affects the amount of heat available as well as the planet’s ability to retain light atmospheric gases, very probably condemned Mars from the start. However, many grey areas remain. The history of the formation and evolution of the Red Planet cannot be told unless, using the essential technique of seismology, we can peer into the planet’s inner depths.

Last updated : 31 july 2017

An interior made up of different layers

You might think that the planets with a solid surface that are revolving around the Sun—like Mercury, Venus, the Earth or Mars—are homogeneous rocky globes, but they are not. If we could cut one of them in two with a giant planetary-scale knife, we would discover something rather like a peach. The first thin, velvety layer of a peach is its skin. If we keep cutting, we find a thicker layer of flesh surrounding a hard stone in the middle. The structure of telluric planets is not that different. The very thin surface layer is known as the crust. Underneath lies a much thicker, hotter layer known as the mantle. In the centre there is a core which can be solid, liquid, or both.

Obviously it is not possible to cut a planet in two like we can with fruit. So how do we know what is hidden deep within our planet? The deepest borehole on Earth, drilled in Russia’s Kola Peninsula, reached a respectable depth of 12,262 metres, or around 12 kilometres. However, the Earth’s average diameter is 12,742 kilometres! The Kola borehole only penetrated 0.2% of the Earth’s globe, and did not even manage to get through the continental crust, which is between 15 and 80 kilometres thick. Despite all our efforts, humans have only managed to slightly scratch the Earth’s surface.

However, geophysicists have a powerful investigation technique that has allowed them over several decades to explore the Earth’s interior and understand its structure. This technique is known as seismology.

Seismology, shedding light on the internal structure of a planet

Whenever a seismic wave changes compartment—for instance when it moves from the crust to the mantle, or from the mantle to the core—part of its energy is transferred from one compartment to the next (this part is known as the transmitted wave), while another part “bounces off” the boundary between them (this part is known as the reflected wave). The respective proportions of these two types of wave depend on the contrast in structure between the two compartments, and are used to identify their boundaries, or “interfaces”.

Likewise, if there are any discontinuities within a given compartment, i.e. if the compartment is not completely uniform, the seismic waves will be similarly affected. Again, geophysicists can determine the structure of a given compartment. They may, for example, reveal stratifications within the crust or anomalies in the mantle. Today, the progress made in geophysics on Earth has been so great that we can now acquire images of the planet’s interior, rather like ultrasound scans. 

The Earth’s internal structure

By observing over the decades how seismic waves naturally generated by earthquakes are propagated through the planet, geophysicists first discovered the existence of the three main envelopes mentioned above: the crust, the mantle and the core. They went on to obtain a very detailed view of the Earth’s interior.

At the surface lies the crust, which may be oceanic or continental. The oceanic crust, mainly composed of basalt rock overlaid by sediments forming the ocean floor, is relatively thin, averaging some 6 to 11 kilometres. However, the continental crust—which forms the skeleton of land masses—is made of granite and is much thicker, ranging from 15 to 80 kilometres under mountain ranges, though the average is 30 kilometres.

Next comes the mantle, which is separated from the crust by a discontinuity discovered in 1909 by Andrija Mohorovicic, a pioneer of seismology. Indeed, the discontinuity is now commonly known as the Moho in his honour. The Earth’s mantle is an invisible yet immense portion of the Earth in terms of volume, because it extends downwards some 2,900 kilometres before meeting the core. It comprises two distinct regions: the upper mantle, which is up to 660 kilometres deep, and the lower mantle, even deeper.

The upper mantle is itself divided into two compartments: a solid, brittle part attached to the crust and forming with it the famous moving lithospheric (or “tectonic”) plates, and a malleable part extending down as far as the lower mantle and able to be deformed. Composed of olivine-rich rocks, the Earth’s upper mantle moves under the influence of convection, participating in the movement of the lithospheric plates it carries on its back.

In the lower mantle, pressure and temperature—which increase with depth—cause modifications in the crystalline structure of minerals that geophysicists call a phase change. One of the biggest phase changes occurs at a depth of 670 kilometres, defined as the limit between the upper and the lower mantle. Here, the olivine becomes denser, transforming into perovskite. This change has major consequences, especially on viscosity, which is a parameter that greatly influences the ease with which matter can move (water, for example, has almost no viscosity compared to honey, which does not flow so easily but still remains more fluid than toothpaste).

The Earth’s internal structure: our planet is divided up into three envelopes: the crust, the mantle and the core. The Earth’s core is liquid on the outside (the outer core) and solid on the inside (the inner core) (© Adobe Stock).

Below the mantle is the core, separated from the mantle by the Gutenberg discontinuity. With an average radius of 3,470 kilometres, the core is also composed of two parts. The outer, liquid, part extends between 2,900 and 5,100 kilometres deep, where the Lehmann discontinuity lies. This boundary separates the liquid outer core from the solid inner core, which is what really lies at the heart of our planet, some 5,100 to 6,371 kilometres below the surface.

Formation of the planet’s internal structure: accretion and differentiation

To better understand the composition and properties of the compartments making up planets, we need to go back to the way they are formed. They acquire their internal structure very early on, during a process known as differentiation, certain aspects of which remain a mystery.

Our solar system, which includes the Sun and the procession of planets orbiting around it, was born some 4.5 billion years ago following the collapse of a huge interstellar cloud of gas and dust. The Sun (which is a star) held centre stage while planets formed around the edges, within the limits of an accretion disk

An accretion disk may be compared to a kind of infernal cauldron from which planets emerge. It is composed of a mixture of gases in which lumps of matter, metal, rock and ice are moving in all directions. The numerous collisions that take place cause some lumps to get bigger, while others are pulverised into smaller particles.

Due to their greater gravitational pull, the biggest fragments are more likely to attract other lumps, thus increasing their size and consequently their power of attraction. Each of these big fragments ends up forming what planetary scientists call a “planetesimal”, which is a kind of huge asteroid. Planetesimals are the building blocks in the formation of planets. Still subject to incessant collisions within the accretion disk, these planetesimals aggregate to form protoplanets. Around the young star we know as the Sun, balls of white-hot matter appeared. One of these was our planet, Earth, and another was Mars.

Artist's concept of the accretion process (© Adobe Stock).

Both on the surface and deeper within, the temperatures involved in the formation of planets are so hot that most of the materials participating in the aggregation of the planet’s body melt. Metals become liquid, rocks become boiling magma and lighter, more volatile elements such as water dissipate into space as burning vapours, being unable to aggregate or remain in the same place.

It is at this stage that the process known as differentiation takes place. Differentiation is fundamental to the planet’s future, because by the end of this process, the body has acquired its internal structure, which is what determines its evolution.

The various elements making up the planet are separated during the differentiation process according to their nature. Iron, of which telluric planets contain a significant quantity, descends into the centre of the planet because of its weight, taking with it other elements such as nickel and sulphur to create a metal core.

Silicates (the main component of rocks) are compounds lighter than metals. They combine with other atoms such as sodium or potassium to form a rocky froth that remains on the surface, becoming a thin stable layer as it cools down. This is the crust.

Between the crust and the core there remains a fairly thick envelope composed of silicates that this time combine with elements such as magnesium and iron to form the mantle.

Finally, very volatile gaseous elements escape through the fissures in the crust and gather around the planet, creating an atmosphere of varying thickness and possibly, if the pressure and temperature conditions are conducive, layers of water (in ice and/or liquid form). The planet can only keep its atmosphere around it if its mass—and thus its gravitational pull—is sufficient. If it is not, the atmosphere can easily and irreversibly dissipate into space.

The differentiation process is similar to what happens in a blast furnace filled with ores in order to extract pure metals. Let us take the example of a blast furnace near an iron mine. The furnace chamber is filled with iron ore made up of iron oxides and other impurities and coke, which is a kind of coal. Everything is heated up to temperatures allowing the mixture to melt. The pure iron that emerges when the mixture melts accumulates at the bottom of the crucible due to its weight, forming cast iron in much the same way as the formation of the planet's core. Silicates being lighter than iron, they form a kind of frothy scum that stays on the surface. This “slag”, as it is known, resembles the crust. Finally, gases whistle as they escape upwards, like those of the atmosphere.

