A reanalysis of an ancient meteorite contradicts the idea that rocky planets acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases during their formation.
A basic hypothesis about planet formation is that planets first collect these volatiles from the nebula around a young star, said Sandrine Péron, postdoctoral fellow in the Department of Earth and Planetary Sciences at the University. of California, Davis, and author of the study.
Because the planet is a ball of molten rock at this point, these elements initially dissolve in the ocean of magma and then outgas into the atmosphere. Later, chondritic meteorites hitting the young planet deliver more volatile material.
Solar vs. chondritic
Scientists therefore expect the volatiles 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 would come mainly from meteorites. These two sources, solar vs. chondritic: distinguished by the ratios of isotopes of noble gases, in particular 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 volatile delivery over the first million years of the solar system,” Péron said. it is a statement.
The results show that Mars’ atmosphere cannot have been formed solely by outgassing from the mantle.
The Chassigny meteorite
Some meteorites that fall to Earth come from Mars. Most come from surface rocks that have been exposed to the Martian atmosphere. 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 careful measurements of minute amounts of krypton (or 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.
“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 those of the solar nebula. This means that the meteorites delivered volatile elements to the forming planet much earlier than previously thought, and in the presence of the nebula, reversing conventional thinking.
“Krypton’s inner Martian composition is almost purely chondritic, but the atmosphere is solar,” Péron said. “It’s very different.”
Formation of the Mars atmosphere
The results show that Mars’ atmosphere cannot have formed simply by outgassing from the mantle, as that would have given it a chondritic composition. The planet must have acquired the atmosphere of the solar nebula, after the magmatic ocean had cooled, to prevent substantial mixing between interior chondrite gases and atmospheric solar gases.
The new results suggest that Mars’ growth was complete before radiation from the Sun dissipated the solar nebula. But the irradiation should also have blown up Mars’ nebulous atmosphere, suggesting that atmospheric krypton must have somehow been preserved, possibly trapped underground or in the polar ice caps.
“However, this would require Mars to have been cold immediately after it accumulated,” said Professor Sujoy Mukhopadhyay, co-author of the paper, which is published in Science. “While our study clearly points to chondritic gases inside Mars, it also raises interesting questions about the origin and composition of the early Martian atmosphere.”
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