A new study of an old meteorite contradicts current thinking about how rocky planets like Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases during their training. The work is published June 16 in Science.
A basic hypothesis about planet formation is that planets first collect these volatiles from the nebula around a young star, said Sandrine Péron, a postdoctoral researcher working with Professor Sujoy Mukhopadhyay in the Department of Earth Sciences. and Planets, University of California, Davis.
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 that crash into the young planet deliver more volatile material.
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 – can be distinguished by the ratios of noble gas isotopes, in particular krypton.
Mars is of particular 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 piece together the history of volatile delivery over the first 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 Mars’ 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 isotopes in samples of the meteorite using a new method pioneered 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 chondritic meteorites, not 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.
“The Martian interior composition of krypton is almost purely chondritic, but the atmosphere is solar,” Péron said. “It’s very distinct.”
The results show that Mars’ atmosphere cannot have formed solely 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 completed before the solar nebula was dissipated by radiation from the Sun. But the irradiation should also have been blowing through Mars’ nebular atmosphere, suggesting that atmospheric krypton must have been preserved somehow, possibly trapped underground or in polar ice caps.
“However, this would require Mars to have been cold immediately after accretion,” Mukhopadhyay said. “While our study clearly points to chondritic gases inside Mars, it also raises interesting questions about the origin and composition of Mars’ early atmosphere.”
Péron and Mukhopadhyay hope their study will spur further work on the subject.
Péron is now a postdoctoral fellow at ETH Zürich, Switzerland.
Reference:
- Sandrine Peron, Sujoy Mukhopadhyay. The krypton in the Chassigny meteorite shows that Mars accreted chondritic volatiles before the nebular gases. Science, 2022; DO I: 10.1126/science.abk1175
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