Martian meteorite upsets theory of planet formation

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A new study of an ancient meteorite contradicts current thinking about how rocky planets like Earth and Mars acquire volatile elements like hydrogen, carbon, oxygen, nitrogen and noble gases as they form. The work is published on June 16 in Science.

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

As the planet is a ball of molten rock at this point, these elements initially dissolve in the magma ocean and then degas back into the atmosphere. Later, chondritic meteorites colliding with the young planet release more volatile materials.

So scientists expect the volatile elements inside the planet to reflect the composition of the solar nebula, or a mixture of solar and meteoritic volatiles, while the volatiles in the atmosphere will come mostly from meteorites. These two sources – solar vs. chondritic – can be distinguished by the isotope ratios of noble gases, in particular krypton.

Mars is of special interest because it formed relatively quickly – solidifying about 4 million years after the birth of the Solar System, while Earth took 50 to 100 million years to form.

“We can reconstruct the history of volatile delivery in the first few million years of the Solar System,” Péron said.

Meteorite from the interior of 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 believed to represent the interior of the planet.

By making extremely careful measurements of minute amounts of krypton isotopes in samples of the meteorite using a new method established at the UC Davis Noble Gas Laboratory, the researchers were able to deduce the origin of the elements in the rock.

“Because of its low abundance, isotopes of krypton are difficult to measure,” Péron said.

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

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

The results show that the atmosphere of Mars cannot have formed purely by outgassing the mantle, as that would give it a chondritic composition. The planet must have acquired an atmosphere from the solar nebula, after the magma ocean had cooled, to avoid substantial mixing between inner chondritic gases and atmospheric solar gases.

The new results suggest that the growth of Mars was completed before the solar nebula was dissipated by radiation from the Sun. But the radiation must also have blown up the nebular atmosphere on Mars, suggesting that atmospheric krypton must have been preserved somehow, possibly trapped underground or in the polar ice caps.

“However, this would require Mars to be cold shortly after its accretion,” Mukhopadhyay said. “While our study clearly points to chondritic gases in the interior of Mars, it also raises some interesting questions about the origin and composition of Mars’ early atmosphere.”

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

Deep mantle krypton reveals the ancestry of Earth’s outer solar system

More information:
Sandrine Péron, Krypton in the Chassigny meteorite shows chondritic volatiles accumulating on Mars before nebular gases, Science (2022). DOI: 10.1126/science.abk1175.

Quote: Martian meteorite disturbs planet formation theory (2022, June 16) retrieved June 17, 2022 from

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