Paris (AFP)

After seven months of travel, NASA's Perseverance rover is due to land on Mars on Thursday, in search of traces of ancient life.

In the solar system, other stars, including icy moons, could also harbor forms of life.

The living ingredients

The keystone of the quest for the living is the presence of liquid water: we then speak of "habitability zone" around a star, that is to say the zone "where it is theoretically possible, with sufficient atmospheric pressure, to maintain liquid water on the surface, ”explains Athéna Coustenis, astrophysicist and CNRS research director at the Paris-PSL Observatory.

But to make life emerge, more ingredients are needed.

The planet must contain organic molecules, the + CHNOPS + (carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur, editor's note) ", the main components of living things, adds Michel Viso, exobiologist at CNES, the French space agency.

Then, for these reactions to lead to a metabolism, and therefore at the beginning of life, a source of energy is necessary.

At the surface, it can come from the Sun, and at depth, chemical reactions or tidal effects.

When water, molecules and energy merge in a stable environment, then life has every chance to emerge.

March

The red planet, considered the twin sister of the Earth, could have met these criteria 3.5 to 4 billion years ago.

Earth sits right in the middle of the habitable zone of our solar system, which may have changed over time.

A cluster of clues shows that life may have appeared on Mars, before being extinguished after the planet lost its atmosphere and its oceans, some scientists say.

Liquid water flowed in abundance on its surface, it is now a certainty.

Large deposits of mud also indicate exchanges between the surface and the depth, proof that the planet is active.

Because of its very low density atmosphere, "there can be no liquid on the surface at the moment, which is why we have to drill the ground", indicates Michel Viso.

Europe

Europe is one of the four moons of Jupiter, the largest planet in the solar system.

"It is the first natural satellite on which we saw traces on the surface suggesting an ocean of liquid water below," says Athéna Coustenis.

These traces, geophysicists interpret them "as displacements of large pieces of ice on the surface".

There are also geysers, resulting from faults on the surface, a sign of cryovolcanic activity (ice volcanoes) and therefore a source of energy.

We can assume that organic elements in the atmosphere (brought by comet impacts) could penetrate the liquid ocean, thus fulfilling the criteria of habitability.

Dedicated to this icy moon, NASA's Europa Clipper probe is under development.

The European probe JUICE will also make observations from 2030.

Enceladus

Frozen moon of Saturn, Enceladus concentrates all the good ingredients (liquid water, organic molecules, energy, stable environment).

The American Cassini probe, orbiting the planet from 2004 to 2017, discovered the existence of water vapor geysers on Enceladus.

As for Europe, "we detected salt in the plumes, organic elements, methane, CO2 ... which suggest a contact between the pockets of liquid water and the core" of this frozen moon, "as on Earth ", suggests the astrophysicist.

No mission is currently scheduled to study Enceladus, the rate of appearance of geysers being too irregular.

Titan

"By going to Titan, we would make a comeback on Earth at its beginnings", during its training, according to Michel Viso, of CNES.

This other satellite of Saturn is indeed the only other place in the solar system where the atmosphere is composed of nitrogen and organic chemistry, as on Earth.

But if there is liquid water on Titan, it seems to be trapped between two layers of ice, unlike Europe and Enceladus.

Therefore, the water could not come into contact with the rock of the core, the source of energy.

A NASA space mission is dedicated to him, Dragonfly, and a probe is due to land there around 2034.

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