(ORDO NEWS) — A new study of an ancient meteorite contradicts current ideas about how the rocky planets Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases as they form.
The basic assumption about planet formation is that they first pick up these volatiles from the nebula around a young star, says Sandrine Peron, a research fellow working with Professor Sujoy Mukhopadhyay in the UC Davis Department of Earth and Planetary Sciences.
Since at this point the planet is a ball of molten rock, these elements are first dissolved in the magma ocean and then degassed back into the atmosphere. Later, chondric meteorites crashing into the young planet bring more volatile materials.
Therefore, scientists expect that the volatiles in the interior of the planet should reflect the composition of the solar nebula or a mixture of solar and meteoric volatiles, while volatiles in the atmosphere will come mainly from meteorites.
These two sources – solar and chondria – can be distinguished by the ratio of isotopes of noble gases, in particular krypton.
The Chassigny meteorite that hit Earth in northeastern France in 1815 is rare and unusual because it is thought to represent the interior of Mars.
By carefully measuring minute amounts of krypton isotopes in meteorite samples using a new method, the researchers were able to determine the origin of the elements in the rock.
Surprisingly, the krypton isotopes in the meteorite correspond to isotopes from chondrite meteorites, not from the solar nebula.
This means that meteorites delivered volatile elements to the forming planet much earlier than previously thought, and in the presence of a nebula, which refutes conventional wisdom.
The results show that the atmosphere of Mars could not have been formed solely by outgassing from the mantle, since in this case it would have a chondrite composition.
The planet must have received an atmosphere from the solar nebula after the cooling of the magma ocean in order to prevent significant mixing of internal chondrite gases with atmospheric solar gases.
The new results suggest that the growth of Mars was completed before the solar nebula was scattered by radiation from the Sun.
But the irradiation must also have blown away the atmosphere of the nebula on Mars, suggesting that atmospheric krypton must have been preserved somehow, perhaps trapped underground or in the polar ice caps. However, for this, Mars had to become cold immediately after accretion.
While the study clearly points to the presence of chondrite gases in the Martian interior, it also raises some interesting questions about the origin and composition of Mars’ early atmosphere.
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