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A Martian meteorite upsets the theory of planet formation

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A new study on an old meteorite contradicts current thinking about how rocky planets like Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases during their training. The book comes out on June 16 a Science.

A basic hypothesis about planet formation is that planets first collect these volatile elements from the nebula around a young star, said Sandrine Péron, a postdoctoral researcher working with Professor Sujoy Mukhopadhyay in the Department of Earth and Planet Sciences. of the University of California at Davis.

Since the planet is now a ball of molten rock, these elements initially dissolve in the magma ocean and then pour into the atmosphere. Subsequently, the chondritic meteorites that crash into the young planet provide more volatile material.

Scientists therefore expect the volatiles within the planet to reflect the composition of the solar nebula, or a mixture of solar and meteorite volatiles, while the volatiles in the atmosphere would come mainly from meteorites. These two sources – solar vs. chondritic – can be distinguished from the isotope ratios of noble gases, especially krypton.

Mars is of particular interest because it formed relatively quickly, solidifying about 4 million years after the birth of the solar system, while the Earth took 50 to 100 million years to form.

“We can reconstruct the history of the volatile delivery in the first million years of the solar system,” said Péron.

Meteorite from inside Mars

Some meteorites that fall to Earth come from Mars. Most come from surface rocks that have been exposed to the atmosphere of Mars. The Chassigny meteorite, which fell to Earth in northeastern France in 1815, is rare and unusual because it is thought to represent the interior of the planet.

By making extremely accurate measurements of tiny amounts of krypton isotopes in samples of the meteorite using a new method first pioneered at UC’s Davis Noble Gas Laboratory, the researchers were able to infer the origin of the elements in the rock.

“Due to their low abundance, krypton isotopes are difficult to measure,” Péron said.

Surprisingly, the krypton isotopes in the meteorite match those of the chondritic meteorites, not the solar nebula. This means that meteorites delivered volatile elements to the forming planet much earlier than previously thought, and in the presence of the nebula, reversing conventional thinking.

“The internal Martian composition of krypton is almost purely chondritic, but the atmosphere is solar,” Péron said. “It is very distinct. “

The results show that the atmosphere of Mars could not have been formed solely by degassing from the mantle, as this would have given it a chondritic composition. The planet must have acquired the atmosphere of the solar nebula, after the magma ocean has cooled, to avoid substantial mixing between the internal chondrite gases and atmospheric solar gases.

The new findings suggest that the growth of Mars was completed before the solar nebula was dissipated by solar radiation. But the irradiation was also supposed to blow through Mars’ nebular atmosphere, suggesting that the atmospheric krypton must have been preserved in some way, possibly trapped underground or in the polar ice caps.

“However, this would require Mars to be cold right after accretion,” Mukhopadhyay said. “Although our study clearly indicates the chondrite gases within Mars, it also raises interesting questions about the origin and composition of Mars’ primordial atmosphere. “

Péron and Mukhopadhyay hope their study will spur further work on the subject.

Péron is now a postdoctoral fellow at ETH Zurich, Switzerland.

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