Meteorite discovery challenges our understanding of how Mars formed

A small piece of rock that once broke off from Mars and found its way to Earth may hold clues that reveal surprising details about the composition of the red planet.

A new analysis of the Chassigny meteorite, which fell to Earth in 1815, indicates that the way Mars obtained its volatile gases — such as carbon, oxygen, hydrogen, nitrogen and noble gases — contradicts our current models of how planets form.

Planets, according to current models, are born from the remains of stars. Stars form from a nebulous cloud of dust and gas when a dense mass of material collapses under the influence of gravity. As it rotates, more material is stored from the surrounding cloud to grow.

This material forms a disk, which orbits the new star. Within this disk, dust and gas begin to clump together in a process that causes a small planet to grow. We’ve seen other planetary systems form in this way, and evidence in our own solar system indicates that they formed in the same way, about 4.6 billion years ago.

But how and when certain elements of the planets were combined was difficult to put together.

According to current models, volatile gases are sucked up by a molten molten, forming a planet from the solar nebula. Because the planet is so hot and mushy at this point, these volatiles seep into the global ocean of magma that is the forming planet, before being partially outgassed into the atmosphere later as the mantle cools.

Later, more volatiles are delivered by meteorite bombardment – associated volatiles in carbonaceous meteorites (called chondrites) are released as these meteorites disintegrate upon entering the planet.

Therefore, the interior of the planet should reflect the composition of the solar nebula, while its atmosphere should mostly reflect the volatile contribution of meteorites.

We can tell the difference between these two sources by looking at the isotope ratios of the noble gases, especially krypton.

And because Mars formed and solidified relatively quickly in about 4 million years, compared to up to 100 million years for Earth, it’s a good record for those very early stages of the planet formation process.

“We can reconstruct the fluctuating delivery history in the first few million years of the solar system,” said geochemist Sandrine Peron, formerly at UC Davis and now at ETH in Zurich.

This is, of course, only if we can access the information we need – and this is where the Chassigny meteorite is a gift from space.

Its noble gas composition differs from that of Mars’ atmosphere, indicating that the piece of rock broke off from the mantle (and blasted off into space, bringing it to Earth), representing the interior of the planets and thus the solar nebula.

Measuring krypton is very difficult, however, and exact isotopic ratios have eluded measurement. However, Peron and her colleague, fellow geochemist Sugoi Mukhopadhyay of UC Davis, used new technology using the UC Davis Noble Gas Laboratory to make a new, precise measurement of krypton in the Chassigny meteorite.

And this is where it gets really weird. The ratios of krypton isotopes in the meteorite are closer to those associated with chondrites. Like, noticeably closer.

“The internal structure of Krypton on Mars is almost purely cartilaginous, but the atmosphere is solar,” Peron said. “It’s very special.”

This indicates that meteorites were transporting volatiles to Mars much earlier than scientists previously thought, before the solar nebula was dissipated by solar radiation.

Thus, the order of events is that Mars gained an atmosphere from the solar nebula after the global ocean of magma had cooled; Otherwise, the cartilaginous and nebulous gases would be more mixed than the team noted.

However, this presents another mystery. When the solar radiation finally removed the remnants of the nebula, it should have also burnt up the nebulous atmosphere of Mars. This means that the krypton in the atmosphere must have been preserved somewhere; Perhaps, the team suggested, in the polar ice caps.

“However, that would require Mars to be cold in the immediate aftermath of its accretion,” Mukhopadhyay said.

“While our study clearly indicates the chordate gases in the interior of Mars, it also raises some interesting questions about the origin and composition of the early Martian atmosphere.”

The team’s research was published in Sciences.

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