Water on Mars?  Best, it was an ocean...

Water on Mars? Best, it was an ocean…

While it is now accepted that Mars’ distant past had liquid water, the existence of an ocean as recently as 3 billion years ago seemed impossible. though, We just published a study in the scientific journal PNAS Which shows the opposite, by making it possible to simulate the climate system of Mars at that time. At the same time on Earth, life evolved to conquer a large number of ecosystems.

It is assumed that the climate was cold on the equatorial continents, but the polar ocean may have remained liquid, as indicated by a number of geological clues. In this scenario, our sister planet might also be greatly beneficial to the evolution of life before it became the Red Planet. A stable ocean provides liquid water essential for life on large time scales as a chemical solvent, but also as protection from stellar radiation. These conditions seem necessary, but they are certainly not sufficient, and the emergence of life is an open question for twenty-first century sciencey a century. It must be remembered that so far, no trace of life has been discovered anywhere other than Earth, either on Mars or anywhere in the universe.

The presence of liquid water on Mars 3.5-4 billion years ago (in the geological age called Noachian) is attested by the presence of branching valleys. These valleys consist of liquid water, often in the form of rain or melting snow. The flow of water results in small, V-shaped erosion-shaped rivers that merge to become larger rivers and so on: the final scene is a network of branching valleys, sometimes flowing into lakes.

On the other hand, glacial valleys have different shapes due to massive glacial erosion, U-shaped and less bifurcated. Why was Mars able to trace these processes so far when that would have been impossible on Earth? On Earth, we have plate tectonics, which brought all the continents together 200 million years ago into a supercontinent: Pangea. Therefore, it is impossible to find ancient landscapes on Earth. On Mars, there are no plate tectonics and the landscape remains static, the scars of time gradually accumulating.

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Ocean in the northern hemisphere of Mars

The existence of a polar ocean in the northern hemisphere is controversial, but many teams have identified a consistent ancient coastline. Recently, a new discovery was made: the identification of huge tsunami deposits, which indirectly indicates the existence of the ocean, and even Identification of the impact crater at the origin of this sediment. Lomonosov crater has a specific shape that indicates its formation in the ocean. The age is estimated at 3 billion years at the end of the geological age called the Hesperian.

How do you estimate the age of the planet’s surface? Astronomers use crater counting based on a very simple principle: craters build up over time. Thus, the old surface is a more cracked surface than the young one. This method gives a relative age (larger/smaller), but to obtain an absolute age it is necessary to have a match between the hole density and the exposure age. This work would have been done on the Moon according to the absolute dating of the samples brought back to Earth. For Mars, modeling work has proven the match between deep crater density and absolute age.

The main scientific debate about the Martian ocean stems from the fact that previous climate models could not simulate a stable ocean at this time: all the water accumulating on the mountains in the form of snow. Our study was published in the journal PNAS, carried out in collaboration between a team from Université Paris-Saclay/CNRS/GEOPS and NASA/GISS, has just built a climate simulation, consisting of two new core components: ocean circulation and glaciers. Adding these two processes, these new climate simulations show a stable ocean in the Northern Hemisphere, even for Martian average temperatures below 0°C. The ocean, despite its polar position, does not freeze thanks to ocean currents that bring warm water towards the poles. On the other hand, this simulation predicts the presence of glaciers that bring ice from heights towards the ocean. These predictions are consistent with geological interpretations of the images that indicate the presence of these glacial valleys.

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Martian climate modeling

Simulating the climate of Mars 3 billion years ago is no small feat. It is necessary to have a numerical representation of the climate, based on physical and chemical principles, of the same kind used to simulate the terrestrial climate, but adapted to Mars. It should be borne in mind that on the one hand Mars is farther from the Sun than the Earth and therefore receives less energy, and on the other hand the Sun provides less light than it does today. Under these conditions, the solar illumination of Mars at that time was only a third of what Earth receives today.

For temperate conditions and liquid surface waters, low solar flux must be compensated for by large greenhouse gases and high atmospheric density. Atmospheric pressure of 1 bar (the same as on present-day Earth, but 100 times more than the current pressure on Mars) is necessary for liquid water, but carbon dioxide2 (currently prevalent on Mars) is not strong enough on its own to reach water’s melting point (0°C). Another powerful greenhouse gas was needed, so the science team used an atmosphere containing 90% carbon dioxide in their model.2 and 10% H2. This powerful greenhouse gas could have been emitted by intense volcanoes at that time or degassed during meteor impacts.

The results showed that the continental climate should have been as follows: a hot and humid area near the shore, with average annual temperatures above 0 °C and rain. A cold, dry region with temperatures below 0°C on all the high plateaus of the southern hemisphere of Mars. In this second area, snow accumulated on the highest mountains which turned into a glacier that flows towards the ocean to complete the cycle. The predictions of this climate are consistent with the existence of networks of branched valleys near the coasts and the presence of large glacial valleys resulting from areas of snow accumulation.

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Map of the climate of Mars 3 billion years ago. Colors indicate climatic zones. Arrows indicate traces of glaciers and branching valley networks that are still visible today.
Schmidt et al., PNAS, 2022
Diagram of the water cycle on Mars, 3 billion years ago. Arrows indicate flows of 1015 kg/year.
Schmidt et al., PNAS, 2022

It is clear that the history of the volatile elements (liquid, ice, and gas) on Mars and water was not fully understood during the history of Mars. How much water is available over time? What does it look like: ice, liquid, permafrost, wet minerals? This scientific publication assumes that there is enough water for the oceans, but what happened to that water next is a huge mystery! Several hypotheses can be formulated: escape from the atmosphere into space, underground storage in the form of ice (permafrost) or wet minerals. It is clear that the waters of present-day Mars reservoirs coming from the polar caps and glaciers are not enough to feed the ocean.

To complete this study, we now wish to examine indices of Martian ice valleys for this period with greater accuracy from satellite images. The Chinese rover Zhurong landed in May 2021 in the ancient ocean area. We expect to find evidence of the ocean in the rocks they will examine. Soon, in less than ten years, the Mars Ice Mapper, NASA’s new mission project equipped with an unprecedented radar probe, will be able to study soil infrastructure. This mission will certainly give us new arguments about the circumference of Mars!

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