A new study of an ancient meteorite challenges current theories about how rocky planets like Earth and Mars acquire volatile elements like hydrogen, carbon, oxygen, nitrogen, and noble gases as they form. The study was published in Science on June 16th. According to Sandrine Péron, a postdoctoral scholar working with Professor Sujoy Mukhopadhyay in the Department of Earth and Planetary Sciences at the University of California, Davis, “a basic assumption about planet formation is that planets first collect this volatiles from the nebula around a young star.”
Because the planet is a molten ball at this point, these elements dissolve into the magma ocean before degassing back into the atmosphere. Later, chondritic meteorites that collide with the young planet deliver more volatile materials.
As a result, scientists anticipate that the volatile elements in the planet’s interior will reflect the composition of the solar nebula, or a mixture of solar and meteoritic volatiles, while the volatiles in the atmosphere will come primarily from meteorites. The ratios of isotopes of noble gases, particularly krypton, distinguish these two sources—solar vs. chondritic.
Mars is of particular interest because it formed relatively quickly, solidifying around 4 million years after the birth of the Solar System, whereas Earth took 50 to 100 million years. “We can reconstruct the history of volatile delivery in the Solar System’s first few million years,” Péron said.
Mars’ interior meteorite
Some meteorites that fall to Earth originate on Mars. The majority come from surface rocks exposed to Mars’ atmosphere. The Chassigny meteorite, which fell to Earth in north-eastern France in 1815, is unique in that it is thought to represent the planet’s interior.
The researchers were able to deduce the origin of elements in the rock by taking extremely precise measurements of minute amounts of krypton isotopes in meteorite samples using a new method developed at the UC Davis Noble Gas Laboratory. “Krypton isotopes are difficult to measure due to their low abundance,” Péron explained.
Surprisingly, the krypton isotopes in the meteorite match those found in chondritic meteorites rather than the solar nebula. That means that, in the presence of the nebula, meteorites were delivering volatile elements to the forming planet much earlier than previously thought, reversing conventional thinking. “The interior composition of krypton on Mars is nearly entirely chondritic, but the atmosphere is solar,” Péron explained. “It’s quite distinct.”
The findings indicate that Mars’ atmosphere could not have formed solely through mantle outgassing, as this would have given it a chondritic composition. To prevent significant mixing between interior chondritic gases and atmospheric solar gases, the planet must have acquired atmosphere from the solar nebula after the magma ocean cooled.
According to the new findings, Mars’ growth was completed before the solar nebula was dissipated by radiation from the Sun. However, the irradiation should have also blown off Mars’ nebular atmosphere, implying that atmospheric krypton must have been preserved in some way, possibly trapped underground or in polar ice caps.
“However, that would imply that Mars was cold in the immediate aftermath of its accretion,” Mukhopadhyay explained. “While our research clearly demonstrates the presence of chondritic gases in the Martian interior, it also raises some intriguing questions about the origin and composition of Mars’ early atmosphere.”
Péron and Mukhopadhyay hope that their research will spark additional research on the subject.
