A new study of an old 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 Sciences.
A basic assumption about planet formation is that planets collect these volatiles from the nebula around a young star first, said Sandrine Péron, a postdoctoral fellow working with Professor Sujoy Mukhopadhyay in the Department of Earth and Planetary Sciences at the University of California, Davis.
Because the planet is a ball of molten rock at this point, these elements initially dissolve in the magma ocean and then degas again in the magma ocean. atmosphere. Later, chondritic meteorites colliding with the young planet they deliver more volatile materials.
So scientists hope that the volatile elements inside the planet should reflect the composition of the solar nebula, or a mixture of solar and meteorite volatiles, while the volatiles in the atmosphere would come mostly from meteorites. These two sources, solar and chondritic, can be distinguished by the isotope ratios of Noble gasesespecially krypton.
Mars is of special interest because it formed relatively quickly: it solidified in 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 million years of the Solar System,” said Péron.
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 Martian atmosphere. The Chassigny meteorite, which fell to Earth in northeastern France in 1815, is rare and unusual because it is believed to represent the planet’s interior.
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 their low abundance, krypton isotopes are difficult to measure,” Péron said.
Surprisingly, the isotopes of krypton in the meteorite they correspond to those of chondritic meteorites, not to the solar nebula. That means the meteorites were delivering volatile elements to the forming planet much earlier than previously thought, and in the nebula’s presence, reversing conventional thinking.
“The interior Martian composition of krypton is almost purely chondritic, but the atmosphere is solar,” Péron said. “It is very different”.
The results show that the atmosphere of Mars cannot have been formed simply by outgassing from the mantle, as that would have given it a chondritic composition. The planet must have acquired atmosphere from the solar nebula, after the magma ocean cooled, to prevent substantial mixing between interior chondritic gases and atmospheric solar gases.
The new results suggest that the growth of Mars was complete before solar energy nebula was dissipated by the sun’s radiation. But the irradiation should also have blown away the nebula’s atmosphere on Mars, suggesting that the atmosphere krypton it must have been preserved in some way, possibly trapped underground or in the polar ice caps.
“However, that would require Mars to have been cold immediately after its accumulation,” 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 the early Martian atmosphere.”
Péron and Mukhopadhyay hope their study will stimulate more work on the topic.
Sandrine Péron, Krypton in the Chassigny meteorite shows chondritic volatiles accumulating on Mars before nebular gases, Sciences (2022). DOI: 10.1126/science.abk1175. www.science.org/doi/10.1126/science.abk1175
Citation: Martian Meteorite Disrupts Planet Formation Theory (June 16, 2022) retrieved June 17, 2022 from https://phys.org/news/2022-06-martian-meteorite-planet-formation-theory .html
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