Our “Dreams of Mars” series in four episodes:
Episode 1: Goodbye planet A, direction planet B?
Episode 2: Expat Life, Millions of Miles
Episode 3: 100,000 years to recreate the Earth
Episode 4: Exploring Time
When opening his cosmography manual, the aspiring schoolteacher of the 1920s discovered this extravagant affirmation to transmit to our dear blond heads: "The inhabitants of Mars, if there are any, are illuminated during certain nights by two moons at the same time" .
The future teacher had just learned, a few lines above, the existence on this celestial body of "seas linked together by canals" ... A century and some fifty missions later, we still have not found any liquid water on the red planet.
By revealing its arid surface in 1965, the Mariner 4 probe shattered the fantasy of a world populated by intelligent beings.
But the fascination with this big stone has remained intact.
Far from uncertain colonization or terraformation projects, scientists are trying to unravel its secrets.
After the putting into orbit of two new devices, Emirati and Chinese, last week, it is the Perseverance robot, from NASA, which must land on its surface this Thursday to try to answer a few questions that remained unanswered.
Certainly, the thesis of seas and canals was tempting.
If Mars deceived our ancestors, it is because the planet has an intriguing geography.
Spans with homogeneous hues and dark scars which, seen only from a telescope, can inspire the most eccentric theses.
But, if there have been rivers and lakes, there are no more rivers and lakes.
How was Mars once able to harbor liquid water, and therefore potentially life, and why is this no longer the case?
Two violent events
The first part of the story is pretty much known: it looks like an evening fight and it's Saturn that started!
In the early days of the solar system, the planet with the rings (rings which we are not sure of the existence then) engaged in a frenzied dance with Jupiter which had the effect of throwing Neptune off the track.
The unfortunate Neptune himself has jostled a horde of small frozen bodies ready to do battle.
The Earth and Mars, solid planets which had nevertheless asked for nothing, were hit hard by these objects coming from the other end of the room: this is, what is called in astronomy, the great late bombardment. (GBT), which occurred 4 billion years ago.
It is believed that this event had the effect of bringing to Earth, in addition to water, not life, but bricks essential to the appearance of life.
And on Mars?
It is very probable, as the astrochemist Caroline Freissinet, of the CNRS explains.
“On the meteorites that we receive on Earth, there are lots of molecules, amino acids.
On our planet, these amino acids are known to be the building blocks of proteins.
But there are also plenty of amino acids on meteorites that have nothing to do with life and are formed chemically.
There is no reason they can't arrive on Mars the same way.
The great late bombardment brought a lot of water and these molecules on the planets, on the Earth, on its satellite, the Moon, but also on Mars.
"
It was not the first time that the early planets had seen themselves sprayed like this.
A longer violent episode, the primordial bombardment, would have taken place from the very first "moments" of the solar system, shortly after its appearance 4.6 billion years ago.
Mars would have already received on this occasion large quantities of water.
And again, the giant planets would have been the source of the brawl.
But smaller than the Earth and therefore exerting a weaker gravity, Mars would have let escape for the first time its atmosphere and its liquid water.
The enigma of the primitive climate
The great late bombardment therefore gave Mars a second chance to own oceans like Earth.
However, today we only find water in the form of ice on the surface of this planet, in the polar regions.
Why, then, is the planet dry again?
It is established that Mars had a denser atmosphere after the GBT and that it again let out much of it.
But the change between the Mars of old, 3.5 billion years ago, and the current Mars is of such magnitude that it remains a mystery.
"It's a real enigma, the primitive climate of Mars, admits the planetologist François Forget.
We do not understand how there could have been lakes and rivers on Mars three or four billion years ago.
We do not know the astonishing climatic mechanism that could have enabled this because Mars is far from the Sun and at the time the Sun was 25% less luminous than today.
You really need a very powerful greenhouse effect to do that.
However, the fact that there were lakes and rivers is indisputable.
"
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“To conduct the survey, we do calculations and study the current climate,” continues François Forget, who collaborates with the Emirati Hope mission, dedicated to the study of the atmosphere.
But studying the geology of Mars, that is, recording past environments, also teaches us a lot.
This is why Perseverance interests me so much.
»This Thursday evening, if all goes as planned, this NASA robot should land in the Jezero crater, a very flat site to increase the chances of a successful landing.
A "photo" of the Earth before
Jezero would also be conducive to the search for traces of life.
"It is an impact crater, of approximately 45 km in radius, which we know to have filled with water," describes the astrophysicist Sylvestre Maurice, who is collaborating on the Mars 2020 mission. was probably twice a lake, in which flowed a river with a slight slope.
This type of river creates deltas, much like the Nile.
There is some flowing water but it is quite shallow.
In this lake, liquid water has left traces.
We are going to land on this promontory, at the foot of this delta and we will go up the river.
"
Carbonates, minerals resulting from the transformation of basaltic rocks, are of particular interest to researchers.
“On Earth, these limestone rocks preserve fossils very well, but also fossil molecules.
It's an interesting place to study what may have happened in the past, especially if there are bacteria that may have formed ”, summarizes François Forget.
According to Sylvestre Maurice, "Perseverance's mission is structured around three main themes: geology, climate and life".
Caroline Fressinet, whose discipline focuses precisely on the study of life, tempers: “The selection of samples will not be ideal for the search for traces of life.
We would have preferred deep samples, less damaged over time, than surface samples.
And there is no instrument on board Perseverance that will allow us to determine whether there is a greater chance that traces of life will be found on a given sample.
"
In her field, Caroline Freissinet expects more from the ExoMars mission, delayed several times, and scheduled for the moment in 2022. But the exobiologist shares the interest of studying the geology of the red planet to do "planetology" compared ”.
“Mars died a few billion years ago.
There is no more tectonics, no more recycling.
All of the land on Mars is ancient and dates from 3.6 to 3.8 billion years ago.
Conversely, on Earth, traces of life from four billion years ago are extremely rare because everything is recycled: plate tectonics means that when we look at the ground on the earth's surface, it is a recent soil.
Mars is a photo of Earth from the past.
It's much easier to understand how the Earth could have evolved while exploring Mars today.
"
