Ancient meteorite upends our ideas of how Mars formed

Meteorite analysis hints that early Mars got important volatile elements like hydrogen and oxygen from meteorite collisions rather than a cloud of gases

By Alex Wilkins

A meteorite that landed on Earth more than 200 years ago is upending our ideas of how Mars formed. A new analysis of it reveals that the interior chemical make-up of the Red Planet largely came from meteorite collisions, rather than from a cloud of gases as was previously thought. This makes Mars’s early formation similar to that of Earth.

Most of what we know about Mars’s mantle, the section of rock outside the planet’s core, comes from three Martian meteorites that landed on Earth after being blasted off Mars by impacts: Shergotty, Nakhla and Chassingy.

Previous analyses of Chassigny, which landed in France in 1815, looked at isotopes of xenon, a chemically inert gas that can survive unchanged for millions of years. These isotopes – atoms that differ by their number of neutrons – occur in specific ratios that can be tied to a place and time.

The isotope ratios from the meteorite seemed to match those of both Mars’s atmosphere and the solar nebula, a large cloud of gas from which the primitive solar system formed. This led to the hypothesis that the Red Planet’s volatile elements, such as hydrogen, carbon and oxygen, came from the solar nebula and that additional elements came from meteorites later.

Now, Sandrine Péron and Sujoy Mukhopadhyay at the University of California, Davis, have analysed a sample from Chassigny to look at isotopes of krypton – another inert gas – which allows for more precise measurements, using a high-resolution mass spectrometer.

“With xenon isotopes, it’s difficult to distinguish the precise source of volatiles, but that’s not the case with krypton,” says Péron. “With krypton, you can better see the difference between potential sources like from solar or meteorites… but krypton isotopes are more difficult to measure than xenon isotopes, so that’s why it had not been previously done.”

The researchers found that the isotopes came from meteorites rather than from the solar nebula. This also implies that the Martian atmosphere, which contains mainly solar nebula isotopes, wasn’t acquired by gases exuding from the solar-derived mantle as we have thought until now, says Péron. So where did those gases in the atmosphere come from? It could be that they were trapped in the ground closer to the surface or in the cold polar caps if the young Mars grew quickly and are being gradually released by impacts, says Péron.

The work could fundamentally change our picture of how Mars was formed, as well as shore up the theory of planetary formation in our solar system, in which Mars seemed an outlier.

“It’s a major change in our understanding of the origin of volatiles in Mars,” says Chris Ballentine at the University of Oxford. “The end result is that Mars looks much closer to the way the Earth formed and the way that Earth acquired volatiles, which gives us a much more consistent view of how planets acquire their volatile elements.”

Finding out how volatile elements are acquired and distributed is also essential for understanding a planet’s chemical make-up, says Ballentine. “The timing and source of the volatiles controls the oxidation state, which, in turn, controls the structure and distribution of elements in the planet, which for our own Earth is why we can live on it.”

Journal reference: Science, DOI: 10.1126/science.abk1175

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