A rare Martian meteorite could rewrite our theory on how planets form

It was 8:00 AM on October 3, 1815 when a space stone was seen falling mercilessly from the sky in Chassigny, north-eastern France, accompanied by loud detonations that shook the ground. The meteorite, which originated on Mars, was named Chassigny, and turned out to be no ordinary rock.

A recent analysis of the meteorite led by Sandrine Péron, a postdoctoral scholar at ETH Zürich, Switzerland, revealed results that hint at how rocky planets like the Earth and Mars acquired volatile (life-forming) elements, including hydrogen, carbon, oxygen, nitrogen, and noble gases.

But these results contradict our fundamental understanding of how our planets form, according to a recent study published in the journal Science.

In other words, this could change much of what we know about planetary science.

Turns out, Mars formed quicker than the Earth

Mars is of special interest to those studying early planetary formation. "Earth's formation took somewhere between approximately 50 to 100 million years," Professor Sujoy Mukhopadhyay in the Department of Earth and Planetary Sciences, University of California, Davis, told IE in an interview. "Mars, on the other hand, formed quicker, in a few million years. Mars can therefore provide us with a window on volatile delivery and accretion in the inner Solar System during the earliest stages of planet formation."

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"We can reconstruct the history of volatile delivery in the first few million years of the Solar System," said Sandrine Péron, working with Mukhopadhyay, in a statement.

Older models and their observations about planet formation

According to current models, planets are born from the debris of a star. The dust that collects contains carbon and iron, which is imperative to the formation of planetary systems. Around a new star, clumps of material crash and collapse into each other in the swirling disk of gas and dust called a solar nebula.

Within the disk, dust and gas clump together in a process that develops into a proto-planet. However, not all of these objects go on to become planets — some clumps remain small and inactive as asteroids and comets.

Models indicated that "as a planet grows and reaches the size of Mars, or somewhat larger, the growing planet can capture nebular gases from the swirling cloud of gas in which the planets are growing and dissolve these gases into a magma ocean," said Mukhopadhyay.

Current hypotheses state that rocky planets contain elements with the same chemical characteristics in both the planet's interior and atmosphere. Some of the volatile elements later degass back into the atmosphere. 

When the magma ocean — which blankets the planet — cools, the "nebular signature" is imprinted into the interior of the planet. Additional volatiles are also delivered into the atmosphere when meteorites crash into the young planet.

"After the nebula dissipates, chondritic volatiles (including water, carbon, nitrogen) are delivered to the planets," said Mukhopadhyay.

These volatiles are essential — on Earth, they helped develop and support life.

Among the planets, Jupiter and Saturn are thought to have a headstart among their "peers." They formed quickly — within roughly the first few million years of the solar system's existence.

After the giant gas planets formed, there was not a lot of gas left for planets like Mercury, Venus, Earth, and Mars. Most of these then took tens of millions of years more to form. However, Mars is of special interest because it is thought to have solidified around 4 million years after the birth of the Solar System, roughly 50 to 100 million years before the formation of Earth.

New study of an old space rock

For their study, Péron and Mukhopadhyay compared two Martian sources of the noble gas krypton, as its isotopes contain information about the sources of volatile substances.

One was from Chassigny, which originated in the Martian interior. The other krypton isotopes used had been sampled from Mars' atmosphere by NASA's Curiosity Rover.

"This particular meteorite, Chassigny, is the only one from the noble gas point-of-view that can give access to the Martian interior composition," Péron told Vice. "All the other Martian meteorites we have currently in the collection are totally or highly influenced by the Martian atmospheric composition. If we want these pure interior components, it’s the only meteorite we have so far."

Now, because of its low abundance, krypton is rather tricky to measure and difficult to separate out from argon and xenon. However, Péron and Mukhopadhyay employed a new technique, which uses a cryogenic method to "cleanly" separate out the gas. "In addition, we used the latest generation of a mass spectrometer to precisely measure the isotopes of krypton," revealed Mukhopadhyay.

Much to their surprise, the krypton signatures did not match.

"Because the atmospheric krypton looks like [that also found in] the Sun, we certainly did not expect to find krypton from chondritic meteorites in the interior of Mars. It seemed a little bit backward to us to have meteoritic gases in the interior and solar (nebular) gases in the atmosphere," said Mukhopadhyay.

(A chondritic meteorite is one that was formed when dust and small grains in the early Solar System accrete and have not melted.)

The team's observations from the meteorite challenged the sequences of events for volatile delivery and accretion "by indicating that chondritic volatiles are not being added just at the end stages of planet formation," Mukhopadhyay said.

Surprising details revealed at the core of rocky planets

The results indicated that Mars' atmosphere could not have formed solely by "outgassing from the mantle, as that would have given the atmosphere a chondritic composition," explained Mukhopadhyay.

The researchers suspect that the planet must have acquired its early atmosphere from the solar nebula after the magma ocean cooled and at least partially solidified.

"We suggested that accretion of the solar gases from the nebula occurred after magma ocean solidification, to prevent substantial mixing between interior chondritic gases and the solar gases in the atmosphere, as magma ocean solidification causes substantial degassing. If Mars had to capture nebular gases to form its early atmosphere after partial solidification of the magma ocean, it indicates that Mars growth was completed before the nebula had dissipated due to irradiation from an early energetic sun," explained Mukhopadhyay.

The order of events would then be, accordingly, that Mars acquired an atmosphere from the solar nebula after its global magma ocean cooled. Else, the nebular and chondritic gasses would be more mixed than what the team discovered.

Mukhopadhyay then added: "Our observations mean that meteorites were delivering volatile elements to Mars much earlier than previously thought and in the presence of the nebula. Our observations also suggest that Mars’ formation was completed before the nebula had dissipated (The nebula dissipates due to the radiation from the early energetic Sun)."

But, this raises another mystery. 

The irradiation from the Sun should have blown off the nebular atmosphere on Mars, "requiring the atmospheric krypton to have somehow been preserved, possibly trapped underground or in polar ice caps. However, that would require Mars to have been quite cold in the immediate aftermath of its accretion," he added.

Breaking the theory on planet formation

The study stresses that there is so much more to learn about planetary formation.

"Our study does raise interesting questions about how Mars’ early atmosphere originated, what its composition was, and whether the surface environments on Mars might have been suitable for habitability early on," said Mukhopadhyay.

Finding out how volatile elements are acquired and distributed is also essential for understanding a planet’s chemical make-up, Chris Ballentine at the University of Oxford told New Scientist. "The timing and source of the volatiles control the oxidation state, which, in turn, controls the structure and distribution of elements in the planet, which for our own Earth is why we can live on it."

The scientists hope to make further observations from other Martian meteorites to get a detailed picture of their interior composition.