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Mars, express formation!

The geochemical study of a meteorite makes it possible to evaluate the formation time of the planet Mars.

Mars, express formation!

Publication date: 28/07/2018

Press, Research

Related themes : Origins

A study by an international team involving researchers from the Institut de Physique du Globe de Paris (IPGP) and the Institut Universitaire Européen de la Mer (IUEM) in Brest has uncovered the oldest Martian materials, zircon grains dating back some 4.476 billion years, discovered in the “Black Beauty” meteorite. These grains are around 100 million years older than the oldest known terrestrial materials. The chemical study of these zircons has also demonstrated that they originate from the melting of an andesitic rocky crust on the surface of Mars, dated to at least 4.547 billion years ago, i.e. less than 20 million years after the formation of the solar system. As the formation of a solid crust corresponds to the final phase of planetary formation, this dating shows that the formation of Mars, the separation of a core and the crystallization of a mantle all took place extremely rapidly.

The emergence of a rocky crust is indeed a fundamental step in the early history of a telluric planet. The primordial crust is considered to be the final product of a sequence of events, including planetary accretion, the formation of a magma ocean, the segregation of a core and finally the differentiation of a mantle and crust. On Earth, plate tectonics and erosion have wiped out most traces of this primordial crust, and all that remains of our planet’s ancient history are a few rare mineral witnesses resistant to the effects of time: zircons. The oldest zircons known to date on Earth come from Australia and are dated at 4370 million years, i.e. around 300 million years after the formation of the solar system.

Meteorite NWA 7034, more commonly known as Black Beauty, was discovered in the Moroccan desert in 2011. This rock, one of the oldest known Martian meteorites, contains numerous zircons. In this study, the researchers extracted and dissolved seven of these zircon grains measuring 50 to 110 µm. By measuring the relative abundances of the lead isotopes contained in these minerals, they estimated their crystallization ages at between 4429 and 4476 million years ago.

These results show that some of these “Martian grains” predate the oldest known terrestrial zircons by almost 100 million years. Even more astonishing: measurement of the isotopic composition of another element, hafnium, in these same crystals has shown that these zircons themselves originate from the melting of an even older crust, formed only twenty million years after the formation of the solar system, and surviving for 100 million years before being melted, probably by meteorite impacts.

The formation of such a crust of andesitic composition in just 20 million years requires Mars to have formed and differentiated very rapidly, which is in line with the most recent models of planetary formation by rapid accretion of millimetric objects.

 

Ref : Bouvier et al. Evidence for extremely rapid magma ocean crystallisation and crust formation on Mars, Nature, 558, 586-589. doi.org/10.1038/s41586-018-0222-z

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