A planet’s internal structure determines its future evolution

After the differentiation process, a telluric planet is composed of a core, a mantle, a crust and, possibly, water layers and an atmosphere. These main envelopes are absolutely fundamental, because together they determine the planet’s future. It is thanks to the Earth’s interior that it is still a living planet, both from a geological and biological point of view, after 4.5 billion years of existence.

The Earth’s geological activity is both varied and intense. Volcanoes scattered over the globe spit out huge lava flows and clouds of ash; earthquakes shake towns and their inhabitants; some mountains continue to rise, while others are worn down by erosion and end up as grains of sand on a beach.

Plate tectonics

The Earth’s unrestrained geological vitality is related to an imperceptible but crucial phenomenon known as plate tectonics.

Very early in our planet’s history, the intense convective activity in the mantle broke the planet’s surface up into some ten huge lithospheric plates, which have ever since been moving away from, colliding with or even slipping under each other with differing degrees of friction. It is the rough sliding of plates that causes earthquakes. When plates move away from each other, the indents left are filled by seas and oceans, and when these same plates finally close together like jaws then end up colliding, these same seas and oceans disappear, leaving scar tissue in the form of mountain ranges.

Over very long time periods (hundreds of millions of years), plate tectonics has had a quite astonishing consequence: the appearance of long-period phenomena known as Wilson cycles. During one of these cycles, the land masses grouped into one “supercontinent” surrounded by an enormous ocean. Pulled on all sides, the supercontinent was finally torn apart into a multitude of moving parts that spread throughout the planet before inevitably coming back together and finally merging. This large-scale geological ballet—which is constantly changing, slowly but surely, the face of the planet—is being played out again and again on the world stage.

Thus, contrary to what you might think, today’s global maps—drawn up following much exploration—are just a snapshot of the Earth’s life. On the scale of Wilson cycles, our geographical maps are only valid for a brief moment. The Earth looked very different hundreds of millions of years ago, and in the distant future, the boundaries of oceans and continents will not look anything like they do today.

Plate tectonics bears witness to the Earth’s constant geological activity (© Adobe Stock).

Yet plate tectonics has other even greater consequences. Its existence has given the Earth two types of crust, mentioned earlier: the oceanic crust which supports the oceans and which is relatively thin, and the thicker continental crust upon which we live. Even if it is only a hypothesis, it is likely that life could not develop and evolve in the same way on an ocean-covered planet (without continents) or a land-covered planet (without any liquid surface). Earth is lucky enough to have both land and oceans, a feature that has no doubt played a significant role in its ability to host and foster the development of life.

Likewise, in the subduction zones where plates are driven downwards into the Earth’s interior, the process of plate tectonics allows a recycling of volatile elements, and especially water. This element lowers the temperature at which rocks melt, so the constant cycle of water between the inner and outer parts of the planet appears vital to maintaining volcanic activity.

Volcanic activity, which on Earth is mainly linked to plate tectonics (virtually all volcanoes being found along the boundaries of lithospheric plates), is one of the features that has allowed our planet to retain its atmosphere. This gaseous layer is directly responsible for surface temperatures. Depending on its thickness and composition, and especially the presence of greenhouse gases that can retain heat, a planet’s atmosphere determines temperatures on the ground. If there is no gaseous layer or if it is too thin, the planet will be very cold, like the Moon or Mars. If it is too thick, it can transform the world into a suffocating inferno like Venus. Again, there is a marked correlation with the Earth’s ability to harbour life.

Everything appears to indicate that if the mechanism of plate tectonics had never occurred on Earth, our world would be very different from what it is today, and it is very likely that we would not even be here to talk about it.

The Earth’s magnetic field protects it from cosmic dangers

Aurora borealis on Earth: at the poles, the energetic particles in the solar wind excite the Earth’s atmosphere, creating magnificent light shows (© Creative Commons).The geological vigour of a planet also fuels another phenomenon crucial to all life forms. Deep in the bowels of the Earth, molten metal is subject to complex convective motions that create a global magnetic field.

Completely invisible unless you have a compass or are lucky enough to witness the northern lights, this magnetic field surrounds the planet with an invisible web of lines of force, and enables us to carry on with our lives without worrying about unsuspected cosmic dangers.

The Earth’s magnetic field acts as a shield, deflecting high-energy particles from the Sun or from the depths of our galaxy, the Milky Way. If these particles were not deflected by a magnetic field, this abrasive radiation could easily “shred” the atmosphere, wearing it down to such a point that it could actually disappear into space. The particles could also cause major damage to any life forms on the Earth’s surface.

Understanding the fate of Mars

Learning about the internal structure of Mars might appear to be a purely academic exercise, at least until we compare the Red Planet to Earth.

When we compare the two bodies, we are bound to be struck by their different fates, despite both of them starting off with the same potential. As already seen, some 4.5 billion years ago, Mars and Earth were formed within a near-identical sector of the accretion disk. As a first approximation, the two planets were assembled in the same conditions and from the same stock of building materials.

A true paradise within the hostile, frozen depths of the universe, Earth was able to host life and nurture its development over several billion years. In comparison, the image we have of the Red Planet today is both troubling and intimidating. Mars no longer has any magnetic field, and with an atmospheric pressure almost 200 times lower than Earth’s, the thin layer of air surrounding the planet cannot do much about heating up the surface. On the ground, conditions are glacial, with a mean temperature of -53°C.

While the majestic volcanic structures on Mars remind us of its glorious past, all of them appear to be extinct now. None of the rovers has come across the tiniest dribble of lava or slightest hydrothermal activity. Finally, if the process of plate tectonics began shortly after the planet’s creation—a subject of much debate among experts—it stopped shortly after, and the Martian lithosphere is not broken up into moving plates like the Earth’s, but forms a single very thick, rocky shell under which lies the mantle and core.

Why such differences? Why did Mars, which had the potential to become a second Earth and host the origins of life, become a desolate, barren planet? The answer lies in the depths of Mars, and it is up to the InSight probe to find it.

Last updated: 14 August 2017

Is Mars cracking from the inside?

Geographical location of faults on Mars. By mapping ancient faults, the movements to which the crust has been subjected in the past can be determined. (© Anderson, 2001).

The vast majority of earthquakes on Earth are related to plate tectonics. The surface of our planet is divided up into plates that are in constant relative motion. Wherever the plates are separating, colliding or slipping, enormous tensions build up. These tensions are subsequently released during earthquakes.

Based on their current understanding, planetary scientists do not believe that tectonic plates have ever existed on Mars. It appears to be a planet whose surface has never fragmented into moving plates. Consequently, any seismic activity is likely to be much less intense than on Earth.

Intraplate seismic activity

While terrestrial earthquakes are concentrated mainly along the boundaries of the plates in the lithosphere, some take place within the plates themselves. Known as intraplate earthquakes, these are mainly caused by the slow cooling of fairly deep rocks. As they lose heat, rocky materials contract and build up tension that is eventually released, resulting in an earthquake. Geophysicists believe that the Martian globe could be a source of tremors similar to intraplate earthquakes. However, do we have any proof that they exist?

Mechanism behind the formation of an extensive fault (© rights reserved / Belin).

So far, no reliable data for assessing Martian seismic activity has ever been obtained. The only functional seismometer ever to operate on the Red Planet, flown on the American Viking 2 probe in 1976, unfortunately failed to provide any convincing information due to disturbances caused by the wind.

Quakes are caused by the build-up of stress in rocky material which, slowly but surely, eventually leads to an abrupt rupture. A slab of rock that once formed a cohesive unit is suddenly split in two, each half being mutually displaced. The rupture zone gives rise to a fault that could quite well reach the surface of the planet, even if the quake took place at some depth. The slabs slip on either side of the fault, sometimes over great distances, and sometimes causing spectacular changes to the landscape.

