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For the first time ever, a space mission is going to investigate the core of Mars

Scheduled for lift-off in May 2018, the InSight mission will be a dream come true for geophysicists, who have long wanted to send a seismometer to Mars to study the tremors that are still rippling through the desolate lands of the Red Planet. They will at long last be able to lay bare the secrets hidden deep inside.

The secret of an extraordinary destiny

The InSight Lander (© NASA/JPL).The InSight lander with its two solar arrays deployed. The seismometer is visible on the ground, attached to the probe by a flexible tether (© NASA/JPL).

While the Martian surface is revealing more and more of its secrets thanks to the satellites that have monitored it from above and the probes that have landed there, the interior of Mars remains unknown, as no past missions have ever explored it.

Yet it is deep in the heart of the planet that lie some of the answers to the mystery surrounding Mars: why did this planet, which was so similar to Earth 3 to 4 billion years ago, have such a dramatically different destiny? Why is what was once a hospitable planet with a protective magnetic field, a thick atmosphere and intense volcanic activity—a place where liquid water could freely run over its surface—now a barren, frozen planet stuck in a torpor that nothing can now overcome? Why is our own planet, Earth, so extraordinarily different?

A planet’s ability to become—and remain—habitable, is intrinsically linked to the power of its internal heat processes.

Seismology, an extremely powerful investigative tool, is the discipline that studies the characteristics of seismic waves emitted by earthquakes and the way they are propagated, even all the way to the centre of the Earth. Over about a century, geophysicists have patiently pieced together the jigsaw so as to understand what lies underneath our feet. Gradually, we have discovered that, just like an apricot, the Earth has a crust (its “skin”), a mantle (its “flesh”) and a core (its “kernel”).

The Earth’s core is composed of metal, the outer part of which is molten. The electrical currents passing through it generate a magnetic field which protects our world from the Sun’s lethal radiation and charged particles. The rocks making up the mantle are so hot they actually soften and partly melt, being able to move around one centimetre a year. Driven by the planet’s inner heat, the mantle is where slow but extremely powerful movements take place that are responsible for shifting continents and oceans.

Martian seismology: an opportunity every 20 years

Unsurprisingly, having set up seismometers on the Moon, geophysicists then turned their attention to Mars, of far greater interest because it is much more similar to Earth from a geological point of view.

Unfortunately, space missions have focused on looking for signs of life (whether past or present), leaving very little room for geophysicists. Seismometers have simply been left out of the picture.

Up to now, only two seismometers have ever reached the Martian surface. Forty years ago, the two Viking missions succeeded in placing seismic instruments on Mars, but their operating conditions were such that no clear, meaningful measurements were able to be taken.

In 1996, 20 years after this first attempt, a new attempt was made with Russia’s Mars 96 probe. To the huge dismay of the teams involved, the spacecraft never even managed to leave Earth orbit.

In 2018, a little over 20 years after Mars 96, it will be the turn of the InSight probe to take up the challenge. Its lander is equipped with a single seismometer which is designed to be as robust as it is sophisticated. Its extremely sensitive but fragile mechanisms should record the planet’s seismic activity for the first time ever.

Artist's concept of the InSight lander (© IPGP/Manchu/Bureau 21).Unlike the Viking probes back in 1976, InSight has a robotic arm to place the seismometer on the ground for more efficient performance. Before its launch, the instrument— currently protected by multiple insulating layers—will be covered by a shield to protect it from wind and temperature variations. On Earth, seismometers are always buried, but this is not yet feasible on Mars (© IPGP/Manchu/Bureau 21).

An amazingly precise yet complex mechanism

The result of 20 years of relentless effort, InSight’s Seismic Experiment for Interior Structure (SEIS) is a technological masterpiece. Its truly unique mechanism has been specifically designed to operate in the hostile Martian environment.

It is one thousand times more sensitive than the seismometer flown on the Viking probes, and will be placed directly on the ground by a robotic arm, whereas the Viking seismometers were attached to the lander.

Under its multiple layers of thermal insulation, designed to protect it from the huge differences in temperature between the Martian day and night, it will for two Earth years patiently listen out for the least tremor on Mars’ surface. Practically nothing will escape its attention. SEIS is so sensitive that it can measure infinitesimally small movements on an atomic scale! If the ground moves by a distance of less than the breadth of one hydrogen atom, the instrument will still record the motion!

Alone on Mars

Durability and robustness are crucial characteristics for any instrument designed to be sent to Mars. SEIS’s additional challenge is that it will be alone throughout its mission.

Unlike terrestrial seismometers, that are networked so that together they can be used to pinpoint an earthquake, for at least two years SEIS will remain alone on its landing site, an equatorial plain in an area of Mars called Elysium Planitia. The rover belonging to the ExoMARS mission, which will be fitted with a seismometer developed by Russian Space Research Institute IKI, will only be launched in 2020.

Yet, by implementing sophisticated data processing techniques along with some clever tricks of the trade, geophysicists will be able to use a single seismic station to do the job that previously required several dozen seismometers.

No tremors? No problem: InSight has two aces up its sleeve

Top view of the InSight probe’s science payload (© NASA/JPL-Caltech/Lockheed Martin)Top view of the InSight probe’s science payload. The ultra-sensitive pressure sensor is visible in white. You can also see the SEIS seismometer’s hexagonal thermal protection and the two red housings that protect the TWINS (Temperature and Winds for InSight) sensors (© NASA/JPL-Caltech/Lockheed Martin).

What if Mars remains silent? What if nothing at all disturbs either the surface or the depths of this planet as it constantly orbits the Sun?

In this case, taken into account by scientists, SEIS will still fulfil its mission thanks to three allies: meteorite impacts, the tides of Phobos (one of Mars’ two moons), and atmospheric tremors.

Meteorites regularly fall from space onto Mars' rust-red surface, and their impact not only forms craters but releases a huge amount of energy. Some of this energy is released as a shockwave, which is exactly what SEIS will be listening out for.

Guided by the seismometer’s indications, the very-high-resolution cameras of the US Mars Reconnaissance Orbiter (MRO) and of the ExoMars mission’s Trace Gas Orbiter will then be able to seek out and photograph the impact, providing information that will in turn be used to considerably refine SEIS data.

The second ace is Phobos, one of the two natural satellites of Mars. On Earth, the tides that are so appreciated by holidaymakers are due to the Moon’s force of gravitational attraction, which mainly affects the Earth’s liquid masses i.e. the oceans and seas. Mars only has two very small natural satellites, yet their mass still manages to deform the Red Planet’s solid surface. Although this motion is minimal, the effect of the nearest moon—Phobos—can still be detected.

