Meteorite Discovery Challenges Our Understanding of How Mars Formed

A little bit of rock that broke away from Mars and landed on Earth may provide clues about the red planet's genesis.

According to a recent investigation of the Chassigny meteorite, which came to Earth in 1815, the way Mars received its volatile gasses — such as carbon, oxygen, hydrogen, nitrogen, and noble gasses – defies existing ideas of planet formation.

Planets are formed from leftover star matter, according to current theories. When a dense clump of material falls under gravity, a nebular cloud of dust and gas forms. As it spins, additional material from the cloud surrounding it spools in, allowing it to expand.

This material whirls around the nascent star, forming a disk. Dust and gas begin to cluster together within that disk, forming a newborn planet in the process. Other newborn planetary systems have evolved in this manner, and evidence shows that our Solar System did as well, about 4.6 billion years ago.

However, piecing together how and when specific elements were introduced into the planets has been difficult.

Volatile gasses are picked up by a molten, developing planet from the solar nebula, according to current theories. Because the planet is so hot and mushy at this point, the volatiles are sucked into the developing planet's global magma ocean, where they are partially outgassed into the atmosphere when the mantle cools.

More volatiles are given later by meteorite bombardment - volatiles locked up in carbonaceous meteorites (called chondrites) are released when these meteorites break apart on impact.

As a result, a planet's interior should resemble the solar nebula's composition, while its atmosphere should mostly represent the volatile input of meteorites.

The ratios of noble gas isotopes, notably krypton, can be used to distinguish between these two sources.

Mars has an excellent record for the very early stages of the planetary formation process since it developed and consolidated very swiftly in around 4 million years, compared to up to 100 million years for Earth.

"We can reconstruct the history of volatile delivery in the first few million years of the Solar System," said Sandrine Péron, a geochemist at ETH Zurich who formerly worked at the University of California Davis.

That is, of course, if we have access to the data we want — and here is where the Chassigny meteorite comes in handy.

Its noble gas composition varies from that of the Martian atmosphere, implying that the piece of rock broke free from the mantle (and was launched into space, triggering its arrival on Earth) and represents the planetary interior and hence the solar nebula.

Because krypton is difficult to quantify, accurate isotope ratios have escaped researchers. However, Péron and her colleague, UC Davis geochemist Sujoy Mukhopadhyay, used a novel approach to produce a fresh, exact measurement of krypton in the Chassigny meteorite, utilizing the UC Davis Noble Gas Laboratory.

And this is when things started to get strange. The meteorite's krypton isotope ratios are closer to those seen in chondrites. Almost unbelievably near.

"The Martian interior composition for krypton is nearly purely chondritic, but the atmosphere is solar," Péron added. "It's very distinct." 

his indicates that meteorites delivered volatiles to Mars far sooner than previously assumed, before the solar nebula was destroyed by solar radiation.

As a result, once the global magma ocean cooled, Mars received an atmosphere from the solar nebula; otherwise, the chondritic and nebular gasses would be considerably more intermingled than what the scientists saw.

This, however, raises a new set of questions. When the remnants of the nebula were finally burned away by solar radiation, the nebular atmosphere of Mars should have been burned away as well. This suggests that any subsequent atmospheric krypton must have been retained elsewhere, probably in polar ice caps, as the scientists hypothesized.

"However, that would require Mars to have been cold in the immediate aftermath of its accretion," Mukhopadhyay explained.

"While our study clearly points to the chondritic gasses in the Martian interior, it also raises some interesting questions about the origin and composition of Mars' early atmosphere."