Mars, from the inside

The mission’s name sums up its goal: InSight Mars should lead to a better understanding of the planet’s internal structure. Since November 2018, data flows have been analysed by scientists, including Véronique Dehant of UCLouvain and the Royal Observatory of Belgium. The first results have just been published in the journal Nature Geoscience.

In November 2018 NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander touched down in a shallow crater located on the Elysium plain of Mars. Since then, the various on-board instruments have been sending a constant stream of data to Earth, from which scientists have just drawn the first results. Two instruments are of particular interest to Prof. Dehant (member of UCLouvain’s Earth and Life Institute and coordinator of Louvain4Space): the SEIS seismometer and the RISE radio science system.

Waiting for The Big One

‘The seismometer is calibrated to record all quakes of an amplitude ranging between those that occur on the Moon and those that occur on Earth’, Prof. Dehant explains. ‘We think “marsquakes” fall within this range.’ The Moon is relatively calm seismically, whereas earthquakes can be violent owing to plate tectonics. This is confirmed in the articles which have just been published in Nature Geoscience.

‘To date, we’ve measured a lot of marsquakes, around 200, but only 20 or so were significant, with a maximum magnitude of three or four. This is important but we can’t qualify them as large.’ You can listen to the two most powerful marsquakes via the mission’s website.

How to explain this seismicity? ‘The significant recorded quakes were due to movements of a fault located not far from the landing site’, Prof. Dehant explains. ‘Mars presents a particular seismicity, undoubtedly strongly linked to the effects of the atmosphere on the surface and, for certain more violent quakes, local tectonics, or fault movements. Remember, plate tectonics don’t exist on Mars as they do on Earth. Mars is a single plate, thus no quakes can be generated by the meeting of different plates. It’s interesting because its entire history is written on its surface, while on Earth the plates penetrate the mantle., causing continuous recycling of rock; there are very few rocks that have remained on the surface since their formation. On Earth, almost everything is recycled, while on Mars surface rocks date from the very beginning of the solar system.’ Scientists, however, are really hoping for the ‘big one’, an even higher magnitude quake powerful enough to reach the lower mantle and core. Because seismic waves are sensitive to the materials they pass through, observing them as they travel towards the planet’s centre is to study the planet’s internal structure, which is the mission’s goal.

Is the egg raw or cooked?

There is however another way to probe the planet’s centre: radio science, in which Prof. Dehant is particularly involved. The principle is simple. Radio waves are sent to Mars; an X-band transponder, the Rotation and Interior Structure Experiment (RISE) instrument, sends the waves back to Earth where they are received by a system of large antennas (70 m in diameter) along the equator. The frequency shift between transmitted and received waves is then measured, which is linked to the relative speed of Mars with respect to Earth.

From this, it’s possible to deduce information on the planet’s rotation and orientation. As the measurements are accurate to within a few centimetres, it’s possible to obtain information on the deep interior of Mars as we do for Earth. But how does the rotation and orientation of Mars affect its internal structure? ‘The simplest analogy involves the egg’, Prof. Dehant says. ‘A cooked egg rotates differently than an uncooked one does. With practice, it’s possible to distinguish one from the other. It’s the same with planets: studying the orientation and rotation of planets makes it possible to deduce information on their internal structure. In particular, we wanted to confirm that the core of Mars is liquid.’

A wish fulfilled? ‘We haven't published anything about it yet because it still requires more observation, our sample of measures is not yet sufficient because it should at least be continued over a Martian year ... which is equivalent to just over two Earth years. And we only have 400 days of observation at the moment.’

Habitability

A priori, however, one would think that the core of Mars is solid because, compared to Earth, it’s smaller and would have cooled faster. There’s another way of knowing whether a planet’s core is solid: the presence of a magnetic field. This is created by movement in a conductive material. This is the case with Earth, whose magnetic field comes from movements in its core consisting essentially of liquid iron. There is no magnetic field around Mars, however.

So there’s no liquid core? ‘Not exactly’, Prof. Dehant says. ‘It means there’s not enough movement to generate a field. We believe that because Mars is a single plate, the lithosphere., which is in one piece, acts as a cover which keeps heat inside. So there could be a liquid core.’

Behind this question hides another: Is the red planet habitable? A fossilised magnetic field has been observed in rocks dating from the beginning of the solar system, between 4.6 billion and four billion years ago. In the beginning, Mars was protected by a magnetic field and water was abundant, as shown by the presence of dry deltas, valleys, etc. But the water began to disappear four billion years ago when the magnetic field disappeared.

‘We don't know what happened’, Prof. Dehant says. ‘We know that the magnetic field protects against erosion by solar winds. It was probably protecting the atmosphere. It’s believed that there was initially a sufficient atmosphere to maintain the surface pressure and temperatures required for liquid water. Then that atmosphere disappeared and we now have pressure of seven millibars, less than a hundredth of the pressure on the Earth's surface. So on Mars, there’s no more liquid water and temperatures oscillate between -150 ° C and + 20 ° C, but the latter is exceptional!’

ExoMars mission postponed

To go further, the scientific community was counting on the Russian-European ExoMars mission, which was to be launched this summer. In addition to a rover which will analyse rock samples, the mission involves sending a platform containing a radio science system called Lander Radioscience (LaRa), which must complete the work of RISE. It’s a very Belgian instrument given that, in addition to Prof. Dehant's teams who interpret the signals, its transponder was manufactured by Antwerp Space (see Figure 1) and its antennas were designed and manufactured by the laboratory of UCLouvain Prof. Christophe Craeye. They’re tiny technological gems: the two transmitting antennas (see Figure 2) each weigh only 115.6 grams and the receiving antenna (see Figure 3) 152.6 grams.

But the mission has just been postponed to ... 2022. We feel Prof. Dehant’s frustration, but also her relief: ‘A series of problems accumulated, particularly regarding the braking parachutes, which couldn’t have been solved in time for a summer launch, unless we didn't test ... but when we don’t test enough, we risk failure ... so I think it's a wise decision.’ For two more years, we’ll have to content ourselves with the signals sent by RISE.

Figure 1 : Modèle de vol du transpondeur LaRa

Figure 1 : LaRa transponder flight model

Figure 2 : Modèle de vol de l’antenne d’émission

Figure 2 : transmitting antenna flight model

Figure 3 : Modèle de vol de l’antenne de réception

Figure 3 : receiving antenna flight model

Henri Dupuis

A new ERC grant

Véronique Dehant hasn’t always been interested in the rotation of other planets in our solar system. Her first love was Earth. She studied the variations of rotation and orientation of our planet until developing a model of them based on the planet’s deep interior. ‘The challenge in modelling,’ she says, ‘is the core and what's going on where it meets the mantle. To study this, we benefited from an ERC grant, which started in 2015 and will end in August of this year.’ But the story doesn’t end there, because her work also led to observing rapid variations in the Earth's magnetic field.

Prof. Dehant thus assembled a team of three: a gravimetry specialist, Annick Cazenave of the Midi-Pyrénées Observatory in Toulouse, because scientists have realised that the movement of masses within the core interferes with measuring earth's gravitation; a specialist in geomagnetism, Mioara Mandea of the National Centre for Space Studies (CNES) in Paris; and Prof. Dehant herself, for whom the Earth’s rotation no longer holds secrets. This crack female trio just won an ERC Synergy Grant to apply the results of their earthly research to the planet Mars.