Thanks to the many satellites orbiting Mars that are flying increasingly powerful and sophisticated instruments, we now have a very good understanding of the characteristics of the Martian surface. The images provided by these orbiting cameras clearly show the presence of fault lines scarring the Martian crust.

Martian faults

A magnificent fault slip on Mars (white line) observed by the American Mars Reconnaissance Orbiter (© NASA/JPL/University of Arizona).

A minutely detailed map of the faults criss-crossing the surface of Mars, spread over the course of Martian geological time, has enabled five major periods to be identified during which tectonic activity has taken place. The energy released during the event has been assessed by determining the length of the faults and estimating both their depth and the rigidity of the rocks.

It has also been possible to identify the regions that were most seismically active in the past by counting the number of faults visible on the surface. Such is the case, for example, of the dome of Tharsis, an immense bulge in the Martian surface that supports giant volcanoes more than 10 km high.

During its formation, the dome of Tharsis exerted huge stresses on the crust, which is fissured in a great number of places. The huge scar of Valles Marineris, a complex of gigantic canyons 4,000 km long and up to 7 km deep encircling Mars’ equator, was probably opened up following the Tharsis uplift. Valles Marineris is, without a doubt, one of the most impressive tectonic structures in the solar system, even though once they were formed, the canyons were widened by other geological processes such as erosion or landslides.

Mars in a Minute video series : Are There Quakes on Mars ? (© JPL-Caltech).

Although geophysicists reckon that most of the quakes that have occurred on Mars up to now have not produced any ruptures at the surface (and therefore cannot be observed by orbiters), analyses confirm that Mars has undergone (and is probably still undergoing) significant seismic activity of an order of magnitude similar to that of intraplate earthquakes. The SEIS seismometer on the InSight probe therefore has a very good chance of observing tremors on Mars for the first time in the history of Martian exploration.

Elevation map of the Martian surface showing the dome of Tharsis (left) and the huge Valles Marineris canyon (centre). These two regions must have undergone intense seismic activity in the past (© MOLA Team/NASA).

Last updated : 10 august 2017

A natural source of seismic waves, meteorite impacts will be a valuable ally for InSight

A recent impact on the surface of Mars (© NASA/JPL/University of Arizona).

To probe the depths of Mars and determine its internal structure, the InSight mission relies first and foremost on the planet’s seismic life.

Although seismologists expect a level of seismic activity much lower than that on Earth, they nonetheless hope to see a few quakes during the mission and take advantage of the seismic waves generated by the tremors to pierce the mysteries of Mars’ planetary interior.

However, it is entirely possible that Mars has stopped trembling for good, having exhausted all its internal heat. The Red Planet may actually be geologically dead, a still and frozen rocky globe circling the Sun. This plausible situation has been taken into account by the scientists involved in the InSight mission. Should it prove to be the case, a celestial phenomenon will nonetheless be used to advantage to probe the interior of Mars: asteroid impacts.

Impacts and collisions

Billions of years ago, when Mars and Earth were formed from the cloud of dust and gas that surrounded the young Sun, their surfaces were continually bombarded by showers of asteroids and comets that left spectacular scars in the form of craters.

Fortunately, the skies today have become much calmer, but occasionally in its voyage around the Sun the Earth encounters rocky fragments wandering aimlessly through interplanetary space. If they are sufficiently massive, these rocks pass through the atmosphere leaving a fiery trail, landing on the surface in the form of meteorites. Rare and highly sought after, these celestial pebbles are a source of great joy to scientists and private collectors alike.

A recent impact on the surface of Mars (© NASA/JPL/University of Arizona).

Meteorites do not fall exclusively onto our own planet. These rocks from the sky also fall on Mars, and vehicles such as Opportunity or Curiosity encounter them from time to time during their exploration of the Red Planet. The rovers then stop to take photographs and carry out a few chemical analyses before quickly continuing on their way. Their mission priority is first and foremost to study Martian rocks, and meteorites are regarded simply as amusing curiosities.

Meteorite impacts on Mars are important to InSight in a completely different way, since each impact is capable of generating the shock waves that seismologists so eagerly await.

To understand the usefulness of meteorite impacts to the InSight mission, it is necessary to look a little more closely at what happens during an impact, on the understanding that there are differences according to whether the meteorite falls on Earth, the Moon or Mars.

The dynamics of an impact

When an asteroid crashes into our planet, it first strikes dense layers of atmosphere at incredible speed, more than ten kilometres per second. Air friction causes the projectile to heat up considerably, at which point most are completely vaporised before they reach the ground. The descent of an impacting body produces first and foremost an acoustic shock wave that propagates ahead of it and strikes the ground violently, leaving a visible trace on seismographs.

If the impacting body is not totally consumed during its passage through the atmosphere, part of its mass may reach the surface and blast out a crater. The energy released by the collision then creates new shock waves that differ from those generated by the atmospheric blast, but which are just as interesting to intercept and study.

Mechanism of an asteroid impact: comparison between Earth, the Moon and Mars (© David Ducros).

However, the mechanism we have just described varies with the nature of the planetary body. Having no atmosphere at all, the Moon obviously provides no resistance to falling meteorites, which arrive intact on the surface and strike the lunar soil with full force. The network of seismic stations left on the Moon by the astronauts from the Apollo missions has detected thousands of meteorite impacts, helping to characterise the inner depths of our own natural satellite. These impacts represent about 20% of the quakes observed by the Apollo seismic network.

Mechanism behind the formation of an impact crater (© rights reserved/ Belin).

With a very thin atmosphere of carbon dioxide (6 mbar of pressure on average, which is nearly 200 times less than that of the Earth’s atmosphere), the situation of Mars is somewhere between that of the Earth and the Moon.

On Mars, the fall of a meteorite is accompanied not only by a shock wave that propagates through the air but also by a cluster of seismic waves related to the surface impact itself and the related excavation of a crater.

Although the Martian soil could significantly attenuate the atmospheric shock wave because of the broken nature of its surface, the energy waves generated by the impact with the surface could on the other hand travel great distances over the surface and be used to advantage in determining Mars’ internal structure.

The main problem is related to the energy released by the impact. The small impacts that create craters of just a few metres in diameter are statistically far more numerous than the impacts that can hollow out a crater 100 metres wide. However, it is the latter that are of interest because of the energy they liberate. Small impacts are less likely to be detected, and hence allow the deep interior of the planet to be probed.

An impact capable of making a crater on Mars 100 metres in diameter occurs roughly every 10 years. It is therefore highly unlikely that InSight will witness such an event during its stay on Mars. Unless the mission is very lucky, it must instead count on much weaker impacts that will only shed light on the near-surface structure between the probe and the crater.

Precise location of quake hypocentres

Besides the fact that they can generate seismic waves similar to those released by conventional quakes, and just as useful for probing the planetary depths, impacts have a second advantage.

With just a single seismometer, InSight cannot triangulate a quake to locate its epicentre. The position of a quake, as well as its depth, on the Martian globe cannot be known to any great precision.

An iron meteorite discovered by Curiosity in the Gale impact crater (© NASA/JPL-Caltech/LANL/CNES/IRAP/LPGNantes/CNRS/IAS/MSSS).

The situation is completely different for impacts, since these leave visible scars on the Martian crust.

One of the US satellites in orbit around Mars—Mars Reconnaissance Orbiter—is fitted with a very powerful telescope that can take high-resolution images of the planet’s surface. Since arriving, Mars Reconnaissance Orbiter and other orbiters have already identified several hundred meteorite impacts that have occurred over the last few decades.

The following scenario might therefore take place. A meteorite impacts the region where InSight has set down. The shock liberates seismic waves that reach the lander and are then registered by the SEIS seismometer. Suitably processed, the seismic signals can approximately locate the point of impact. The coordinates are transmitted to Mars Reconnaissance Orbiter, which then makes several fly-bys to photograph the target region.

