Water on Mars? At best, it’s a sea …

Water on Mars?  At best, it's a sea ...

Although it is now accepted that the distant past of Mars contained liquid water, the existence of the ocean 3 billion years ago seemed impossible. However, We have published a study in the scientific journal PNAS By simulating the Martian climate system of the time, it shows the opposite. At the same time on Earth, life evolved to capture numerous ecosystems.

The climate on the equatorial continents was indirectly cold, but the polar ocean may have been fluid, as shown by several geological traces. In this scenario, our brother planet could have been greatly favored for the development of life before becoming a red planet. A stable ocean as a chemical solvent will provide the liquid water needed for life in large quantities but will protect it from stellar radiation. These conditions seem necessary, but certainly not enough, and the origin of life is an open question in the science of XXI.Th Century. To date, no traces of life have been found anywhere on Mars or in the universe other than Earth.

The presence of liquid water on Mars 3.5-4 billion years ago (called Nocian in the Geological Era) confirms the existence of branch valleys. These valleys are formed by liquid water, usually in the form of rain or melting snow. The water flow forms small rivers with a V-shaped erosion profile, which converge into large rivers and so on: the final terrain is a network of tributary valleys, Sometimes thrown into lakes.

Glacier valleys, on the other hand, are of different shapes due to massive glacial erosion, U-shaped and less branched. Why has Mars been able to monitor these processes so far at an impossible time on Earth? On Earth, there is the plate tectonics that brought all the continents into a super continent 200 million years ago: Pangea. Therefore, on Earth, it is not possible to find old landscapes. On Mars, the plate has no tectonics and the terrain is stable, gradually accumulating the scars of time.

An 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 beach. Recently, a new discovery was made: the identification of megatsunami deposits indirectly indicates the presence of the ocean. Identification of the impact abyss at the origin of this deposit. The Lomonosov crater has a specific shape, which indicates its formation in an ocean. The estimated age at the end of the geospheric era known as the Hesperian is 3 billion years.

How to estimate the age of a planet’s surface? Astronomers use the groove counting method based on a very simple principle: grooves accumulate over time. So the old surface is a grooved surface rather than a young surface. This method gives the relative age (old / young), but in order to have a full age, it is necessary to have a correlation between the groove density and the exposed age. This work may have been done on the moon according to the complete date of the samples brought back to Earth. For Mars, the modeling work has established a correspondence between the groove density and the absolute age.

The great scientific controversy over the Martian Ocean arises from the fact that earlier climate models could not simulate a stable ocean at this time: all the water that has accumulated in the mountains in the form of ice. Our study was published in the journal PNAS, In collaboration with a team from the University of Paris-Chockley / CNRS / Geops and NASA / GISS, has developed a climate simulation that incorporates two new essentials, ocean cycles and glaciers. Combining these two processes, these new climate simulations show a stable ocean in the Northern Hemisphere, even when the average Martian temperature is below 0 C. The ocean, despite its polar state, does not freeze due to ocean currents that bring warm water toward the poles. On the other hand, these simulations predict the presence of glaciers that bring ice from the mountains to the sea. These predictions are consistent with the geographical descriptions of the images that indicate the presence of these glacial valleys.

Climate modeling of Mars

Simulating the climate of Mars 3 billion years ago is no small feat. The same type used to simulate terrestrial climate, but in accordance with Mars, it is necessary to have a numerical representation of the climate, based on physico-chemical principles. On the one hand Mars is farther away from the Sun than the Earth, so it receives less energy, on the other hand it should be taken into account that the Sun gave less light than it does today. Under these conditions, the sunlight of Mars at that time was only one-third of what Earth receives today.

To obtain moderate conditions and liquid surface water, low sunlight must be offset by significant greenhouse gases and high atmospheric densities. Atmospheric pressure 1 bar (present on Earth, but currently 100 times higher than on Mars) requires liquid water, but CO2 (Currently dominating Mars) is not powerful enough to reach the melting point of water (0 C). Another powerful greenhouse gas is required, so the scientific team used the atmosphere with 90% CO in their sample.2 And 10% H2. This very powerful greenhouse gas may have been emitted by the intense volcano of the time or by meteorite impacts.

The results show that the continental climate should be as follows: warmer and humid zone near the coast, with average annual temperatures above 0 C and precipitation. A cold and dry zone with temperatures below 0 ° C in all the high plateaus of the southern hemisphere of Mars. In this second zone, the ice that accumulated at the highest mountains was transformed into ice that flows towards the ocean to complete the cycle. These weather forecasts coincide with the networks of ramified valleys near the coast and the presence of large glacial valleys as a result of ice accumulation zones.

Climate map of Mars 3 billion years ago. Colors indicate climatic zones. Arrows traces of glaciers and networks of branch valleys can still be seen today.
F. Schmidt et al., PNAS, 2022

Map of the water cycle on Mars 3 billion years ago. The arrows represent 1015 kg runs per year.
F. Schmidt et al., PNAS, 2022

The history of the volatile elements (liquid, ice, gas) on Mars and in water was not fully understood during the history of Mars. How much water is available over time? In what form is it: ice, liquid, permanent ice, hydrated minerals? This scientific publication assumes that there is enough water for an ocean, but what happened after that water is a big mystery! Many hypotheses can be formed: Escape into atmospheric space, Underground storage in the form of ice (permafrost) or hydrated minerals. Apparently, the water from the current Martian reservoirs coming from polar caps and glaciers is not enough to feed an ocean.

To conclude this study, we would like to examine more precisely the symbols of the Martian glacial valleys of this period from satellite images. The Chinese Jurong Rover landed in Paleo-Sea area in May 2021. We expect to find evidence for the sea in the rocks they explore. Soon, within ten years, NASA’s Mars Ice Mapper, equipped with an unprecedented radar sounder, will be able to study the structure of the soil. This work will definitely give us new arguments about Mars!

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