Meteorite find challenges our understanding of how Mars formed

(ORDO NEWS) — A small piece of rock that once broke away from Mars and landed on Earth may hold clues to surprising details about the formation of the red planet.

A new analysis of the Chassigny meteorite that fell to Earth in 1815 shows that the way Mars produced volatile gases – such as carbon, oxygen, hydrogen, nitrogen and noble gases – contradicts existing models of planet formation.

According to existing models, planets are born from the remnants of stellar matter. Stars form from a nebular cloud of dust and gas when a dense clump of material collapses under the influence of gravity. As it spins, it draws in more and more material from the surrounding cloud to grow.

This material forms a disk that orbits the new star. Inside this disk, dust and gas begin to stick together as the planet grows.

We have seen other baby planetary systems form in this way, and evidence from our own solar system suggests that it formed in the same way about 4.6 billion years ago.

But how and when certain elements were included in the composition of the planets is not easy to figure out.

According to existing models, the volatile gases enter the molten forming planet from the solar nebula. Since the planet is very hot and soft at this stage, these volatiles are sucked into the global magma ocean of the forming planet and then partly expelled into the atmosphere as the mantle cools.

Later, even more volatiles enter through meteorite bombardment – volatiles bound in carbonaceous meteorites (called chondrites) are released when these meteorites break apart on impact with the planet.

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

The difference between these two sources can be determined by the ratio of noble gas isotopes, in particular krypton.

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

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

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

Its inert gas composition is different from that of the Martian atmosphere, suggesting that this piece of rock broke away from the mantle (and was ejected into space, leading to its arrival on Earth) and is representative of the planetary interior and thus the solar nebula.

However, krypton is quite difficult to measure, so the exact ratio of isotopes eludes measurement. However, Perón and her colleague, UC Davis geochemist Sujoy Mukhopadhyay, applied a new technique using the UC Davis Noble Gas Laboratory to make a new, accurate measurement of the krypton in the Chassigny meteorite.

And this is where things got really weird. The krypton isotope ratios in the meteorite are closer to those associated with chondrites. For example, surprisingly close.

“The Martian interior composition of krypton is almost entirely chondrite, but the atmosphere is solar,” Perón said. “It’s very noticeable.”

This suggests that meteorites delivered volatiles to Mars much earlier than scientists thought, before the solar nebula was scattered by solar radiation.

Thus the order of events is that Mars gained an atmosphere from the solar nebula after its global magma ocean cooled; otherwise, chondrite and nebular gases would be much more mixed than what the team observed.

However, this presents another mystery. When the solar radiation eventually burned the remnants of the nebula, it must have burned the nebular atmosphere of Mars as well. This means that the atmospheric krypton present later must have been preserved somewhere; perhaps, the team speculated, in the polar ice caps.

“However, for this, Mars had to be cold immediately after accretion,” Mukhopadhyay said.

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

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