The aim is to identify the traces of an impact on the surface. However, this operation is only possible if that sector has already been imaged before. If an impact appears on an image it will be impossible to know if it has just been produced or if, on the other hand, it dates back several years without having been noticed.

Artist’s impression of a impact on Mars near the InSight probe (© IPGP/Manchu/Bureau 21).

In preparation for the arrival of InSight, Mars Reconnaissance Orbiter has been carrying out the painstaking work of criss-crossing a region of roughly 5,000 km2 around the landing site. The data collected will be used to create a map to serve as a baseline for identifying new impacts.

Collaboration between InSight and Mars Reconnaissance Orbiter for the localisation of an impact-type seismic source (© IPGP/David Ducros).

Hence, if in the sector indicated by SEIS an impact crater should appear on a previously virgin surface, the position of the source of seismic waves will have been determined with remarkable precision. The mathematical algorithms used to process the signals registered by the seismometer will be refined, thus making future location operations even more reliable.

By documenting the rate of impacts currently striking the surface of Mars, InSight will also be able to refine the dating of the Martian terrain (the basic rule is that the older the terrain, the bigger and more numerous craters it contains). The danger inherent in falling meteorites for crewed missions will also be better qualified. Last but not least, it should be noted that InSight can only measure the meteorite flux in terms of events (number of impacts per year) and not in terms of the mass deposited on the surface by the impacts.

Last updated : 4 april 2018

Can seismic activity be studied with a single seismometer?

By definition, seismology is a science that depends on networks. Earth is currently covered by a grid of over 20,000 seismometers that form a gigantic web around the globe and allow an earthquake to be located and characterised almost instantly, regardless of its size and origin. Massively interconnected, this network gives scientists direct access to data and considerably facilitates interaction between geophysicists around the world.

Some of the great seismological discoveries have been made with a small number of stations. The discovery of the Earth’s metallic core by Oldham in 1906 needed only 14 earthquakes, whose tremors were intercepted by a few dozen listening stations. However, the installation of a network of stations is imperative in seismology. For example, one of the basic rules stipulates that, in order to locate the depth and epicentre of a quake, at least three seismometers are required for triangulation.

Principle of triangulation to locate the epicenter of an earthquake (© IPGP / David Ducros).

Hence, the first planetary seismological projects on the Moon or Mars required network of stations to be set up. Instruments placed on the Moon during the Apollo missions gave geophysicists a mini network of five stations: four seismometers placed during the Apollo 12, Apollo 14, Apollo 15 and Apollo 16 missions, plus the gravimeter on Apollo 17). After the Moon, the first attempt to probe the centre of Mars was in 1976 with the Viking mission. The seismological experiment depended on two seismometers, one for each lander, but gave no useful results.

The post-Viking Martian projects were particularly ambitious with regard to the number of stations to be landed: MESUR consisted of 16 seismometers, and Netlander was planned to land another four. In 1996, Mars 96 took off with two seismometers on board, but failed to leave Earth orbit. The fact that the InSight lander is flying only one seismometer perfectly demonstrates the gulf that separates the desires of scientists, who want to deploy the greatest possible number of seismometers, and the stern limits of reality.

Seismology using a single station

Detection of the core by direct propagation of seismic waves. Note that on the bottom illustration, S waves convert into P waves for the liquid external core. (© rights reserved).

Despite the fact that SEIS is an ultra-sensitive gem of technology that can measure ground displacements smaller than the diameter of an atom, it is legitimate to wonder about the instrument’s limitations, given that it is on its own on the surface of Mars.

Operating on its own, without the help of any companions, can SEIS really significantly advance our understanding of the depths of Mars and the processes going on in its interior?

Astonishing as it may seem, the scientists and engineers behind SEIS have done all that they can to gather a maximum of data. Thanks to several clever tricks that take advantage of the special features of Mars as well as sophisticated signal processing techniques, most of the limitations inherent in operating solo, without a network, will be addressed.

Seismic activity

As we saw previously, Mars appears to be a celestial body with little promise for seismic study. On Earth, the vast majority of quakes are related to plate tectonics. However, this phenomenon is absent on Mars, so we can expect the planet to be affected by seismic activity roughly 100 times weaker than that on Earth. Consequently, marsquakes will be less intense and far less numerous than earthquakes. To pierce the secrets of Mars’ interior, the ideal would be to make use of a multitude of quakes releasing a large amount of energy.

Optimistically, geophysicists expect to be able to observe a few quakes of magnitude 5 or 6 over the course of one year on Mars (equivalent to two years on Earth), whereas Earth has roughly 1,500 seismic events of magnitude 5 per Earth year.

On the other hand, Mars is much quieter than Earth: there is neither human activity nor the noise of pounding ocean waves on Mars. Atmospheric circulation can obviously pose problems, but given the absence of vegetation, there are no trees to transmit the sound of the wind down into the ground. This silence is a real boon when it comes to listening to tremors of feeble intensity, provided, of course, that the available instrument is sufficiently sensitive, which is the case of SEIS.

Location and magnitude of a quake

Let us assume that a quake occurs and that we wish to locate it on Mars with the only station there is on the surface, SEIS. How do we go about it?

The distance between the epicentre, i.e. the point on the surface immediately above the zone from which the tremor originated (known as the hypocentre) and the receiving station (i.e. InSight), can easily be calculated by measuring the difference between the arrival time of the P-waves (which arrive first at the seismometer) and the arrival time of the S-waves (which arrive second). The error here is only about 10%.

To locate the epicentre, we need to determine not only the tremor’s distance from the station but also its direction, i.e. its “azimuth”. The direction from which the seismic waves arrive can be determined due to the fact that SEIS measures seismic signals in all three spatial directions. Using the horizontal plane data provided by the seismometer’s pendulums, direction can be calculated to within 10°.

Finally, the magnitude of the tremor—a measurement of the amount of energy released—can be calculated by measuring the amplitude of ground displacements, taking into account the distance between the source of the quake and the seismometer, knowing that whether the seismic waves travel through the interior of the planet or around the surface, they will always be attenuated.

Even though it will be difficult to locate small quakes, SEIS will nevertheless provide data that will enable a tremor to be related to a particular region of Mars and hence be correlated with the geological structures that are visible on the surface (e.g. faults or volcanoes), as well as larger-scale geophysical features.

A quick trip around Mars before disappearing

In order to locate quite large tremors, InSight has other tricks up its sleeve. The diameter of Mars is half that of the Earth, which is advantageous when observing quakes. Since its circumference is smaller, seismic waves have less far to travel from the point of origin (i.e. the hypocentre) to the receiver. With shorter propagation distances, the attenuation is reduced since less energy is lost.

Circulation of surface (Rayleigh) waves R1, R2 and R3 (© rights reserved).If the tremor has sufficient energy (greater than magnitude 4.5), the surface waves can even pass the InSight landing site several times before finally vanishing due to attenuation. On Earth, quakes of magnitude at least 6 are necessary for such a phenomenon to be observable, i.e. a difference of roughly an order of magnitude. In other words, a quake on Earth must release more than 40 times as much energy than on Mars to be able to generate surface waves capable of travelling the whole way around the planet once.

The illustration above shows the propagation of surface waves for a marsquake. The surface waves that travel from the quake towards the InSight lander, and which are the first to reach the seismometer, are known as R1. The waves that travel in the opposite direction around the planet, and which have a greater distance to cover before reaching the lander, arrive second; these are known as R2. Finally, if the quake has sufficient energy (not less than magnitude 4.5), then the R1 waves—after having once passed the station—are capable of travelling all the way around the planet once more, thus passing the lander for the second time. These are the R3 waves.

In such a scenario, the distance separating InSight from the source of the quake, its time of origin, and the average speed at which the waves propagate over the planet’s surface, can be estimated by combining the arrival times of the R1, R2 and R3 waves. As before, the azimuth (the direction of the quake) is estimated from the signals recorded by the seismometer’s horizontal sensors. Quakes of magnitude 4.5 or greater are relatively rare on Mars, but geophysicists believe that over the duration of the mission (one Mars year, i.e. two Earth years), it should be possible to observe between 3 and 5 such events.

