March: "More than a pure water lake, it's a brine"

The radar of the European Mars Express spacecraft has discovered a large lake of liquid water on the red planet. The astrophysicist Francis Rocard, responsible for the exploration of the solar system at Cnes and connoisseur of the planet Mars, decrypts this sensational news for Le Point.fr.

Le Point.fr: Is the discovery of a lake on Mars such a big surprise?

Francis Rocard: I would say that the MARSIS radar (Mars Advanced Radar for Subsurface and Ionosphere Sounding) Mars Express, in operation for 14 years, has been "sold" for this. ESA then broadcast an image showing a liquid zone in the depths of Mars. It was his goal: to find an underground water table in the basement of the red planet. At the time, the argument revolved around models predicting that liquid water could be found between 2 and 3 kilometers below the surface. For 14 years, MARSIS did his job but found nothing. The conclusion was: either there is no ice that turns into liquid water 2 or 3 kilometers deep on Mars, or the radar waves of MARSIS do not penetrate enough to have an echo of what is in these depths. Simply because the composition of Martian soil is unknown and radar operators can not estimate the penetration depth of the waves sent by their instrument. So we stayed there for a long time, a very long time, and we forgot MARSIS! At least, until the day before yesterday when it was learned that he had found something, curiously near the South Pole. Although on reflection, it is not so surprising …

How did the researchers make this discovery? What exactly did they see?

They simply saw a brighter echo where the so-called lake is. More technically, their discovery is based on what we call the dielectric permittivity: basically, the penetration / reflection coefficient of the radar wave. When you have liquid water that behaves like a mirror, the wave does not penetrate at all. Whereas when you have pure ice, the permittivity is close to 0 and the radar wave pbades like butter. That's why it's not surprising that this lake was seen near the South Pole. For a radar like MARSIS, it is much easier to detect a subglacial lake at the pole than an underground lake at the equator. Basically, we found this lake where there was light. That is to say, there may be somewhere else that MARSIS is not able to see.

But let's get to the facts. On Mars, the permittivity of the soil is generally between 7 and 10, the pure liquid water is 80, the ice 0. But there, they found a permittivity of 30. What is very above the value usual for the surface of Mars, even if it is not the value corresponding to liquid water. So we are in a somewhat intermediate zone. From what, we enter the field of interpretation using what physics tells us.

"We regularly discover bacteria living in incredibly hostile places, where we thought it would be totally impossible "

And what does it tell us?

Thanks to it, we can roughly predict the pressure and the temperature according to the depth. Since what has been seen is 1.5 km deep, it can be inferred that the temperature should be around -75 degrees. However, to have a liquid at -75 degrees, it must be ultra-loaded salts (calcium, magnesium, sodium, etc.). We also know that there are many salts on Mars, remember the perchlorates. Conclusion: we have more to do with brine in the liquid phase, which is extremely rich in salts, than with a lake of pure water.

Read also Mars: what does the discovery of salty liquid water change?

The question that burns all lips: Could this lake shelter life?

The first comments are not very positive. If it's so full of salts, it seems hard to imagine that there are bacteria bubbling in. Now, this is the whole issue of extremophiles [des espèces qui vivent dans des conditions mortelles pour la plupart des autres espèces, NDLR] and it is a field that evolves very quickly. We regularly discover bacteria living in incredibly hostile places, where we thought it would be totally impossible. Life has an ability to adapt that is pretty amazing. So, the subject is on the table … Now, I think extremophile specialists will be interested in this problem and we will know more in a few months or years. They will probably, for example, try to put bacteria in extremely salty environments and see if some subsist. Think of the tardigrades, they are not bacteria, but mites. Well, they are the all-round world champions of survival in a hostile environment. You put them in boiling water, they survive. From the international space station, we put it in the interplanetary vacuum where they were irradiated to levels that would kill a man for sure and … we bring them back to Earth, alive! Why ? Because they have an ability to repair their DNA quite extraordinary.

Can we hope to know more about this lake in the years to come?

I'm afraid not. There were 29 crossings of MARSIS over this area to characterize it. The team has been working on this for three years, it's a long-term job. She also asked to use almost all the memory of Mars Express for these observations. The work has been done. There is a second radar in orbit around Mars which is called SHARAD and which is aboard the Mars reconnaissance Orbiter probe. But it works at higher frequency and its waves penetrate so less well. And he sees absolutely nothing. Is it because he does not see deep enough? Perhaps. Or is there anything else? It will be necessary that the radar specialists of the SHARAD and MARSIS teams discuss and manage to make all this coherent. Rovers are not at all in the area.

After that, there is no way to study. Except to do as we did in subglacial lakes, Lake Vostok type, and arrive with a Derrick to dig … But that's not for tomorrow the day before. First, because I do not see the men who will go to Mars to position themselves near the South Pole. This seems to me a place not at all suitable for them. It's too cold! So I do not think we're going to dig into this lake. On the other hand, one could imagine digging where these men will be. From the moment they are going to stay at least a year and a half and where they will have a lot of time, putting them a big drilling system to work on the long term would be extremely interesting.

MARSIS, he will continue to search other lakes rather around the north and south poles of Mars since obviously it is there that he can succeed to see them. We can thus expect that we find others, there is no reason that it is the only one. As for a MARSIS 2, it is not planned but I want to say: the news is so recent, it can arouse things, provided that we are not already at the limits of the possibilities of development of this type of radar, the MARSIS antennas are already 40 meters!