By accumulating data over an Earth year, SEIS will measure the bulging of the crust due to Phobos, which will provide information on the nature of Mars’ metallic core. Even without tremors of any kind, the core must surely reveal the mystery of whether it is liquid, solid or both.

Finally, the surface of Mars is relentlessly swept by winds, and its atmosphere disturbed by whirlwinds and turbulence that constantly generate variations in pressure at ground level. If we compare a quake to a drum stick, this turbulence is like a drum brush: a source of constant quivering which ends up making the planet hum and vibrate even without any tremors. This distinctive excitation of the planet by its atmosphere will also shed light on its interior structure.

The InSight probe on the Martian surface, with its payload deployed (© David Ducros).The InSight probe on the Martian surface, with its payload deployed (© David Ducros).

InSight: a comprehensive geophysics observatory

In its quest to reveal the mysteries buried deep inside Mars, SEIS will be accompanied by several other instruments. The InSight spacecraft is a comprehensive geophysics observatory, with its own meteorological station, a probe designed to measure heat flowing from the interior up to the surface, and a magnetometer. The radio signals will also be used to accurately determine how Mars spins on its own axis and to pinpoint with GPS precision the station’s position with respect to Earth.

InSight offers no less than a journey into the very heart of Mars, and by extension, into the heart of planetary worlds that have been orbiting the Sun for billions of years and have constantly accompanied the Earth on its interminable cosmic voyage.

Last updated: 18 september 2017

An instrument sensitive to tiny movements in the ground

A seismometer is a device that is sensitive to vibrations. It works on the principle of a pendulum: a heavy, inert mass with a certain resistance to movement (i.e. inertia) due to its weight is suspended from a frame by a spring that allows movement. The energy from any seismic activity excites this “proof mass” as it is called by geophysicists, making it vibrate.

What actually moves? That depends on your point of view!

This view of a mobile mass is valid if you consider that the frame to which the mass is attached—and which is firmly fixed to the ground—does not move. However, when a quake occurs, or a tremor is produced by any kind of shock, it is actually the ground—and therefore the frame attached to it—that moves!

If you change your viewpoint and look at the mass, we may consider that when a tremor occurs, the mass—which has inertia because of its weight—will only move after a certain time, whereas the frame will move in keeping with the ground motion.

In addition to the mass, the spring and the frame, a seismometer needs a device to constantly record the motion of the mass relative to the frame. This is a central part of the seismic sensor and the difference between seismometer technologies, as some measure the speed of the mass and other its displacement. In both cases, the recording that shows ground motion over time is known as a seismogram.

An even simpler technique to represent this signal consists in attaching a pen to the pendulum. The pen touches a roll of paper wound around a rotating drum. This is known as a seismograph, an instrument which directly plots the signal rather than recording it in digital form.

Simple pendulums and inverted pendulums

Principle of operation of a seismometer (© Adobe Stock).Principle of operation of a seismometer (© Adobe Stock).

The first seismometers developed were based on a simple pendulum in which the moving mass is suspended vertically from a frame.

To increase sensitivity, this type of device was then mounted upside down, which is why it is known as an inverted pendulum. In this case, the pendulum’s centre of mass is above the pivot point.

Unlike the first design, this assembly is naturally unstable and the least disturbance will make that mass leave its point of equilibrium and move left or right as gravity attracts it downwards.

The inherent instability of an inverted pendulum nonetheless is what makes this device react to the slightest movement, however small. As you will see later, the InSight spacecraft’s SEIS instrument is based on the inverted pendulum principle.

Plotting a seismogram

The seismometer provides data as a seismogram, which is a recording of the magnitude of ground motion over time while regularly measuring the offset between the position of the mass and the frame to which it is attached, in relation to an equilibrium position i.e. when the device is at rest in the absence of any seismic activity.

Historically, the first seismograms were produced by basic means: a metal needle attached to a mobile mass left a track of its movements on paper covered in soot. Later, the needle was replaced by an ink pen that recorded movements on a rotating roll of paper. Nowadays, in the digital age, seismometers provide digital signals recorded by computers. The seismometers flown to the Moon by the Apollo missions were among the first of this kind, because even in the early 1970s, most seismic stations on Earth were still fitted with seismographs.

The role of modern sensors is to follow the motion of the mass, and they can take three types of measurement: changes in the position of the mass relative to its “zero” equilibrium position (displacement), speed of the mass (i.e. motion over time), and finally a variation in speed over time (i.e. acceleration or deceleration).

Recording of a seismic signal (© Adobe Stock).Recording of a seismic signal (© Adobe Stock).

Some types of sensor are more appropriate than others depending on what type of measurement you wish to make. Inductive sensors are geared to measuring speed, while capacitive sensors are ideal for measuring the position of the mass. InSight’s SEIS seismometer uses an extremely sensitive capacitive sensor.

Number of axes

As ground motion occurs in three-dimensional space (along the vertical axis or two horizontal axes), displacements need to be recorded using three separate pendulums. This is the only way that seismic activity can be comprehensively documented.

Sophisticated seismometers usually measure all three axes, one for each direction in space. Simple one-axis seismometers usually only measure vertical motion; horizontal surface motion is ignored since it is not measured.

The SEIS instrument designed to fly aboard InSight to Mars is a three-axis seismometer. An interesting detail is that the three axes of the seismometer are not aligned with the horizontal and vertical, which would perhaps be “logical”.

Although each of SEIS’s three axes is positioned at a 90° angle to the others, the whole assembly is inclined by approximately 30.5° with respect to the horizontal axis. There are many, sometimes complex, reasons for this but to simplify matters, let us just say that such a configuration minimizes the effect of noise.

The sensitivity of a seismometer depends on the relationship between the seismic signal that you wish to record and the various disturbances that interfere with this signal. This “noise”, which can go as far as preventing measurements, is similar to the snow effect you can see on a television screen.

The 30.5° angle of InSight’s Very BroadBand (VBB) pendulums corresponds to an angle where the vertical component is measured with the best sensitivity relative to the instrument’s own noise. Tipping it towards the horizontal axis would reduce the amplitude of vertical acceleration, while tipping it away would increase the instrument’s self-noise, with increasing oscillation leading to a loss in long-period sensitivity.

Ideal set-up conditions

One of the issues to face when using a seismometer is how to ensure that it can optimally monitor a vibration even when the vibration lasts a long time, from minutes to hours.

Seismometers are by definition extremely sensitive, recording all that is going on around them whether seismic or not. However, the slightest drift of any kind can prevent a seismometer from continuing to monitor and plot a vibration, especially if the latter is a long-period event (the recording of short-period events being easier to handle).

