The atmosphere on Mars may have formed in a way that contradicts current theories, say researchers at the University of California, Davis, US. The team formed this conclusion thanks to a new analysis of the Chassigny meteorite, which fell to Earth in north-eastern France in 1815 and is believed to represent the Martian interior.
Current theories of planet formation suggest that rocky planets like the Earth and Mars acquired volatile chemical elements such as hydrogen, carbon, oxygen, nitrogen and noble gases such as krypton from the nebula surrounding their parent star during the early stages of their formation.
Initially, these elements dissolved (technically, they “ingassed”) in the planets’ mantle, which at that point existed as an ocean of molten rock, or magma, on the surface. Later, when the magma ocean crystallized, the ocean “degassed” these solar nebula-derived volatiles back into the atmosphere, where they gradually dissipated into space. Finally, at an even later stage, meteorites called chondrites delivered additional volatile materials by crashing into the young planets.
“It is therefore expected that the interior of the planets would mainly be composed of solar volatiles, or a mixture of solar and chondrite volatiles. The volatiles in the atmosphere, on the other hand, would come mainly from meteorites,” explains study team leader Sandrine Péron.
Martian interior contains chondrite krypton
That prediction, however, is not consistent with the team’s findings, which are based on measurements of krypton isotopes in samples of the Chassigny meteorite. Because the ratio of krypton isotopes in solar nebula-origin krypton and chondrite-origin krypton are different, analysing the isotope ratios allowed researchers to determine how Chassigny – and, by extension, the interior of Mars – got its krypton.
“Our study shows that the Martian interior contains chondrite krypton, which contrasts with the [solar-krypton-like] atmospheric composition,” Péron tells Physics World. “The current scenario therefore does not hold anymore.”
Precise measurements of isotopes
Before they could perform their measurements, the researchers first had to eliminate a third source of krypton. Chassigny spent 11 million years travelling through space before it fell to Earth – quite long, Péron says. During this time, it was exposed to cosmic radiation, which can generate krypton and other noble gases from other elements via spallation reactions.
To remove this so-called “cosmogenic” krypton from their sample, the researchers heated the meteorite in stages from around 200 to 1500 °C. This step-heating technique works because cosmogenic and Martian krypton are released at different temperatures.
Another important part of the analytical procedure was to separate krypton from the other noble gases present in the meteorite. The researchers did this by analysing the noble gases one after the other using mass spectrometry. “As we want to avoid interference issues, we need a nearly pure krypton phase (without argon and xenon) in the mass spectrometer,” Péron explains. “To achieve a clean separation of krypton from argon and xenon, we developed a new separation protocol at UC Davis involving a new cryogenic trap.”
This protocol, combined with the step-heating, enabled the team to obtain precise krypton isotopic measurements of the Chassigny meteorite, Péron says.
Meteorites delivered volatile elements much earlier
The fact that the krypton isotopes in Chassigny correspond to those found in chondrite meteorites, rather than in the solar nebula, implies that chondrites were delivering volatile elements to the infant Mars much earlier than previously thought, while the solar nebula was still present. “Solar volatiles in the atmosphere cannot originate from mantle degassing as previously assumed, but were likely captured from the solar nebula before the nebula dissipated (in around 10 Myr after the solar system was born), and after most of Mars had accreted,” Péron says. “This overturns current thinking.
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“A challenging aspect is how to retain these solar volatiles in the atmosphere, since they should have been lost due to radiation emanating from the early Sun,” she continues. “A possible scenario is that Mars was cold after accretion and part of the solar gases got trapped underground or in the polar ice caps.”
The researchers hope their work will motivate further studies on how planetary atmospheres, and in particular the Martian atmosphere, form. For their part, they plan to better characterize the composition of the Martian mantle to determine whether it is heterogenous. “Another aspect is to better understand where the Martian atmosphere originated and how it evolved, taking into account the constraints from our study,” Péron says. “This will involve determining the conditions that allow solar krypton and xenon to be retained at the surface of the planet.”
The research is detailed in Science.
