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A meteorite reveals a warm climate on Mars 4.4 billion years ago

In a study conducted by IPGP, CNRS and Université de Paris, and published on October 30, 2020 in Science Advances, an international scientific team has shown that 4.4 billion years ago, the young Martian atmosphere was hot enough to maintain water in a liquid state. Analysis of a Martian meteorite has revealed that oxidation of Mars' crust due to the planet's numerous impacts could have induced, through the greenhouse effect, warming of the atmosphere despite a weaker Sun than today.

A meteorite reveals a warm climate on Mars 4.4 billion years ago

Publication date: 30/10/2020

Press, Research

Related themes : Origins

A great deal of geomorphological evidence from observations made by space missions shows that liquid water flowed on the surface of Mars very early in the planet’s history (more than 3.7 billion years ago). However, the presence of liquid water remains an enigma, since at that time our Sun was 30% weaker in terms of energy production than it is today and did not heat the Red Planet sufficiently to maintain water in a liquid state. This question, which also applies to the Earth, is known as the “paradox of the weak young Sun”.

The models developed to date suggest that the atmosphere of Mars is warming as a result of the outgassing of greenhouse gases by intense magmatism. But the validation of these models is hampered by the scarcity of ancient Martian samples. However, while awaiting the return of samples from space missions, it is already possible to study some Martian rocks using meteorites.

One meteorite is of particular interest to scientists: the Martian meteorite NWA 7533, also known as the “Black Beauty”. This meteorite, a fragment of Martian rock ejected during an impact, arrived on Earth after a long interplanetary journey and was discovered in 2011 in Morocco. “Black Beauty” is unique (among the 300 or so Martian meteorites found so far) for its chemical composition, which is similar to the rocks of the southern hemisphere of Mars, and also for the fact that it contains the oldest known fragments of the crust of Mars, dating back as far as 4.4 billion years.

Martian meteorite NWA 7533, nicknamed "Black Beauty" (© Luc Labenne)

By examining this particular meteorite, scientists from the University of Paris, the IPGP and the CNRS, together with colleagues from the Universities of Western Brittany, Copenhagen and Tokyo, have found clues as to the mechanism that would have enabled a warm climate to be maintained on the surface of Mars during the period of the “young weak Sun”.

By applying new chemical and isotopic tools developed at the Institut de Physique du Globe in Paris, the scientists discovered that the ancient fragments of the Martian crust were formed during the powerful impacts that the young planet experienced, and that these fragments underwent progressive oxidation as they cooled.

For the first time, they have analysed the isotopic abundances of titanium contained in the fragments of meteorite crust, and observed that the processes that led to their particular composition bear the signature of impact melting and oxidation.
At the same time, the isotopic analysis of the oxygen contained in the various inclusions of the meteorite shows a change in oxidation over time, confirming the hypothesis of oxidation of the crust linked to impact melting in the presence of water, already present in the form of ice or brought in by impactors.

The study, published on 30 October in Science Advances, also explains that this early oxidation of the Martian crust by water led to the release of gaseous dihydrogen (H2) into the Martian atmosphere. A high quantity of H2, a greenhouse gas, in a thick atmosphere of CO2 (like that of Mars) has, in reaction, caused the surface of Mars to heat up by several tens of degrees.

While the transient warming of the planet due to the kinetic energy associated with impacts only lasts a few years, the greenhouse effects associated with impact processes can induce warm climates, conducive to the presence of liquid water, which can last for tens of millions of years, despite a “weak young Sun”.

This study thus provides a solid argument for the hypothesis of a primitive Martian crust remelted by impacts, as well as for that of heating by the greenhouse effect allowing liquid water to flow onto Mars more than 4 billion years ago.

Ref : Z. Deng, F. Moynier, J. Villeneuve, N. K. Jensen, D. Liu, P. Cartigny, T. Mikouchi, J. Siebert, A. Agranier, M. Chaussidon, M. Bizzarro, Early oxidation of the martian crust triggered by impacts. Sci. Adv. 6, eabc4941 (2020). https://advances.sciencemag.org/content/6/44/eabc4941

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