a wet planet long before Earth

If current Martian conditions no longer allow the establishment of oceans on its surface, it was very different when the planet formed. A new study suggests that the Red Planet’s primordial atmosphere was particularly dense, rich in water vapor and dihydrogen. Conditions that favor the very rapid appearance of vast expanses of liquid water and perhaps the appearance of life.

Today there is no longer any doubt that the Martian landscape was, in the distant past, shaped by the presence of liquid water. Traces of ancient rivers feeding ancient lakes are clearly visible from orbit. In addition, the Curiosity and Perseverance rovers have provided irrefutable proof of this, thanks to rock analysis and precise photographs of the sedimentary units typical of lakes. Today however, liquid water has completely disappeared from the planet’s surface.

In this great environmental change that occurred about 3 billion years ago, the Martian atmosphere played a crucial role.

A primitive atmosphere rich in water vapor.

The analysis of the atmosphere shows us that currently the Red Planet is unable to maintain liquid water on its surface. But flow traces and lake sediments tell us that the situation was drastically different more than 3 billion years ago. However, it is difficult to define what the atmospheric conditions were at that distant time.

Therefore, scientists have set out to produce the first numerical model that traces the evolution of the Martian atmosphere from the formation of the planet to the establishment of the first oceans and lakes.

This model suggests that, like Earth, the Martian atmosphere was originally rich in water vapor, concentrated in clouds in the lower layers of the atmosphere. The upper, outermost parts would have been much drier. In contrast, dihydrogen molecules (Htwo), too light to be held, would have quickly escaped into space.

A dense atmosphere rich in H2 that allows the rapid establishment of oceans and lakes.

These results, published in the journal Earth and Planetary Science Letters, allow us to define the atmospheric conditions that prevailed on Mars shortly after its formation. For scientists from Seti Institute, this primordial atmosphere must have been very dense, about 1,000 times denser than the current Martian atmosphere. The surface of the planet was then occupied only by an immense ocean of magma, like the primitive Earth, and the atmosphere composed mainly of dihydrogen. However, under high pressure conditions, Htwo It is a powerful greenhouse gas.

Unlike today’s thin atmosphere, the primitive atmosphere of Mars would have been, therefore, a true blanket of lead, which would have allowed the rapid formation of oceans of liquid water on the surface of the planet. These warm oceans would have been stable for several million years before Htwo it is gradually lost to space, and the Martian atmosphere gradually loses its density. Therefore, Mars would have been “wet” shortly after its formation, long before the same conditions appeared on Earth.

A situation favorable to the appearance of life, but which did not last long

The data used to build this model comes from analyzes of Martian rocks carried out by the Curiosity rover, but also from the study of certain Martian meteorites. Through analysis of the deuterium to hydrogen isotope ratio (D/H), scientists were able to show that Earth and Mars began life under relatively similar atmospheric conditions. However, the two planetary environments would have differed 3,000 million years ago, when Mars began to lose its light isotopes of hydrogen in space.

These results are particularly important from the point of view of the search for extraterrestrial life. It has been shown that the prebiotic molecules at the basis of life would form quite easily in an environment rich in Htwo like that of the early Martian atmosphere. Therefore, Mars would have had all the ingredients available during its youth to see life unfold on its surface.

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