Geophysicists attach great importance to the way seismometers are set up. However sensitive an instrument is, it will only give good results if it is set up correctly.

Seismometers are generally placed on very hard surfaces such as granite or concrete slabs. The surface must be as flat and as horizontal as possible.

Ideally, the place chosen must be quiet. It is not a good idea to set a seismometer up next to a road or near an underground railway station. Temperature is also very important. It must be as stable as possible, as any variation may affect the seismometer mechanics, especially the force generated by the spring on the mass.

On Earth, seismometers are preferably set up in the pits or shafts of former mines, hundreds of metres below the surface. These are good locations as they provide ideal operating conditions.

The sources of interfering noise, which propagate very easily near the surface (such as the continual hum of human activities, the constant roar of the oceans, and atmospheric turbulence), are reduced as far as possible. As for temperature, it is incredibly and naturally stable. The quietest site in Europe is in the Black Forest, in the workings of a former mine. The temperature there only varies by a few thousandths of a degree per year!

Once in an ideal location, a seismometer can feel and monitor any seismic wave, whether very short or, on the contrary, lasting a matter of minutes or even hours.

Seismic wave sources

A seismometer is designed to record seismic waves. On Earth, these waves are mainly caused by earthquakes that regularly and sometimes forcefully shake certain parts of the world, causing major damage depending on the amount of energy they carry.

Apart from major earthquakes that make newspaper headlines, the Earth’s surface is constantly subject to very small movements that we do not detect but which are revealed on seismograms.
Our planet can actually act like a bell, and when certain events occur, can even resonate in rhythm. The Earth then produces its own music, a telluric melody imperceptible to our ears but able to be picked up by the most sensitive seismometers.

Whether the seismic waves have been released during major seismic activity that shakes certain regions of the world or whether they are just due to the imperceptible oscillations of our planet, geophysicists use every opportunity to broaden our knowledge of the Earth’s interior.

A seismometer is rather like a doctor’s stethoscope. By determining the way that seismic waves propagate within our planet, depending on how they are reflected or refracted by the materials making up the planet’s interior structure, whether rocks or metal, it becomes possible to construct an image.

Passive and active sounding

When it is necessary to study a particular sub-surface area when prospecting for oil, for instance, you cannot always count on an earthquake at the right time. Seismic activity does not occur on demand.
There are two solutions in this case: either you listen for long periods of time in the hope that seismic activity will generate seismic waves as input for the seismometers (this is the solution chosen for the InSight mission, which will last two Earth years), or you create your own vibrations. The first is known as passive sounding, the second as active sounding.

On Earth, oil prospectors regularly create artificial seismic waves using various means, from exploding dynamite to triggering air guns or using trucks fitted with heavy vibrating plates. This technique was also used on the Moon. Explosive charges were triggered to create seismic waves that were then recorded by geophones set up on the surface by the astronauts of the Apollo 14, 16 and 17 missions.

In the space sector, geophysicists have many entertaining ways of making noise: they can deliberately send the upper stage of a rocket on a collision course once it has finished its useful life or, along the same lines, crash an obsolete spacecraft into the surface of a planet.

On Mars, these somewhat brutal techniques cannot be applied because scientists wish to avoid contaminating the surface with objects that have not previously been sterilized. InSight and the SEIS instrument will have no other choice then but to count on their luck of the draw, even though the penetrator of the Heat Flow and Physical Properties Package (HP3) will be used for a small active seismology experiment, not powerful enough to sound more than a few dozen metres below the Martian surface.

Last update: 7 november 2016

Seismic activity produces different types of seismic wave, each of which has its own particular characteristics.

What is a quake?

A quake is a geological event during which mechanical stresses of varying magnitudes within a planet lead to a break in rock, suddenly releasing energy.

A significant part of this energy is carried by seismic waves, which will propagate in all directions within and on the surface of the globe.

Formation of a seism (© IPGP/David Ducros)Formation of a seism (© IPGP/David Ducros).

These waves are like the ripples you make when you throw a pebble into a pond: they form at a single point then spread out over the surface, creating ripples of gradually decreasing height (amplitude). When a quake occurs, similar ripples appear around the focus. However, unlike our example, they are not limited to one plane but spread out in all three spatial dimensions.

The wavefront generated by a tremor immediately meets rock, which then temporarily deforms. Once the seismic wave has passed, the deformed material returns to an equilibrium state. Near the source, this state may be different from the pre-quake state because of the deformation created by the fault. Further away, however, the equilibrium position of the materials is almost the same as before the tremor. Geophysicists consider this reversible deformation “elastic”.

Schematic representation of a seismic wave (© IPGP/David Ducros).Schematic representation of a seismic wave (© IPGP/David Ducros).

Seismic waves are divided into two main types: those that travel within the planet, and which are capable of crossing its whole breadth (even through a metallic core); and those that travel along the surface and do not seek to penetrate the subsurface.

Body waves and surface waves (© rights reserved)Body waves and surface waves (© rights reserved).

P- and S-waves (body waves)

Principle of propagation of a P wave (IPGP / David Ducros). Principle of propagation of a P wave (IPGP / David Ducros).

Body waves, capable of propagating through the interior structure of a planet like Earth or Mars, are divided by geophysicists into two types: P-waves (primary waves), which are the first to reach seismometers when there is a quake, and S-waves (secondary waves) that come after the P-waves and therefore reach seismic stations second.

P-waves always reach seismometers first simply because they travel quicker than S-waves. They travel at speeds of around 6 to 14 kilometres per second on average, and can cross any material, whether solid or liquid. The denser the material, the faster they travel. They slow down when the material is less dense, which is the case near the surface.

S-waves are slower than P-waves (around 4 to 6 kilometres per second on average), and cannot cross through liquid. The Earth’s outer core, which is molten metal, is therefore an impenetrable barrier for them.

By measuring the speed of P- and S-waves resulting from a tremor, and by calculating the time between the P-wave wavefront and the S-wave wavefront reaching a given seismic station, it is possible to estimate the distance between the station and the seismic focus. By combining data recorded by at least three seismic stations, the epicentre may also be located with greater precision.

Principle of propagation of a S wave (IPGP / David Ducros).Principle of propagation of a S wave (IPGP / David Ducros).

P- and S-waves do not deform rocks in the same way. When a P-wave wavefront hits rock, the rock compresses then extends before returning to its initial shape.

This cycle of expansion/compression to which rocks are subject is parallel to the direction in which the wave is travelling. Once the wave has passed, the material returns to its original shape. The effect of the propagation of a P-wave is easily observed by extending then releasing a spring (mind your fingers!). The wave that travels through the rings of the spring is a compression/expansion wave, which explains why P-waves are known as compressional waves.

