Mars has not always been the cold, reddish, arid planet we know today. In the past it was much warmer and wetter, as evidenced by dry riverbeds and relict coastlines. There was a time when large volumes of liquid water flowed across its surface, although very little remains today, most of it frozen in the polar ice caps.

Until now, several studies have suggested that ancient Martian water escaped into space over billions of years, a claim supported by the ratio of hydrogen (H) to one of its isotopes, deuterium (D), in its atmosphere.

However, measurements of the current rate of atmospheric water loss are too low for this leakage alone to explain all of its loss. Geological reasons seem to be behind it, according to a new study presented at the Lunar Planetary Science Conference (LPSC) being held these days virtually and also published in the journal Science.

Researcher Eva Scheller from Caltech (USA), together with colleagues from Caltech and the JPL laboratory, reports how much of Mars’ initial water, up to 99 %, could have been incorporated into the minerals of its crust, and was not lost to space.

The large volume of water that covered Mars, where there were rivers and oceans, decreased by 40-95% between 4.1 and 3.7 billion years ago, according to observational data and simulations used in a new study.

Using observations from spacecraft orbiting the red planet, as well as data from rovers travelling across its surface and from Martian meteorites, the authors have calculated the amount of water in the past, as well as a D/H (deuterium-hydrogen) model that takes into account its present-day ratio, atmospheric escape of water, volcanic degassing and hydration of the crust through chemical weathering.

The team found that about 4 billion years ago, Mars held enough water to have covered the entire planet with an ocean between 100 and 1,500 metres deep, a volume equivalent to half of Earth’s Atlantic Ocean.

But by simulating the loss of Martian water over geological time and under different possible conditions, the model results come to a conclusion: “The volume of water participating in the hydrological cycle decreased by 40-95% during Mars’ Noachian period (between 4.1 and 3.7 billion years ago)”.

Crustal hydration

Between 30 and 99% of Martian water was sequestered by crustal hydration,” the authors add, “demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets.

The results indicate, therefore, that the large percentage of Mars’ initial water was incorporated into minerals and buried in the red planet’s crust, and that the rest escaped into space, which would explain the D/H ratio currently found in its atmosphere.

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