It is important to note that the effectiveness of the techniques to be used for InSight have been validated on Earth with data from single stations. These have been able to reconstruct the Preliminary reference Earth model (PREM), a model of the Earth’s internal structure commonly used by geophysicists with acceptable error bars.

There are unknowns, however, and the validity of the technique summarised above can be confirmed only when InSight lands on Mars. The Red Planet may also have a few small surprises in store for us. On the Moon, geophysicists were astonished to discover that the lunar crust caused enormous diffraction of the seismic waves, preventing the existence of surface waves. Since the Martian crust, just like the lunar crust, was exposed to a massive bombardment of asteroids early on in the formation of the solar system, its pulverised nature and numerous craters, especially in the planet’s southern hemisphere, could also cause seismic waves to be diffracted; seriously complicating analysis. Fortunately, however, the effect is smaller at the low frequencies to which very broad band seismometers such as SEIS are sensitive.

The origin of marsquakes

Three possible scenarios as to the geographical location of marsquakes: from left to right, uniform distribution over the Martian globe; clustering in the volcanic provinces of Tharsis and Elysium; or a non-uniform distribution (©rights reserved).

Apart from the quakes per se, other sources of seismic wave can come to the aid of the SEIS seismometer in its quest to investigate the depths of the Red Planet. Whether on Earth or Mars, these planets are still exposed to a hail of meteorites, mostly from the asteroid belt. A study of the high-resolution images of the Martian surface provided by Mars Global Surveyor and, more recently, Mars Reconnaissance Orbiter, has identified several very recent impact craters caused by meteorites of various sizes. With a little luck, a few impacts will generate a seismic wavefront sufficiently powerful to shed light on the planet’s interior.

It is also possible, though unlikely, that volcanic events can give rise to seismic activity. Although certain regions of Mars were volcanically active a mere 10 million years ago, no eruption or hydrothermal event has ever been detected on the surface. However, it is possible that there is still some magmatic activity, with magma continuing to shift through subterranean tunnels or chambers.

As we shall see in due course, the continual circulation of the atmosphere around the globe causes it to resonate. While this seismic “hum,” as it is known by geophysicists, is very subtle, it is perfectly audible to instruments sensitive to very low frequencies, like SEIS. The study of these eigenmodes (see below) will offer very promising possibilities in the investigation of Mars’ mantle.

As it orbits the Sun, Mars is accompanied by two small moons, Phobos and Deimos. Although much smaller than our Moon, they are nonetheless bound gravitationally to Mars and exert a (rather weak) force of attraction on it. Phobos, the largest natural satellite and the nearest to Mars, causes deformations in the ground that can be measured by SEIS. A study of these tides, whose forces are here exerted not on expanses of water but on rock, could provide critical information about Mars’ core.

To conclude this overview of the phenomena that InSight can exploit to achieve its mission, we note the interactions between the surface and the atmosphere. The action of the wind at the landing site will generate micro-seismic noise that will contaminate the signals collected by the seismometer, but ever-resourceful geophysicists can even make use of this to probe the first few hundred metres below the surface.

Finally, although SEIS is by nature a passive experiment, i.e. the seismic waves needed to study the depths of Mars are generated by natural phenomena without any human intervention, active measurements will nonetheless be taken when the drill attached to the HP³ instrument starts to penetrate the ground. The tremors generated by the staccato advance of the penetrator could actually help SEIS determine what lies hidden close to the lander’s feet.

Of course, these active measurements will not be in the same league as those made on Earth using vibrating trucks on the ground or seismic air guns in the sea. Neither can they be compared with those taken on the Moon during the controlled crash of rocket stages or lunar modules. However, in the world of space science nothing is wasted, and scientists seize the slightest opportunity for any additional. This is why the vibrations generated by the HP3 heat flow package will play a small but not insignificant role in determining the structure of the Red Planet’s surface.

Investigating the crust: dispersion of surface waves and receiver function

One of InSight’s major objectives is to measure the thickness of the Martian crust and identify the presence of discontinuities, i.e. the existence of layers that are different in terms of composition and structure (lava strata, frozen water, etc.) that will influence the propagation speed of seismic waves.

Geophysicists have several tools at their disposal to meet this objective. A study of the reflection of seismic waves within the crust will be particularly useful, as will techniques involving analysis of the dispersion of surface waves or “receiver function” techniques.

The surface wave dispersion technique is quite a powerful analysis tool for probing a planet’s crust. When a seismic wave packet travels through the crust after a quake, the low-frequency waves travel faster and are able to sample greater depths. Conversely, higher-frequency waves travel more slowly, and tend to remain slightly more grouped on the surface.

By analysing this variation in wave propagation speed as a function of frequency (called “dispersion”), geophysicists can obtain a great deal of significant information about the rocky surface structure of a planet’s crust, and even the underlying mantle if the low frequencies can be measured. This technique is particularly sensitive to changes in thickness, so a variation of just 10% in the crust’s thickness causes changes of 5% in the group velocity.

The “receiver function” technique is used to improve the signal-to-noise ratio by studying the echoes in the seismic signals. This entails in particular identifying those waves that have undergone a conversion (e.g. conversion of a P-wave into an S-wave) when passing through an interface. These generally less energetic waves require the signals to be summed so that they can emerge from the background noise and become visible.

Meteorite impacts and surface deformation caused by atmospheric disturbances will also provide important information about the Martian crust.

Investigating the mantle

The Martian mantle, especially the upper mantle down to about 600 km, will be probed using bulk seismic waves capable of traversing the planet (i.e. P- and S-waves). The surface wave dispersion analysis technique mentioned above could also prove to be useful.

The slow natural oscillations of Mars that geophysicists call eigenmodes will also be used to advantage. Like Earth, Mars can resonate like a bell if excited by a major quake or, more subtly, by the continuous circulation of the atmosphere. Geophysicists can use eigenmodes to tease out a whole mass of relevant information about the structure and composition of the mantle. One of the great advantages of eigenmodes is that no knowledge of the location of the source of seismic waves is necessary. However, only very wide band instruments such as the SEIS seismometer are capable of perceiving the very low-frequency hum associated with eigenmodes.

Quakes capable of exciting Mars’ eigenmodes and causing the planet to resonate are relatively rare. They must be of at least magnitude 5.5. From the beginning to the end of the InSight mission, only one or two tremors of this type can be expected. The advantage here is that the energy transmitted to the planet will be big enough to distinguish the oscillations from the instrument’s noise floor.

Unlike major quakes, the excitation of eigenmodes due to atmospheric turbulence is continuous over time. However, in terms of energy content, this excitation lies below the limit of detection of the SEIS seismometer. Hence it will be necessary to accumulate data over several months of observation to benefit from atmospheric excitation and see through the window that this very subtle phenomenon opens up into the depths of the Red Planet.

Investigating the core

Three models of the Martian core (© rights reserved).

Although deep down in the centre of the planet, Mars’ core is nevertheless within reach of InSight’s SEIS instrument.

It can be studied by monitoring the seismic waves (i.e. bulk waves) that propagate within the planet and are reflected off the surface of the core like a mirror before travelling upwards.

A detailed study of the rotation of Mars by the RISE geodesy experiment will also be able to tell whether the core is in a liquid or solid state, and to estimate its size. The tides of the Phobos moon should provide confirmation of these two aspects.

Last updated : 27 february 2018

The tidal forces exerted by Phobos, one of Mars’ two moons, will shed light on the planet’s core

Mars is accompanied on its journey around the Sun by two small irregularly-shaped moons, Phobos and Deimos. Phobos is some 27 km long from pole to pole, whereas the even smaller Deimos is just 15 km long.

The two moons look very much like asteroids: shaped like a potato, they have a very dark grey, almost black, surface marked by many craters. At first glance, they look very different from the Earth’s moon, which is much bigger and spherical. Yet the two Martian moons share a feature in common with the Moon, because all three are gravitationally locked to the planet around which they orbit, and therefore exert tidal forces on that planet.