S-waves, on the other hand, are shear waves that vibrate the ground perpendicular to their direction of travel. To simulate an S-wave, just shake a blanket or rug to get rid of the dust: S-waves are just like the wave that ripples along the rug when it is held by two hands at one end then shaken.

Surface waves

Principle of displacement of a surface wave (© IPGP/David Ducros).Principle of displacement of a surface wave (© IPGP/David Ducros).

The second type of wave is known as a surface wave. As their name suggests, these waves only travel along the ground, on the surface. They are slower than the body waves we have already looked at (only up to 4 km/s on average), but have a greater amplitude. They can cause absolute havoc on their path, and are responsible for the mass destruction that often accompanies violent earthquakes.

Again, geophysicists distinguish two main types: Rayleigh waves and Love waves.

Rayleigh wave (© IPGP/David Ducros).Rayleigh wave (© IPGP/David Ducros).

The motion of Rayleigh waves is quite complex. On Mars, SEIS will use these waves to locate quakes, a feat impossible for a single seismic station waiting to record body waves.

Unlike its counterparts on Earth, which form part of a large network made up of thousands of seismic stations, the SEIS seismometer placed on the Martian surface by InSight will be all alone on the Red Planet. Thanks to Rayleigh waves, and especially those with enough energy to travel all the way around the planet, SEIS will be able to determine the propagation speed without needing to know the exact position of the tremor. The strategy involves detecting the first arrival of the surface wave, then measuring it again after it has been once around the planet. The circumference of Mars is known, so the difference between the two arrival times will be used to calculate the propagation speed of the surface Rayleigh wave.

Love wave (© IPGP/David Ducros).Love wave (© IPGP/David Ducros).

Surface Love waves are rather like a simpler version of secondary body waves, i.e. S-waves. On Earth, Love waves are responsible for most of the damage caused by violent earthquakes. The way they travel makes them particularly apt to destroy buildings not designed to withstand seismic activity.

Short-period and long-period waves

Seismic waves are not only divided into body waves and surface waves, but also by their period, which can be long or short.

To understand this concept, we can imagine a seismic wave like a wave rippling through the ground. When the wave arrives, the surface of the ground is deformed and rises until it reaches its maximum amplitude, after which it descends again until it returns to its initial level and flatness.

 High frequency seismic wave (© IPGP/ETHZ).High frequency seismic wave (© IPGP/ETHZ).

For a short-period wave (that geophysicists also call a high-frequency wave), the rise and fall will occur quickly, for example in less than one second. Faced with a long-period wave (or “low-frequency” wave), on the other hand, the ground will rise slowly (taking around a minute, for instance), then descend just as slowly (again around one minute).

Mid frequency seismic wave (© IPGP/ETHZ).Mid frequency seismic wave (© IPGP/ETHZ).

To accurately measure these two types of wave (long- and short-period waves), geophysicists use seismometers that are geared to each type. The more sensitive a seismometer is to a wide range of periods, the higher its bandwidth. Seismologists then call this kind of instrument a “broadband” seismometer.

Low frequency seismic wave (© IPGP/ETHZ).Low frequency seismic wave (© IPGP/ETHZ).

In order to maximize scientific information, the InSight mission’s SEIS instrument contains two types of seismometer. One, which constitutes SEIS’s central hub, is a very broad band (VBB) seismometer. Particularly sensitive to long-period waves, the VBB seismometer is coupled to a short-period (SP) seismometer, thus extending its range to shortwaves.

Planetary "ultrasound" scans

Propagation of seismic waves inside Mars (© IPGP/David Ducros).Propagation of seismic waves inside Mars (© IPGP/David Ducros).

Seismic waves are of great benefit to geologists. Just as doctors use a stethoscope to listen to the activity of a patient’s internal organs, or an ultrasound scan to see the image of a baby in its mother’s womb, so listening to Earth’s vibrations by a dense network of seismometers provides a way of piercing Earth’s inner secrets, inaccessible by any direct means.

The seismic waves produced by a quake behave like rays of light. When they hit a given environment, they can be reflected in a different direction, like bouncing off a mirror, or refracted which means that they are both deviated and slowed down.

Created at the heart of a quake—the “focus”—seismic waves will spread out in all directions and follow trajectories that are complex to a greater or lesser degree. Depending on the geological structures that they meet on their journey—and that may be considered as obstacles—they will arrive at the multiple seismic stations worldwide at different times.

By carefully analysing the data collected, geophysicists can track their journey and understand what they came across as they travelled through the Earth. This is how it was discovered that Earth is not the homogeneous rocky ball that people imagined, but has a complex interior structure made up of—from the centre out—a metallic core, the outer part of which is liquid, a partly molten mantle and a very thin rocky crust.

Last updated: 17 april 2018

A brief history of seismology

While the SEIS instrument is a technological marvel, as indeed is the InSight spacecraft that is going to place it on the Martian soil, the data that scientists hope to obtain on the planet’s interior structures are likely to be very different from those that geophysicists daily collect on Earth.

Seismic recording on smoked paper (© IPGP).Seismic recording on smoked paper (© IPGP).

The inner depths of Mars are as yet completely unknown, despite the armada of robotic explorers that have been sent there over the past 50 years. Seismology has been somewhat neglected by previous Martian missions—unlike lunar missions—having focused from the start on the search for life and habitability. The only seismometer that actually worked on the Red Planet, flown on the Viking 2 lander, was sent 40 years ago, and did not provide any clear results. This explains why, although the surface of Mars is becoming increasingly well-known and characterized from a topographical, geological and climatic point of view, we know practically nothing about the planet’s internal structure.

The InSight mission finds itself in the paradoxical situation whereby an ultra-sophisticated robot bristling with sensors is designed to carry out scientific experiments that were completed on Earth between the end of the 19th and the first quarter of the 20th century.

Explaining earthquakes in ancient times

In ancient times, earthquakes were thought to be due to winds imprisoned inside the Earth and trying desperately to escape by furiously whipping through an interior maze. This “pneumatic” theory gave way to a theory based on the sudden, violent emissions of steam caused by the heating of pockets of water by heat deep within the planet.

Engraving showing an eruption of the Saint Rose volcano on Reunion Island (© rights reserved).Engraving showing an eruption of the Saint Rose volcano on Reunion Island (© rights reserved).

