Discovery of meteorites challenges our understanding of how Mars formed

A small chunk of rock that once broke away from Mars and found its way to Earth could hold clues that reveal surprising details about the red planet’s formation.

A new analysis of the Chassigny meteorite, which crashed to Earth in 1815, suggests 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 are born, according to current models, from leftover stellar material. Stars form from a nebular cloud of dust and gas when a dense clump of material collapses under gravity. Spinning around, it wraps more cloud material around it to grow.

This material forms a disk, revolving around the new star. Inside this disk, dust and gas begin to clump together in a process that creates a baby planet. We’ve seen other baby planetary systems form this way, and evidence from our own Solar System suggests it formed the same way, around 4.6 billion years ago.

But how and when certain elements were incorporated into the planets has been difficult to understand.

According to current models, volatile gases are absorbed by a molten planet, forming from the solar nebula. Because the planet is so hot and pasty at this stage, these volatiles are sucked into the global magma ocean that is the planet in formation, before later being partially degassed into the atmosphere as the mantle cools.

Later, more volatiles are delivered through meteorite bombardment – ​​volatiles bound to carbonaceous meteorites (called chondrites) are released when these meteorites break up on introduction to the planet.

Thus, a planet’s interior must reflect the composition of the solar nebula, while its atmosphere must primarily reflect the volatile contribution of meteorites.

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

And since 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 the early stages of the planetary formation process.

“We can reconstruct the history of volatile delivery in the Solar System’s first few million years,” said geochemist Sandrine Péron, formerly of the University of California Davis, now at ETH Zurich.

That, of course, is only if we can access the information we need — and that’s where the Chassigny meteorite is a gift from space.

Its noble gas composition differs from the Martian atmosphere, suggesting that the chunk of rock detached itself from the mantle (and was launched into space, precipitating its arrival at Earth) and is representative of the planetary interior and therefore the solar nebula.

Krypton is quite complicated to measure, however, the precise proportions of the isotopes have eluded measurement. However, Péron and his colleague, fellow geochemist Sujoy Mukhopadhyay at UC Davis, employed a new technique using the UC Davis Noble Gas Laboratory to perform a new, accurate measurement of krypton in the Chassigny meteorite.

And that’s when it got really weird. The proportions of krypton isotopes in the meteorite are closer to those associated with chondrites. Like, noticeably closer.

“The composition of the Martian interior for krypton is almost purely chondritic, but the atmosphere is solar,” Péron said. “It’s very distinctive.”

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

The order of events, therefore, would be that Mars acquired an atmosphere from the solar nebula after its global magma ocean cooled; otherwise, chondritic gases and nebular gases would be much more mixed together than the team observed.

However, this presents another mystery. When solar radiation ended up burning the nebula’s remnants, it should have also burned up the nebular atmosphere of Mars. This means that the atmospheric krypton present later must have been preserved somewhere; perhaps, the team suggested, in the polar ice caps.

“However, this would require Mars to be cold shortly after its accretion,” Mukhopadhyay said.

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

The team’s research was published in Science.

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