Tidal forces

Phobos, one of Mars’ two moons (© NASA).

When you talk of tides, the first image that comes to mind is that of a beach with waves rolling up the golden sand before returning to the sea. What is less well known is that this regular phenomenon, which sets the pace of numerous activities on Earth, is due to the influence of stellar and planetary bodies (mainly the Sun and Moon) on liquid masses.

Despite the distance separating it from the Earth—some 384,400 km—the Moon is surrounded by a field of gravity that locks onto our own planet and grasps it in invisible claws, with astonishing consequences.

The effect is especially visible on seas and oceans because they are made of liquid. The difference between high and low tide is measured in metres. When the Moon passes over land, it also bulges, but the effect on land is much harder to see than the effect on water. The “tidal bulge” of land surfaces results in a deformation measured in millimetres or, occasionally, centimetres.

From its orbit 6,000 km above Mars, and despite being so small, Phobos also exerts tidal forces on the Red Planet. Its presence subtly deforms the planet, which bulges by a fraction of a millimetre along the path followed by Phobos.

Mechanism of action of the attraction force of the Moon Phobos on Mars (© IPGP / David Ducros).

You might think that the bulge on Mars caused by Phobos’ attraction should lie exactly underneath the moon, but things are not that simple. The bulge does not occur exactly when Phobos flies over a given region, but appears after a certain delay, due to the fact that part of the energy transmitted by Phobos is absorbed by Mars.

As the law of conservation of energy applies to this celestial dance between Mars and Phobos, the exchange of energy between the small moon and the Red Planet has an important effect on the moon’s orbit. Each year, Phobos drops about one centimetre closer. Slowly but surely, Mars’ moon is falling toward its surface.

In several dozen millions of years, Phobos will be so close to the Red Planet that it will literally be ripped apart by these relentless tidal forces. When the moon crosses the Roche limit, the tidal forces will be greater than the moon’s force of internal cohesion, and the moon will break up. A shower of fragments will then fall onto Mars, though the majority of Phobos’ remains may form a ring around the Red Planet.

Phobos: a trump card for InSight

Phobos above the Martian surface (© NASA).

Phobos is of particular benefit for the InSight mission. The SEIS seismometer flown aboard the probe is so sensitive that it will be able to detect and measure the smallest gravitational attraction that Phobos exerts on Mars whenever it passes over the Elysium Planitia landing site.

Phobos is of special interest because its orbit around Mars is not linked to other phenomena such as the day/night cycle. The small satellite revolves once around the planet every 7 h and 39 min, so passes over the same area three times a day.

The signal generated by Phobos will be recorded in particular by the seismometer’s vertical axis. However, the deformation related to tidal forces is so small that it will be impossible at first to distinguish it from the instrument’s self-noise.

By superimposing data covering several months or, even better, a full year, seismologists should nonetheless be able to extract and quantify the tidal forces generated by Phobos. This is the trickiest experiment of the whole mission. To be able to distinguish the influence of Phobos, the noise level must remain acceptable. If it is ten times greater than the signal, it will not be possible to identify even if long-term measurements are available for integration.

Exploring the Martian core

Characterizing the tidal forces exerted by Phobos opens up the possibility of probing the depths of Mars, and more particularly its core. Depending whether the core is liquid or solid, it will not be deformed in the same way. Its radius will also be determined within around 60 km, these data complementing those provided by RISE.

Even if InSight is unlucky enough not to witness a single seismic event, marsquake or meteorite impact throughout the mission, the probe will still be able to fulfil some of its objectives through Phobos.

In the world of space exploration, scientists try to take advantage of each and every phenomenon, however discreet it may be. Although orbiting several thousand kilometres above the Martian surface, the larger of Mars’s two natural satellites could make a significant contribution to the investigation of the planet’s interior structure.

Last updated : 18 september 2017

Atmospheric disturbances

Artist’s impression of a dust storm on Mars (© IPGP/Manchu/Bureau 21).

Unlike on Earth, the sources of noise able to disturb the acquisition of seismic signals are limited on Mars.

The Red Planet has no oceans or seas, a major source of noise on our own planet. Obviously there is no human activity to contend with either. Yet the job of the InSight probe’s SEIS seismometer will be complicated by the Martian atmosphere.

The interaction of the Martian atmosphere and its surface will leave a trace on seismic records. For a first approximation, this phenomenon may be negatively considered as a source of disturbance against which little or nothing can be done. SEIS will of course be sheltered from winds by its protective cover—the WTS—which will be efficient against most wind effects without being able to cancel them out completely. Atmospheric activities have other indirect effects that cannot be offset. They may be classified into two major categories.

Mars’s constant hum

The first is due to the global atmospheric circulation. Constantly flowing around the planet, the Martian air excites the planet, making it vibrate like a bell at precise frequencies.

Geophysicists call this the planet’s “hum”—a constant buzz that only seismometers sensitive to long periods (like SEIS) can hear.

Despite the fact that this constant hum may be considered an interfering background noise, it is of particular interest to geophysicists because it can be used to probe the upper layers of Mars from a depth of several dozen metres down to several hundred kilometres (giving information on the mantle), even when there are no quakes.

Some large-scale weather events, like the impressive dust storms that sometimes completely envelop Mars in an opaque, impenetrable veil, are likely to make the planet vibrate significantly. SEIS will be ready to measure the resulting hum.

Orbital view of a dust storm near the huge Valles Marineris canyon (© NASA).

The last global dust storm occurred in 2007, and these repetitive events occur roughly every 3 Mars years (the equivalent of around 6 Earth years). In November 2018, InSight will land during a period conducive to dust storms, and it is possible that the probe will witness this spectacular phenomenon.

During a global dust storm, the significant drop in sunshine and the accumulation of dust on the solar arrays could easily jeopardize a lander, but InSight has been designed to withstand such a disastrous scenario.

Turbulence and dust devils

The second atmospheric source of micro-seismic noise is local: the landing site is affected by gusts of wind or the arrival of a dust devil near the probe.

In both these cases, the Martian air either exerts pressure on the ground or removes it. A dust devil 10 m wide thus reduces the pressure on the soil of an amount equal to that of a small car being removed from the surface.

The surface’s static deformation will affect the seismometer, and more particularly the pendulums that measure horizontal motion. Although the ground also moves up and down, the effect of sideways movement is maximal, and this will be picked up by the SEIS instrument’s sensors.

Simulation of ground deformation around the InSight lander (© IPGP/David Ducros).

When exerting a load—to whatever degree—the atmosphere will create very slight slopes in the surface that SEIS will inevitably record. It is as if the atmosphere was using the surface of Mars like a drum, the multiple variations in pressure on the Red Planet’s soil being like fingers constantly but erratically tapping a drum skin.

The passage of a dust devil or turbulence also has another effect. It creates both a sound wave that propagates through the air and a high-frequency surface wave that travels along the Martian ground.

Once again, both these waves will be plotted on the seismograms. The instrument will therefore “see” an atmospheric disturbance of any kind—for example a dust devil, whether or not it actually raises dust and can therefore be seen or not by the lander’s technical cameras—before it actually arrives at the landing site, and monitor it until it moves away and finally disappears.

Dust devil photographed by the US Mars Reconnaissance Orbiter over the dull plains of Amazonis Planitia (© NASA/JPL/University of Arizona).

The passage of a dust devil within the vicinity of the InSight probe may then be confirmed by the powerful camera on board Mars Reconnaissance Orbiter, capable of identifying the marks on the ground left by these miniature whirlwinds that zigzag freely over the Martian surface. The ultra-sensitive pressure sensor of the probe’s weather station will also provide precious data.

The good news is that by analysing atmospheric disturbances around the landing site both in the high- and low-frequency range, geophysicists will be able to “virtually” penetrate the subsoil down to a few hundred metres in order to study its properties. They will, for example, be investigating the depth of the regolith—the layer of soil pulverised by the impact of innumerable asteroid and meteorite impacts—and looking for discontinuities between layers of material with different physical properties, then estimating their elasticity (which depends on the composition of the rocks). In other words, they will be turning a disadvantage into an advantage.

