Leveraging Meteorite Outgassing Experiments to Constrain the Initial Atmospheres of Terrestrial Exoplanets

At present, there is no first-principles understanding of how to connect a planet’s bulk composition to its atmospheric properties. Since terrestrial exoplanets likely form their atmospheres through outgassing, a novel step towards building such a theory is to assay meteorites, the left-over building blocks of planets, by heating them to measure their outgassed volatiles. Our Solar System presents a wide variety of meteorite types, including carbonaceous chondrites which are believed to be representative of the bulk material in the solar nebula during planet formation. In addition, carbonaceous chondrites contain the highest proportions of volatiles relative to other remnant materials from terrestrial planet formation that can be directly studied in the laboratory. To inform the initial chemical composition of terrestrial planet atmospheres, we present the results of our outgassing experiments in which we heated carbonaceous (CM) chondrite samples to 1200 ℃ and measured the abundances of released volatiles (e.g., H₂O, CO, CO₂, H₂, H₂S, CH₄) as a function of temperature and time. Our experimental set-up consists of a residual gas analyzer, a type of mass spectrometer particularly sensitive to trace amounts of gas, connected to a furnace to heat samples at specified rates. We also perform complementary bulk element analysis on the samples before and after the heating experiments using inductively coupled plasma mass spectrometry to monitor outgassing of heavier elements (e.g., Na, Mg, S, Fe). We compare these experimental results to thermochemical equilibrium models of outgassing from the same types of chondrites and determine how these experiments will improve the models. Common assumptions currently made in terrestrial exoplanet atmosphere models include some multiple of the solar abundances, the current atmospheric compositions of Solar System rocky planets, and ad-hoc abundances (e.g., H₂O-only or CO₂-only atmospheres). Our outgassing experiments suggest initial secondary atmospheres of terrestrial exoplanets may differ significantly from some of these common assumptions. This experimental work takes an important step forward in connecting terrestrial planet interiors and atmospheres and places important constraints on the chemical abundances in initial outgassed atmospheres of terrestrial exoplanets.