Fiery tempests, underground fires and telluric explosions caused by the combustion of sulphurous or bituminous substances were next evoked during the Renaissance period. This was an attractive idea because it could be related to volcanoes, which were then considered as the planet’s safety valves. In the 18th century, electricity—which revolutionized everyday life—was briefly accused of the tremors shaking the Earth due to the building up of an electrical charge in underground cavities.

It was only in the 19th century that tectonics were first mentioned, bringing a definitive answer to scientists’ questions. Earthquakes are caused by the violent rupture of rocky masses deep below the surface that are subjected to strong mechanical stresses. When they break, this rocky material releases seismic waves that move through the rock at the speed of sound, typically several kilometres per second and up to several dozen kilometres per second deep within the planet. These waves spread not only through the interior structure of the planet, but also on its surface. The latter are the ones mostly to blame for the huge damage caused in inhabited areas.

The first seismometer

Historically, the first device designed to study earthquakes was conceived by Chinese scholar and inventor Zhang Heng.

His seismoscope, a metal urn (not unexpectedly decorated with dragons), could only indicate the direction in which an earthquake had occurred, but it testified to the intuition and ingenuity of its inventor, and enabled emergency rescue teams to know where to head after a violent quake.

The first real seismograph as such, capable of recording the occurrence of seismic waves as a seismogram, was developed by the Italian Nicola Zupo in 1784. He used a vertical pendulum composed of a spherical lead weight to which a needle was attached. The needle recorded displacements of the mobile mass in ash when an earthquake occurred.

From that point on, through the 19th and 20th century, numerous seismometer models were developed and numerous innovations made, including recording quakes by plotting displacements on a roll of paper wrapped around a drum, studying ground motion with horizontal pendulums rather than remaining limited to the vertical axis, and using inverted pendulums to improve sensitivity.

All these instruments use a mass held by a spring or similar device that moves when there is a tremor. Its own motion is interpreted to deduce that of the ground. This basic principle is still used by the InSight probe’s seismometer, but its numerous refinements have led to a sensitivity way beyond that of its honourable ancestors.

First recording of a quake

First recording of an earthquake by Ernst von Rebeur-Pacshwitz on 17 April 1889 (© rights reserved).First recording of an earthquake by Ernst von Rebeur-Pacshwitz on 17 April 1889 (© rights reserved).

The first recording of an earthquake took place on 17 April 1889 at Postdam, Germany, when Ernst von Rebeur-Pacshwitz’s seismograph measured significant tremors in the ground that he attributed to a strong earthquake in Tokyo, a city some 9,000 kilometres away as the crow flies. By comparing the signals recorded simultaneously at Wilhelmshaven, the scientist estimated for the first time that seismic waves travel at a speed of around 7 km/s.

In 1897, British geologist Richard Dixon Oldham managed to characterize seismic wave trains. He defined P-waves (primary waves) that arrive first, S-waves (secondary waves) that follow, and finally surface waves that differ from the first two because, as their name suggests, unlike the first two types they do not travel deep within the Earth.

In 1906, by analysing the propagation time of seismic waves generated by numerous earthquakes, Oldham concluded that the Earth had a core less than 0.4 times that of its diameter, i.e. around 5,100 kilometres (the Earth’s metallic core has since been found to have a diameter of 6,960 kilometres).

This was the first time in history that seismic waves were used to sound the Earth’s inner structure. Indeed, seismology has two complementary focuses: the study of quakes as such (position and magnitude), and the exploitation of seismic waves to investigate the inner depths of a planet or even a star, which are completely inaccessible by any other means. In 1906, the Earth’s core was thus identified but doubts remained as to its exact size and nothing was known about its composition.

Earth’s structure

In 1909, Croatian meteorologist then seismologist Andrija Mohorovicic identified a sudden acceleration in P-waves when studying the seismographs of an earthquake that occurred in Zagreb. He put this down to a discontinuity in the upper part of the Earth.

Journey to the centre of the Earth (© rights reserved).Journey to the centre of the Earth (© rights reserved).

This discontinuity, called the “Moho” in his honour, is a boundary around 35 kilometres deep on average (5 to 10 km under the oceans and 20 to 90 km under the continents) that marks the borderline between two major components enveloping our planet: the Earth’s crust, and its underlying mantle.

The Moho is the shallowest physical discontinuity of our planet that we know. However, despite several attempts to drill down to it through either land or sea, nobody has ever reached it directly, and only seismic waves have allowed us to “touch” it indirectly. So much, then, for the dreamers hoping to travel down through the Earth’s mantle or core. Despite the romance of novels like Journey to the centre of the Earth, or Hollywood films such as The Core, seismology is our only way of exploring the Earth’s innermost depths!

The Earth’s core once again became the subject of immense interest when German geophysicist Beno Gutenberg used seismic waves to reveal another discontinuity, this time between the bottom of the Earth’s mantle and its core. The “Gutenberg discontinuity”, as it is still called today, is located 2,900 kilometres below the surface and accurately delimits the volume occupied by the Earth’s core.

The interface between the Earth’s mantle and its core plays a critical role in our planet’s geological dynamics because it is from here that mantle plumes begin their journey to the surface. It is when they manage to reach it that they produce huge volcanic eruptions, along with other associated geological events.

Earth tide

In 1926, when investigating not earthquakes but tides due to the Moon’s gravitational attraction on the seas and oceans, British geophysicist Harold Jeffreys put forward the idea that the Earth’s core was only relatively solid and could even be liquid!

Finally, the last major discovery on the Earth’s interior structure was made in 1936 and concerns the Earth’s metallic core. Danish seismologist Inge Lehmann revealed from her study of seismographs the existence of a solid inner core separate from the outer liquid core by a discontinuity that was named after her.

As far as our own planet is concerned, seismology has cleared up the major mysteries, so it can now turn to Mars to make history all over again.

Some of InSight’s major goals are to measure the thickness of the Martian crust and determine any stratification (multiple layers of different densities), characterize the mantle’s mineralogical composition, accurately determine the core’s radius and establish whether it is liquid, solid or a combination of the two states. As you can see, this corresponds to the key discoveries about Earth’s interior structure that we have just briefly reviewed. Again, seismic disturbances and tidal forces will be used to unveil the mysteries that the pioneers of seismology have already pierced on Earth.

InSight’s contributions to knowledge will therefore be a truly historic event for geophysics.

Making new history on Mars

There are numerous hurdles to overcome to repeat on Mars what geophysicists managed to do in the 20th century on Earth, and they have constantly challenged the ingenuity of SEIS instrument team members.

