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Most lakes on Mars have not been identified

If the presence of liquid water in the past of Mars is no longer in doubt, a new study underlines that we are far from having identified the majority of the lakes having existed on the Red Planet.

For some time now, and in particular thanks to data collected by Mars rovers, we know that Mars once had lakes of liquid water, often nestled in the heart of ancient impact craters. The Jezero crater, which Perseverance is currently crisscrossing, is a perfect example of this, notably with the presence of a sedimentary succession of deltaic origin.

The study of these ancient lakes, whose existence dates back several billion years, is extremely important for determining the climatic conditions that prevailed on the planet at the beginning of its history. They are also preferred places to hope to find traces of primitive life. Because lake sediments, often clayey, are the most favorable environments for the conservation of biological molecules.

The ancient Martian lakes are of course identified thanks to satellite data and images acquired from orbit by the various probes which constantly scan the Martian surface. Today, about 500 ancient lakes have been listed on the surface of Mars. But for some scientists, this figure is certainly far below reality.

70% of ancient Martian lakes still unidentified

More than 1,000 paleolacs could indeed have escaped the eye of satellites. It all seems like a big story. It should be noted that almost all of the ancient lakes identified so far are particularly large, generally over 100 km2. However, on Earth, only 30% of lakes belong to this category. The remaining 70% are much smaller.

If we consider that this statistic is valid for the Red Planet, it is therefore highly probable that we have so far missed the vast majority of Martian lakes. This gap could be explained quite simply by the difficulty in identifying these old small basins by satellite imagery. However, they could, just as much as the larger lakes, deliver an important mass of information on the past environment of the Red Planet.

Ephemeral and short-lived lakes

This issue was the subject of an article in the journal Nature Astronomy. For Joseph Michalski, of the University of Hong Kong and lead author of the study, referencing all the ancient lakes could in particular make it possible to better date the presence of liquid water on the surface of the planet, but also to better understand global water conditions and their variations over time. Because if the lakes currently identified are between 3.5 and 4 billion years old, it appears that each lake would only have had a lifespan of the order of 10,000 to 100,000 years. This means that most of the time the planet would have been arid and cold, with only episodic warmer periods during which the lakes would have formed ephemerally.

Martian lakes would also be relatively different from those observed on Earth. The low gravity of the planet would indeed not have favored the rapid deposition of sediments on the bottom. Instead of crystalline lakes, it would seem more accurate to imagine lakes with turbid waters, loaded with sediments, and in which the pale light of the young Sun would have had difficulty penetrating. The absence of vegetation, which normally helps stabilize sediments, would also have favored this phenomenon. Add to that the fact that remote Mars receives much less solar energy than Earth and you have a lake environment that is certainly not conducive to the development of photosynthetic life.

The best chances of finding biosignatures remain in large and deep lakes

In this context, and even if all of the ancient Martian lakes are far from having been identified, the large lakes remain the most interesting sites to visit in order to find biosignatures. They are indeed those which are likely to have remained in water over long periods of time. They also have the advantage of presenting several types of environments, with potentially the presence of hydrothermal systems capable of providing a source of heat and energy supporting the emergence of microbial life.