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