In the 17th century, scholars thought that the Earth was hollow and contained many fiery hot spots. These were thought to lead to numerous volcanoes all over the world. One of the best representations of these telluric infernos is Athanasius Kircher’s Pyrophylaciorum (© rights reserved).In the 17th century, scholars thought that the Earth was hollow and contained many fiery hot spots. These were thought to lead to numerous volcanoes all over the world. One of the best representations of these telluric infernos is Athanasius Kircher’s Pyrophylaciorum (© rights reserved).

SEIS is the only seismometer to be flown on InSight, so it will have to operate alone on Mars, whereas seismologists on Earth were quickly able to benefit from networks that have never ceased to grow, starting with several dozen, then several hundred, and now several thousand seismic stations.

Today, Earth is being constantly monitored by over 20,000 seismometers deployed throughout the world, most of which transmit data in real time. As early as the late 19th century, Rebeur Paschwitz had understood the fundamental advantage of being able to study an earthquake from several points on the globe.

Progress in signal processing and some clever tricks based partly on the fact that Mars is quite small, will nonetheless enable SEIS to take measurements that even recently were considered impossible.

Another astonishing feature of SEIS is that not only has the instrument been designed to be very sensitive but it is also extremely robust.

In terms of sensitivity, SEIS bears comparison with terrestrial standards. It is also thousands of times more sensitive than the seismometers flown to Mars by the Viking probes, and dozens of times more sensitive than those taken to the Moon by the Apollo mission astronauts.

The space environment being what it is, the InSight seismometer must be ultra-resistant so as to withstand the extreme conditions to which it will be subjected: vibrations and shocks during the launch and landing phase, the space vacuum, glacial or extremely high temperatures, and the harmful radiation to which it is exposed on its journey to Mars to name but a few.

The phenomenal bangs and bumps that the probe will go through on lift-off from Earth and landing on Mars appear to totally contradict sensitivity requirements which are even more stringent in the case of a planet with little seismic activity, which may be the case for Mars.

Yet engineers have successfully met most of the challenges and will deliver to NASA in mid-2017 a technological gem that should revolutionize our knowledge of the inner depths of the Red Planet while writing a new chapter in the history of seismology.

Last updated: 7 november 2016

The first planetary seismology experiments were carried out on Earth’s natural satellite, the Moon

  • Ranger probe (© NASA)Ranger probe (© NASA)

    Planetary seismology really took off in the early 1960s with the US Ranger mission, designed to send a series of spacecraft to the Moon on a collision course so that they could photograph the lunar surface up to the moment of impact. To fulfil their mission, the probes were bristling with cameras, but seismometers had also been flown on the second series, Ranger 3, 4 and 5.

    The Ranger probes’ brutal approach to the Moon does not appear very compatible with the exploitation of a seismometer, a device that by definition is extremely sensitive to the slightest bump. NASA engineers had nonetheless designed a balsa wood sphere able to resist a rough landing on the lunar soil, and it was inside this protective sphere that the seismometer was placed.

    The Ranger programme was a serious setback for NASA, because of the 9 probes launched from Earth, none managed to fulfil its mission due to various malfunctions, either on launch or during the guidance phase on their journey to the Moon.

    Flying a seismometer, Ranger 3 missed its target after its launch on 26 January 1962 and was lost somewhere in outer space. Ranger 5’s fate was exactly the same, 9 months later. A failure in Ranger 4’s onboard computer deprived the spacecraft of its solar arrays and navigation capabilities, so it unfortunately crashed into the Moon and no scientific data were ever recovered.

    Last updated: 25 October 2016

  • Model of the Surveyor lunar probe (© NASA)Model of the Surveyor lunar probe (© NASA)

    For the scientists investigating seismic activity on the Moon, the logical follow-on to Ranger would have been to fly seismometers on the Surveyor spacecraft.

    The Surveyor programme absolutely had to succeed, NASA having no further room for mistakes after the Ranger fiasco. A seismometer was therefore developed, but the project was finally shelved, temporarily curbing the ambitions of geophysicists.

    Last updated: 25 October 2016

  • Portrait of astronaut Buzz Aldrin during the Apollo 11 mission (© NASA).Portrait of astronaut Buzz Aldrin during the Apollo 11 mission (© NASA).

    Apollo 11

    The first lunar seismometer was finally placed on the Moon’s surface by man rather than a robot, during the Apollo missions. In July 1969, Buzz Aldrin set up the Passive Seismic Experiment (PSE) on the Sea of Tranquillity. As its name suggests, this seismometer listened out for natural seismic noise. It did not use active techniques such as impacts or explosions to generate seismic waves.

    PSE, the device that Aldrin set up on the lunar soil, was powered by solar arrays, so it could only operate in sunlight. It ran out of power during the lunar night, so could take no further measurements. During the second lunar day, the midday Sun caused the instrument to overheat and finally fail. It was still able to record seismic activity over 21 Earth days, which is a little less than one lunar day (which lasts 28 Earth days).

    The PSE seismometer was sensitive to vibrations in three-dimensional space for long-period waves (three axes), and along one axis for short-period waves. With a mass of 11.5 kg, its power consumption varied between 4.3 and 7.4 watts. It was made of beryllium, a very lightweight metal that is extremely challenging and costly to work with due to its extreme toxicity to humans.

    The correct operation of a seismometer is highly dependent on the quality of its installation, and it was thanks to the lessons learned from Apollo 11 that greater care was taken when setting up later seismometers to ensure a more accurate orientation and configuration. The instruments were also covered with thermal insulation to prevent overheating. The solar arrays of the first seismometers were replaced by a radioisotope thermoelectric generator—a device that generates heat through the radioactive decay of unstable atomic nuclei—thus enabling them to continue to operate at night, in the dark.

    A network of seismometers set up by Apollo 12, 14, 15 and 16

    Magnetic tape containing the recordings of active seismic events during the Apollo 16 mission (© IPGP).Magnetic tape containing the recordings of active seismic events during the Apollo 16 mission (© IPGP).

    The second, improved, seismometer was set up on the Ocean of Storms by Apollo 12. It was soon joined by other seismic stations at Fra Mauro (Apollo 14), Hadley (Apollo 15) and finally in the highland region north of the ancient Descartes crater (Apollo 16). Together they made up a network of four seismic stations forming a grid over the centre of the visible side of the Moon, thus enabling them to sound its interior structure and characterize the propagation of seismic waves deep within its rocky layers.

    In December 1972, the last Apollo mission—Apollo 17—set up a gravimeter in the Taurus-Littrow valley. The objective of this experiment was to measure the gravitational waves predicted by Einstein’s general theory of relativity (which were actually detected for the first time in February 2016 by the US Laser Interferometer Gravitational wave Observatory (LIGO)).