Last updated : 1^er february 2018

The mystery of Mars’ interior

Internal structure of Mars (© NASA).

Unlike our own planet, whose internal structure has been well defined, our knowledge of the interior of other planets in the solar system is as yet very limited. Geophysicists gaze at them longingly, dreaming of shedding light on their internal structure just as they have the Earth’s. Because of its similarities and differences with our own planet, Mars is a high-priority target for them.

Up to now, the only measurements relating to Mars’ internal structure have provided information on its mass and diameter (needed to calculate density) and a parameter that geophysicists call the moment of inertia. This was determined from research on the planet’s rotation using probes placed on its surface (see InSight’s RISE experiment). This type of measurement gives geophysicists an idea of the way mass is distributed inside a planet.

Orbiting probes, sensitive to the slightly bumpy nature of globes and the unequal distribution of mass (which modifies the intensity of the gravity field), have only managed to confirm that Mars has the same basic envelopes as the Earth and to estimate the average thickness of the Martian crust.

The presence of a global magnetic field is generally a good indicator of the existence of a molten metal core subjected to convective motions. However, neither Mars nor Venus has such a field.

Numerous uncertainties remain about the structure and activity of the Martian mantle (© Ana Plesa/DLR).Finally, fragments of rocks ejected from the surface of a planet into space can sometimes land up on the Earth. If they are picked up and identified, these extraterrestrial stones (known as meteorites) can provide geophysicists with valuable information. The mineralogical analyses carried out on Martian meteorites have allowed extrapolations to be made on the nature of the crust and mantle. Important contributions have been made by geochemical measurements taken by orbiting satellites, in addition to in situ analyses carried out on the surface of the Red Planet by the Curiosity rover among others.

However, despite the efforts undertaken to attempt to understand what is hidden in the depths of the Red Planet, there remain many unknowns. The limits and constraints imposed by the measurements carried out to date do not enable us to design even a very simple model of Mars’ internal structure. The exact size and composition of the internal envelopes remain a mystery, because measurements of gravity do not result in a single model of the internal structure. Several solutions—sometimes significantly different—can explain, or at least remain coherent with, the measurements taken.

The acquisition of new information through the application of techniques such as seismology, suited to sounding the inner depths of planets, is thus crucial to extend our knowledge of the mechanisms governing the formation and evolution of planets.

Yet, as paradoxical as it may appear, of all the missions that have landed on Mars up to now, only one—Viking 2—enabled an operational seismometer to be placed on the surface. Unfortunately, throughout its operational life, the instrument only recorded a single event that could have been caused by a marsquake. However, even today—40 years later—this hypothesis is a subject of controversy among the scientific community, and the mystery of the true nature of the vibrations recorded remains to this day.

Mars's internal structure : core, mantle, crust and atmosphere (© IPGP/David Ducros).

Mars, a planet similar to Earth?

As we saw in the previous article, the internal structure of Mars is similar to that of Earth. It has three distinct layers: going inwards from the outside, there is a relatively thin rocky crust, a mantle and finally a metal core.

Many questions remain unanswered, however, when we try to define this structure in more detail. How big is the core, and what is it made of? Is it completely solid, still liquid or, like Earth’s core, does it have a solid inner core surrounded by a liquid envelope?

What is the Martian mantle made of? Is it subject to convective motions or does it remain immobile, transferring the heat accumulated in the centre of the planet outwards quite simply by thermal conduction? Are there any major discontinuities in the mantle, linked to significant changes in its composition?

How thick is the Martian crust? Is it homogeneous or, on the contrary, stratified (with ice lenses or alternating strata of sedimentary rock and volcanic flows)? All these questions, and many more, are essential because they are directly linked to the conditions present at the origin then development of the Red Planet over billions of years. There is also a very strong relationship with the planet’s habitability, i.e. Mars’ ability to host and sustain the development of life.

A hypothetical sequence showing the evolution of Mars’ internal structure (© rights reserved).

The Martian crust

The first Martian meteorite found on Earth in 3D (Chassigny, France) (© IPGP/MNHN).

A planet’s crust is formed by the melting of rocky material making up the upper mantle, followed by an upwards expulsion of liquid magma which then crystallizes as it cools down.

When you start looking in more detail, the mechanisms behind the formation of planetary crusts are complex. Thus it is that on Earth, the oceanic crust upon which the oceans are founded is formed differently from the continental crust, which is much older and of a different composition.

The Martian crust is thought to be between 30 and 100 kilometres thick depending on the hemisphere, with a mean thickness of 65 kilometres, a value that is closer to that of the Earth’s continental crust. However, its composition—basaltic rather than granitic—resembles that of the Earth’s oceanic crust. InSight’s seismic measurements will measure the crust’s thickness to within a few kilometres, and will reveal whether there is any stratification, a phenomenon which has not yet been observed.

Simulation of Martian magma plumes (© rights reserved).

Another fundamental difference is that the Martian crust appears to be completely motionless, whereas Earth’s surface is divided up into lithospheric plates that are constantly moving relative to each other.

It may appear that the geological machinery in motion on Mars has been simpler than that on Earth. The Red Planet is thought to be surrounded by a single plate that forms a thick, rigid outer shell. However, the mechanism behind the formation of this single crust remains a mystery to this day.

Several different scenarios may be hypothesized, each leading to a different crust thickness and a specific efficiency with regard to the planet’s capacity to dissipate internal heat.

If we assume that a very early, fleeting plate-tectonics process occurred on Mars (remembering that this is a subject of much controversy), the Martian crust would be fairly thin, and the planet would then have been able to efficiently evacuate its internal heat, at least at first.

A second possibility is that, without breaking up the lithospheric surface into moving plates yet involving the convection of materials in the mantle, the planet formed a moderately thick, motionless crust. In this “stagnant lid” regime, heat is evacuated normally but less efficiently than with plate tectonics.

Finally, there is a third case, linked to the appearance of an ocean of magma. Shortly after its accretion, Mars was constantly colliding with great violence against asteroids and huge bodies. Glowing red-hot as a result of the energy released, the rocky materials are thought to have melted, becoming a widespread ocean of magma of unknown depth. As it cooled down, this ocean is assumed to have formed a very thick crust, some traces of which may remain today on Mars.

Deeper down, in the mantle, a specific mechanism may have been at work. The upper part of the mantle may have solidified, forming aggregates denser than the lighter, underlying materials. Naturally unstable, these blocks of agglomerated matter could have sunk down towards the centre, causing material in the depths to rise to the surface and possibly even triggering convection by turning the mantle upside down. In this scenario, the very thick crust acts as an insulating layer, preventing heat exchange with the exterior. This problematic situation could quite easily, as we shall see later, be responsible for the disappearance of the magnetic field created by the core.

By accurately determining the thickness of the Martian crust, and measuring today’s heat flow, it will be possible to dismiss certain scenarios. Needless to say, the reality is no doubt more complex than these scenarios, and it is entirely possible that in the 4.5 billion years of its existence, Mars may have gone through different stages, all inextricably linked to one another. Distinguishing and understanding them may prove a lengthy and complicated business.

The mantle

Peridotite in terrestrial basalt (© IPGP/Philippe Labrot).

Sandwiched between the crust and the core, the Martian mantle must have a similar composition to that of Earth, going by the composition of Martian meteorites that have crashed down onto our planet. It could also be made up of olivine-rich rocks known as peridotites.

However, numerous questions remain as to its composition and structure. It could theoretically contain a discontinuity similar to that between the Earth’s upper and lower mantle, with a change in the olivine’s phase. However, because Mars is smaller than the Earth and its field of gravity is weaker, the Martian mantle is perhaps not deep enough for such a discontinuity to exist (it being absolutely crucial on Earth).

