Meteor discovery challenges our understanding of how Mars formed

A small chunk of rock that once broke loose from Mars and made its way to Earth may contain clues revealing surprising details about the formation of the red planet.

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

Planets are born from leftover stardust, according to current models. Stars form from a nebula of dust and gas when a dense mass of material collapses under the influence of gravity. Spinning, it washes more material from the cloud around it to grow.

This material forms a disk that swirls around the new star. Within that disk, dust and gas begin to clump together in a process that makes a baby planet grow. We’ve seen other baby planetary systems form in this way, and evidence in our own solar system suggests it formed the same way, about 4.6 billion years ago.

But how and when certain elements were incorporated into the planets was difficult to put together.

According to current models, volatile gases are taken up by a molten, forming planet from the solar nebula. Because the planet is so hot and mushy at this stage, these volatiles are slurped into the global magma ocean that forms the planet, before later partially releasing into the atmosphere as the mantle cools.

Later, more volatiles are delivered via meteorite bombardment – volatiles bound in carbonaceous meteorites (called chondrites) are released when these meteorites disintegrate upon introduction to the planet.

Thus, a planet’s interior should reflect the composition of the solar nebula, while the atmosphere should largely reflect the volatile contribution of meteorites.

We can tell the difference between these two sources by looking at the ratios of isotopes of 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 planetary formation process.

“We can reconstruct the history of volatile delivery in the first few million years of the solar system,” said geochemist Sandrine Péronformerly of the University of California Davis, now at ETH Zurich.

Of course, that’s only if we have access to the information we need — and this is where the Chassigny meteorite is a gift from outer space.

Its noble gas composition differs from that of the atmosphere of Marssuggesting that the piece of rock from the mantle broke up (and was thrown into space, hastening 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 precise isotope ratios have eluded measurement. However, Péron and her colleague, fellow geochemist Sujoy Mukhopadhyay of UC Davis, used a new technique using the UC Davis Noble Gas Laboratory to make a new, accurate measurement of krypton in the Chassigny meteorite.

And this is where it got really weird. The krypton-isotope ratios in the meteorite are closer to those of chondrites. Like, remarkably closer.

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

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

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

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

“For that, though, Mars would have had to have been cold in the immediate aftermath of its accretion,” Mukhopadhyay said

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

The team’s research was published in Science

#Meteor #discovery #challenges #understanding #Mars #formed

Leave a Comment

Your email address will not be published. Required fields are marked *