A Martian meteorite upsets the theory of planet formation

A new study of an ancient meteorite contradicts current thinking about how rocky planets like Earth and Mars absorb volatile elements like hydrogen, carbon, oxygen, nitrogen and noble gases during their formation. The book will be published on June 16 in Science.

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

Because the planet is a ball of molten rock at this point, these elements first dissolve in the ocean of magma and then spill out into the atmosphere. Later, chondritic meteorites impacting the young planet yield more volatile material.

Scientists therefore expect that the volatiles in the planet’s interior would reflect the composition of the solar nebula or a mixture of solar and meteoric volatiles, while the volatiles in the atmosphere would come primarily from meteorites. These two sources – solar vs. chondritic – can be distinguished by the ratios of noble gas isotopes, especially krypton.

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

“We can piece together the history of ephemeral delivery over the first million years of the solar system,” Péron said.

Meteorite from inside Mars

Some meteorites that fall to earth come from Mars. Most come from surface rock that has been exposed to the Martian atmosphere. The Chassigny meteorite, which fell on Earth in north-eastern France in 1815, is rare and unusual as it is believed to represent the interior of the planet.

By meticulously measuring tiny amounts of krypton isotopes in samples of the meteorite using a new method developed at UC Davis Noble Gas Laboratory, the researchers were able to deduce 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 chondritic meteorites, not those of the solar nebula. This means that the meteorites introduced volatile elements to the forming planet much earlier than previously thought, and in the presence of the nebula, reversing conventional thinking.

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

The results show that the Martian atmosphere could not have formed from mantle outgassing alone, as this would have given it a chondritic composition. The planet must have assumed the atmosphere of the solar nebula after the magmatic ocean cooled to prevent significant mixing between inner chondrite gases and atmospheric solar gases.

The new results suggest that Mars’ growth was complete before the solar nebula was dissipated by solar radiation. But the radiation should also have been blowing through Mars’ nebula atmosphere, suggesting that atmospheric krypton must have been conserved somehow, possibly trapped underground or in polar ice caps.

“However, this would assume that Mars would have been cold immediately after accretion,” Mukhopadhyay said. “While our study clearly points to chondrite gases in the interior of Mars, it also raises interesting questions about the origin and composition of the early Martian atmosphere. »

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

Péron is now a postdoc at ETH Zurich, Switzerland.

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