Theoretical simulations show that the Martian mantle should still be subject to convective motions, the planet having a sufficient amount of heat to fuel such motions. However, unlike the Earth, this convection has perhaps not been able to correctly homogenize the mantle’s material, so it is possible that the Martian mantle contains several independent deposits or reservoirs that were never in contact with each other. Geophysicists are also placing their hopes in InSight to resolve this question.

The mantle is a compartment that plays a major role in the development of a planet. By partly melting, the rocky material in the mantle forms magmatic fluids that infiltrate upwards until reaching the crust. There, they can trigger huge volcanic phenomena able to change the climate or foster life forms able to survive. Furthermore, as we shall see, by playing the role of heat exchanger between the core and the exterior, the mantle also has a huge influence on the planet’s magnetic field.

One of the InSight mission’s important objectives is thus to accurately determine the Martian mantle’s thickness, its composition and the presence or absence of strata.

Core and magnetic field

Ten-centimetre slice of a Glorieta Mountain pallasite (© Luc Labenne / Société Labenne Météorites).

Nowadays, Mars no longer has a global magnetic field like the one surrounding Earth.

The only measurements taken from orbit by the US Mars Global Surveyor probe have revealed the existence of a fossil magnetization in the rocks of the southern hemisphere, which go back several billion years. The younger rocks in the northern hemisphere, however, show no trace of any remanent magnetic field.

The observations made appear to clearly indicate that in a distant past, shortly after its formation, Mars must have had a global magnetic field capable of protecting the atmosphere from the abrasive solar wind and thus helping to establish the climatic conditions allowing liquid water to flow. Such a magnetic shield was no doubt generated by a dynamo effect, i.e. the convection of a molten metal in the planet’s core. Over 4 billion years ago, Mars’ core must have been partly or fully liquid, and the contrast in temperature between the core and the mantle sufficiently strong to have triggered such convective motion.

There is no such magnetic field today, which indicates that something dramatic happened at the very heart of the planet around 4 billion years ago. At this point in time, we can only speculate about what actually occurred. It is possible, for example, that the planet cooled down so quickly that the core became motionless. The liquid layer crystallized, becoming so thin that convective motion stopped, leading to the definitive disappearance of the planet’s global magnetic field. In this case, the Martian core would have completely solidified. In the light of the latest data collected by satellites orbiting around Mars, this is not the most likely scenario. The planet’s gravitational response, as observed by orbiting probes, tends to favour the presence of a liquid state. The recordings of the SEIS seismometer flown on the InSight mission will help clear up this nagging uncertainty.

While a massive cooling of the planet’s interior can lead to the shutting down of Mars’ dynamo, another directly opposed scenario is also feasible. Convection can only occur if there is a sufficiently large temperature difference between the core’s centre and its edge or surface.

The core’s heat must be evacuated efficiently up through the mantle for convection to occur. On the other hand, if the mantle suddenly stops conducting heat effectively and forms an insulating layer, the core can no longer dissipate its own heat and the convection motions among the molten metal gradually disappear, along with the global magnetic field.

The Martian core could therefore be fully or partially molten despite the fact that the magnetic field disappeared billions of years ago. The situation is made more complicated by the fact that the core’s fate depends not only on the properties of the mantle, but also the composition of the core itself.

Mars in a Minute video series : What's inside Mars ? (© JPL-Caltech).

The study of iron meteorites having landed on Earth from the asteroid belt shows that planetary cores are mainly formed of iron mixed with a small quantity of nickel. As incredible as it sounds, these meteorites are thought to be fragments of the metal core of stillborn planets; bodies that had begun to form around the Sun but which were then torn apart by huge collisions. They therefore provide clues as to the composition of cores which, buried deep in the heart of planets, are completely inaccessible to direct human investigation.

Telluric planets such as Mars or the Earth contain not only iron and nickel, but traces of other elements such as silicon, sulphur or even hydrogen. Although in the minority, these elements could play a significant role in the fate of planets.

The geochemical models deduced from the analysis of SNC meteorites from Mars (shergottite, nakhlite and chassigny classes) suggest that the Martian core contains more sulphur than the Earth’s. A significant amount of this element may have migrated into the planet’s interior along with the molten iron, with major consequences. The more sulphur there is, the more liquid the core tends to remain, even when the temperature decreases. If the Martian core does contain a significant amount of sulphur, it may still be completely liquid. However, if the core contains little sulphur, the molten metal could have completely or partially solidified. The liquid and/or solid state of the Martian core therefore depends on its composition as well as the planet’s thermal evolution (i.e. its cooling over time).

The core’s size, which is unknown with any degree of accuracy, is however related to the conditions in which the planet was formed. It is also linked to the differentiation process, the crucial phase in the formation of a planet when very dense metallic elements separate from the rocky material to descend into its centre and agglomerate to form a core. The Martian core is considered to have a radius of about 1,700 kilometres, but its exact size is not known precisely, it being estimated to within 250 to 300 kilometres. InSight will reduce this uncertainty by at least a factor of four.

The Red Planet itself has a radius of 3,389 kilometres, so the core appears to take up quite a large part of its inner volume, even if proportionally speaking it is smaller than the Earth’s. This difference in size is probably due to the fact that part of the iron that could have sunk down into the core may have remained in the mantle, incorporated in iron-rich minerals.

As well as measuring the core’s dimensions, InSight will also provide information allowing the core’s density to be assessed more accurately. For the moment, it is estimated at 7 g/cm³ (+/- 1 g/cm³), a high density associated with a small core, in order to preserve the planet’s total mass.

By enabling geophysicists to accurately determine the size, state and composition of the Martian core, InSight will greatly advance our knowledge of an envelope of fundamental importance to the Red Planet and which no doubt holds the key to the specific fate of this stillborn body.

Three models of the core’s development

Three scenarios of the Martian core’s evolution (© rights reserved).

To conclude, let us consider three different models of the internal structure of Mars, all three of which are plausible in the light of the data available today but which may prove completely false when the first results of InSight’s SEIS seismometer reach the Earth..

The first model suggests a sulphur-rich metal core. This scenario suggests that once formed, Mars had a completely liquid core whose convective motions generated a global magnetic field. The mantle was also subject to convection, but the Martian crust did not break up into floating plates and no plate tectonics ever occurred.

As the planet cooled down, the liquid core’s convection decreased and finally stopped, leading to the disappearance of Mars’ magnetic field. The convective motions in the mantle may have continued, but it is also possible that the material making up the mantle also became immobile over time.

The second model is based on a sulphur-poor core. Mars’ internal structure would thus be similar to that proposed by the first model: a crust comprising a single plate covering the whole of the planet and a mantle subject to convective motions. Instead of being completely liquid, the core in this scenario has an external part that is liquid and whose convection is sufficient to create a global magnetic field, and an inner part that is solid due to the crystallization of the initial molten metal. The difference between this model and the first one is based on the hypothesis that, following the planet’s cooling, the core rapidly continued to gain mass, only leaving a liquid layer too thin to be able to host convective cells able to generate a magnetic field.

The third model, independent of the quantity of sulphur in the core, involves the appearance of a major mechanism that changes everything: plate tectonics, that allows the planet’s internal heat to be evacuated efficiently. In this scenario, the Martian surface—like that of the Earth—is divided up into mobile lithospheric plates. Under the crust, the mantle is subject to convective motions that contribute to plate movements. The core is composed of a convective liquid outer core and a solid inner core.

Following the efficient cooling of the planet’s interior, or due to a specific event such as a major collision, the mantle’s convection stops. This automatically leads to the cessation of plate tectonics and has a dramatic effect on the mantle’s convection, which also stops. The domino effect leads convective motions of the outer core’s molten metal to stop, leading to the disappearance of the global magnetic field.

The final result is the same for all the models, which is normal given that all three scenarios must lead to the final situation as we know it: Mars loses its protective magnetic field. Exposed to the abrasive solar wind, the atmosphere is lost in space, surface temperatures plummet, liquid water no longer flows and a global, definitive desertification process is triggered.

Last updated : 3 june 2019