Martian meteorite holds clues to how planets form atmospheres

Analysis of krypton isotopes trapped in a Martian meteorite could jumble astronomers’ understanding of how the planet’s atmosphere formed.

By piecing together how Mars formed the atmosphere it has today, scientists could glean insights into the early chemistry of our solar system and how Earth obtained an atmosphere favorable to life, says geochemist Sandrine Péron.

A prevailing hypothesis among astronomers is that Mars gained much of its atmosphere from the gases released by the magma oceans that once covered the young planet. Lighter elements and isotopes in this off-gassed atmosphere were depleted over time due to other physical processes to produce the Martian atmosphere we observe today. Scientists have collected meteorites from Mars made of the same minerals as the planet’s mantle to try to understand the composition of those early magma oceans.

Krypton remains inert over millennia so that the ratio of Kr isotopes trapped within these meteorites reflects the chemical composition of the volatile compounds that joined the Martian atmosphere during this molten period. So Péron and her colleagues at the University of California, Davis developed a system to precisely measure the isotopic composition of krypton from within a meteorite called Chassigny.

The team heated a sample of the meteorite to isolate these ancient isotopes and then measured the ratio of ⁸⁶Kr to ⁸⁴Kr. They compared that ratio to data collected from the planet’s atmosphere by the Curiosity rover.

The researchers found that the ⁸⁶Kr-to-⁸⁴Kr ratio was larger in Chassigny than in the current atmosphere (Science 2022, DOI: 10.1126/science.abk1175). If most of the Martian atmosphere derived from its magma oceans, scientists expected the current atmosphere to have a bigger ⁸⁶Kr-to-⁸⁴Kr ratio than the meteorite because of the depletion of lighter isotopes over time. The fact that they found the opposite suggests that Mars continued to obtain Kr and other volatile compounds from the gaseous disk around our young Sun. “We show that the Martian atmosphere can’t be explained simply by outgassing of the mantle as was commonly used in models of Martian atmosphere formation,” Péron says.