    No gravitational waves were observed on the Moon, but geophysicists realised much later that the instrument was not very sensitive because it was acting like a short-period seismometer and that the data sent back to Earth were such that it could actually be considered a fifth seismic station, which was a real windfall. As far as seismology is concerned, the more the merrier!

    Lunar experiments ended in September 1977, after nearly 8 years of observation amounting to 600 gigabits of data. Although the seismometers were passive, active seismology experiments were also carried out on the Moon to generate seismic waves on demand.

    Active seismology

    Impact site of the Apollo 16 mission's S-IVB rocket stage (©LRO/NASA).Impact site of the Apollo 16 mission's S-IVB rocket stage (©LRO/NASA).

    By deliberately crashing a heavy object, such as the last stage of a Saturn V rocket or the ascent stage of the lunar module (once used, of course) into the Moon, geophysicists could cause impacts to produce seismic waves. The key advantage of this over passive measurements lies in the fact that the source of the tremor could be accurately located on the lunar surface, and the instant of impact known to a split second. Nine controlled-impact experiments were carried out on the Apollo 14, 16 and 17 missions. For this purpose, geophones (which are similar to seismometers) were fitted with small explosive devices.

    A review of lunar observations

    Artist's concept of the Apollo 15 ALSEP layout (© NASA).Artist's concept of the Apollo 15 ALSEP layout (© NASA). The first planetary seismology experiments carried out on the dusty grey surface of our natural satellite revealed how noisy the Earth is. With no atmosphere and therefore no weather, no oceans with their breaking waves and pounding of surf along the shoreline, no cities, highways or underground railways, the Moon is a near-silent world. Up there, there is practically nothing to disturb a seismometer, which can therefore measure the slightest ground motion, an impossible achievement on Earth.

    Throughout the measurement period, the lunar seismology network recorded a little fewer than 2,000 impacts made by meteorites of different sizes and over 10,000 tremors. Most of these low-magnitude tremors (generally below 2 on the Richter scale) originated in the depths of the Moon, with a focus between 800 and 1,000 kilometres deep.

    Studying the propagation of seismic waves inside the Moon has also led to a definition of its interior structure, the end goal of all seismic measurements.

    Like other large bodies in the solar system, the Moon is not a homogeneous sphere but is made up of several layers. It has a rocky crust three times thicker than that of Earth, a mantle then a metallic core. Although fairly small in diameter (less than 450 kilometres), the outer part of the lunar core is nonetheless molten. The inner core is solid. The weak attenuation of seismic waves crossing through the mantle suggests that it is cold and dry (low water content).

    The Moon confirmed the potential of seismology for studying other planets in the solar system, and the surprises it held for geophysicists—which included the diffraction of seismic waves by the regolith, a layer of heterogeneous broken rocks making up the lunar crust—presages more unexpected discoveries on other planetary bodies, and especially Mars, the Red Planet.

    Last updated: 18 september 2017

 

Viking was the first (and only) mission to have landed a seismometer on Mars

  • The seismometer flown aboard Viking (© NASA)The seismometer flown aboard Viking (© NASA)

    In 1975, NASA launched an armada to Mars. No fewer than four sophisticated spacecraft (two orbiters and two landers) headed off to the Red Planet. The Viking mission, as it was known, revolutionized our vision of Mars for several decades and even today is one of the most spectacular programmes ever undertaken by NASA, the US space agency.

    The Viking spacecraft, and more especially the landers, were designed to look for signs of life on Mars, and the mission therefore focused on exobiology. However, geophysicists managed to edge in on the mission, and each of the two ground stations contained a seismometer.

    However, the geophysicists had to comply with several requirements and some of the resulting trade-offs had a significant impact on the seismic experiments. The seismometers, for example, were attached to the lander deck, and could not be deployed on the ground. The necessary contact with the Martian surface was through the lander legs, fitted with shock absorbers, which was far from ideal when seeking to record seismic waves travelling from the ground to the instrument.

    Each seismometer had a mass of 2.2 kg and consumed about 3.5 watts. Sensitive to short-period waves in all three directions, the Viking lander seismometers were about 10 times less sensitive (of the order of a nanometre) than those deployed on the Moon during the Apollo missions. Furthermore, because of the slow data rate between the Earth and Mars, the seismic recordings had to be compressed before transmission to Earth, thus further reducing performance.

    On 20 July 1976, for the first time in the history of Martian exploration, the Viking 1 spacecraft gently landed on Mars’ rust-red surface. Regrettably, the seismometer’s mechanical unlocking system failed to work, making the instrument unusable.

    Being extremely sensitive to vibrations, seismometers must be protected during the mission phases when the spacecraft suffers major shaking or shocks, such as during lift-off or landing. In the case of the Viking seismometers, the moving part was mechanically locked in place so as to avoid any damage or other incidents. Unfortunately, once landed, the engineers could not manage to free the moving mass from its prison. The Viking 1 seismometer was the only inoperable instrument of the Viking mission. All the others—whether aboard the orbiters or landers—operated without a hitch.

    Last updated: 25 October 2016

  • View of the Viking 2 landing site on Utopia Planitia (© NASA)View of the Viking 2 landing site on Utopia Planitia (© NASA)

    The rapid loss of the first seismometer made the deployment of the instrument flown on the Viking 2 lander even more critical. Fortunately, after a successful touchdown on 3 September 1976 on the Utopia plain, the unlocking system worked the first time, freeing the instrument to acquire seismic data. However, geophysicists were confronted with another nasty surprise.

    Riveted to the lander deck and imperfectly linked (or “coupled” in the language of geophysicists) to the ground via the lander legs, the instrument faithfully recorded events that had nothing to do with planetary tremors.

    The sometimes intense activity on the lander’s deck often prevented the instrument from recording the Red Planet’s seismic life. The seismometer reacted to everything, from the rotation of a high-gain antenna or cameras to the movement of the robotic arm or even the clicks of the tape recorder when recording data. Even worse, because of its unpredictable nature, the slightest gust of wind made the spacecraft platform vibrate, which was again picked up by the seismometer’s mobile mass.

    During the day, the interfering background noise due to airflow over the Viking lander was enough to prevent any good seismic measurements. The seismometer on the Viking 2 lander actually became a good weather station throughout its period of operation, i.e. 560 Martian solar days (known as “sols”), or 19 Earth months, from 1976 to 1978. The instrument did an excellent job complementing the space probe’s meteorological station, transmitting numerous data on winds and atmospheric circulation that meteorologists enthusiastically exploited. So, to adapt a well-known saying, a geophysicist’s loss can sometimes be a meteorologist’s gain.

