A new study of an ancient meteorite contradicts current thinking about how rocky planets such as Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases when they form. The work was published on 16 June 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 fellow working with Professor Sujoy Mukhopadhyay at the Department of Earth and Planetary Sciences, University of California, Davis.
Because the planet is a sphere of molten rock at this time, these elements first dissolve in the magma ocean and then degas back to the atmosphere. Later, chondritic meteorites that crash into the young planet deliver 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 energy and meteorite volatiles, while the volatiles in the atmosphere would mostly come from meteorites. These two sources – solar energy vs. chondritic – can be distinguished by the ratio of isotopes of noble gases, especially krypton.
Mars is of special interest because it formed relatively quickly – solidified within about 4 million years after the birth of the solar system, while it took 50 to 100 million years to form the earth.
“We can reconstruct the history of volatile delivery in the first millions of years of the solar system,” Péron said.
Meteorite from Mars’ interior
Some meteorites that fall to Earth come from Mars. Most come from surface rocks that have been exposed to Mars’ atmosphere. The Chassigny meteorite, which fell to the ground 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 small amounts of krypton isotopes in samples of the meteorite using a new method set up at the UC Davis Noble Gas Laboratory, the researchers were able to deduce the origin of elements in the rock.
“Because of their low abundance, krypton isotopes are challenging to measure,” Péron said.
Surprisingly, the krypton isotopes in the meteorite correspond to those from chondritic meteorites, not the solar nebula. This means that meteorites delivered volatile elements to the formed planet much earlier than previously thought, and in the presence of the nebula, reversed conventional thinking.
“Mars’ interior composition for krypton is almost purely chondritic, but the atmosphere is solar energy,” Péron said. – It is very distinct.
The results show that Mars’ atmosphere could not have been formed solely by degassing from the mantle, as it would have given it a chondritic composition. The planet must have acquired atmosphere from the solar nebula, after the magma ocean cooled, to prevent significant mixing between internal chondritic gases and atmospheric solar gases.
The new results suggest that Mars’ growth was completed before the solar nebula was scattered by radiation from the sun. But the radiation must also have been blown by the nebulous atmosphere on Mars, which indicates that atmospheric krypton must have been preserved in some way, possibly trapped underground or in polar ice caps.
“However, it will require Mars to have been cold in the immediate aftermath of the accretion,” Mukhopadhyay said. “Although our study clearly points to the 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 earth’s outer solar system ancestors
Sandrine Péron, Krypton in the Chassigny meteorite shows Mars-accredited chondritic volatiles before nebulae, Science (2022). DOI: 10.1126 / science.abk1175. www.science.org/doi/10.1126/science.abk1175
Citation: Mars meteorite disrupts planet formation theory (2022, June 16) retrieved June 17, 2022 from https://phys.org/news/2022-06-martian-meteorite-planet-formation-theory.html
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