    At night, from midnight to around six o’clock in the morning, the winds dropped and the ambient quiet around the landing site nonetheless allowed geophysicists to collect a few data. However, their analysis proved very disappointing because the Viking seismometer was not as sensitive as hoped and the influence of the lander vibrations could not be estimated or cancelled out.

    Even today, although Viking 2 data have been reanalysed with more sophisticated data processing techniques and means than those available at the time of the Viking missions, seismologists still wonder if the lander’s seismometer actually picked up a single tremor.

    Apart from one event that occurred on sol 80, which is doubtful from a seismic point of view, Mars appears to have remained as silent as the grave. The uncertainty surrounding the sol 80 detection arises from the fact that no meteorological data were collected that day by the weather station, precluding the possibility of knowing whether a vibration detected by the instrument was due to a gust of wind or actual seismic activity. This highlights the need to acquire meteorological data in order to correctly interpret seismic data.

    The sudden loss of the Viking 1 seismometer following a failure to unlock the pendulum had major knock-on effects for seismology experiments. Henceforth alone on Mars, measurements of tremors taken by the remaining seismometer on Viking 2 could no longer be confirmed by another instrument. It was no longer possible to use triangulation techniques for positioning either.

    It is interesting to note that, once on Mars, the InSight seismometer will be—like that of Viking 2—the only seismic station in operation. We shall see in another section the clever tricks geophysicists have come up with to maximize its efficiency.

    Last updated: 25 October 2016

Venus

  • Measuring seismic activity on Venus: a real challenge

    While acquiring seismic signals on Mars is already extremely difficult, the task is even trickier on Venus. Not only is its surface roasted by temperatures around 460°C, but it is also crushed by a very dense atmosphere that exerts a pressure of 90 bar, which is 90 times that of Earth’s atmospheric pressure, and more than the pressure felt at a depth of 900 metres under water on Earth. In such extreme conditions, spacecraft only survive a few hours.

    Twin sisters

    Venus and Earth could almost be considered twin sisters. They have practically the same diameter and a similar mass. However, a closer look reveals glaring differences that Venus’s closer proximity to the Sun does not explain.

    Venus observed by the Pioneer Venus Orbiter (© NASA)Venus observed by the Pioneer Venus Orbiter (© NASA)

    For one thing, on Venus’s surface, temperatures can reach a record 460°C. These unbearable conditions are caused by a very thick, suffocating atmosphere mostly composed of carbon dioxide (over 90 bars). It is the greenhouse effect that leads to these infernal temperatures.

    Another major difference is the complete absence of water. Earth is known as the Blue Planet for a very good reason: it contains about 100,000 times more water than Venus. Since water plays a vital role in plate tectonics, the extremely arid nature of Venus would explain why this mechanism never developed despite the planet’s huge reserve of internal heat, dissipated through numerous volcanic structures that punctuate its charred surface.

    And as if that were not enough, Venus does not turn in the same direction as the other planets in the solar system, and its rotation is extremely slow. It has no natural satellites to keep it company on its journey around the Sun, whereas the Earth has a massive natural satellite—the Moon—and Mars has two small ones. Finally, and strangely, Venus has no magnetic field.

    Seismic measurements

    In the light of the horrendous conditions on Venus, it is near-impossible to even attempt to measure any seismic activity there. For the moment, only the Soviets have tried. In 1982, the Venera 13 and Venera 14 space probes managed to land on Venus, but only survived a few hours in its hellish conditions.

    Although the Venera probes are most well-known for the magnificent and troubling colour images that they sent back to Earth, they also flew the Groza 2 instrument, a combination of a microphone and rudimentary single-axis seismometer able to measure vertical displacements of the Venusian surface to micron precision.

    Attached to a ring around the probe which was in contact with the surface, the Groza 2 seismometer’s coupling with Venus was less than perfect. The very short lifetime of the two stations was also a major drawback for measuring seismic activity. With a chance of survival limited to a matter of hours, the primary objective was to determine background noise and any micro-seismic activity.

    The charred surface of Venus photographed by Soviet probe Venera 13 (© IKI)The charred surface of Venus photographed by Soviet probe Venera 13 (© IKI)

    Groza 2 managed to send some data back to Earth, but their interpretation ran up against the same problems as those of the Viking landers, as the measurements were complicated by outdoor disturbances such as small stones whipped up by the violent winds that were unfortunately blowing at the time of landing, or disturbances caused by the spacecraft itself (expansion and cracking of materials due to the heat and pressure). The validity of the data acquired was therefore called into question and no convincing results were obtained.

    Future projects

    The spectacular divergence in the respective destinies of Venus and Earth, two celestial bodies formed in similar conditions yet which gave rise to two radically different worlds—one a stifling hell and the other a paradise—fascinates planetary scientists. There is no doubt that the answers to the mysteries of the formation and evolution of Venus are sealed in the depths of the planet, and to explore them, seismology is once again vital.

    In the absence of plate tectonics—the phenomenon responsible on Earth for most earthquakes—it is logical to wonder if there is still any seismic activity on Venus. The relative youth of the Venusian surface (one billion years on average, which is more recent than the crust of the Moon, Mars or Mercury), the existence of strangely-shaped volcanic structures and the presence of tectonic structures such as rifts and faults indicate that the ground must tremble, obviously not as much as on Earth, but more often than on Mars.

    The challenge lies in recording this seismic activity. The development of motion detection or displacement sensors with a similar performance to the pendulums of SEIS aboard the InSight mission to Mars and capable of operating in the hellish conditions encountered on Venus would keep engineers busy for several years, if not decades.

    There are other efficient ways of trying to unveil the mysteries of Venus’s interior structure. The very thick atmosphere around the planet creates efficient coupling with the ground. When a quake occurs, the ground tremors are conveyed to the air and generate infrasonic wavefronts that are inaudible to the human ear but that can be detected by instruments designed for the purpose and which could be flown in the gondola of sounding balloons. The latter could orbit at altitudes where the extreme temperatures and pressure of Venus finally become easier to withstand.

    Orbital observations are another possibility. The interaction of infrasonic waves with the upper atmosphere and more especially the ionized layer (ionosphere) could be measured and monitored from space by satellites.

    At the current time, all the seismic survey projects focusing on Venus are still on the drawing board. In the field of planetary seismology, the InSight mission—designed to deploy an ultrasensitive seismometer on Mars—is the boldest project to date. Geophysicists interested in the Moon and Venus will be keeping a close eye on the mission. The lessons learned will no doubt serve for future missions back to the Moon or to explore the interior structure of Venus, a planet so familiar, yet so enigmatic.

    Last updated: 25 October 2016

 

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