Building a Framework for Interpreting Terrestrial Exoplanet Atmospheres Through Meteorite Outgassing Experiments

Planetesimals are the building blocks of terrestrial planets and their early atmospheres. Meteorites provide a link for understanding how the bulk compositions of planetesimals influence the compositions of exoplanet atmospheres. Here, we discuss our experimental approach to determining the composition of gases released as meteorites are heated under various conditions. This study has implications for the volatile inventory of planetary building blocks and the initial composition of terrestrial planet atmospheres.

In our first study, Thompson et al. (2021) heated powdered chondrite samples up to 1200°C under vacuum and measured the abundances of released volatiles as a function of temperature using a residual gas analyzer. They determined that if a planet’s bulk composition is similar to that of CM chondrites, its initial outgassed atmosphere should be water-rich with significant amounts of CO, CO₂, H₂, and H₂S. To understand the outgassing behavior of heavier gaseous species like Na, Fe, and Ni, we analyzed the bulk composition of the material leftover (residues) from our heating experiments using wet chemistry. We ran the outgassing experiments under vacuum and under 1 atm pressure. We find that outgassing under these conditions produces significant amounts of Na and S, but Zn outgasses only under vacuum conditions. Na outgasses significantly faster than the chemical equilibrium models predict. We do not observe detectable changes in the Fe, Ni, Mn, or Co concentrations, which are predicted to outgas under these conditions. Although our experiments do not achieve equilibrium conditions, our results are generally consistent with model predictions and provide an experimentally determined boundary condition for the lower-temperature or lower-pressure paths, approximately simulating the initial heating phase during planet formation. These results provide a framework for understanding atmospheric formation and evolution, haze and cloud composition, and bulk composition of an exoplanet.

We focused these first studies on CM chondrites because they are some of the most volatile rich meteorites available and because they are similar in composition to the Sun’s photosphere. Future studies will analyze a wider range of meteorites (e.g., additional carbonaceous chondrites along with enstatite and ordinary chondrites) and we are exploring avenues to improve our experimental technique.

Reference: Thompson, M.A., et al. Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments. Nat Astron 5, 575–585 (2021). https://doi.org/10.1038/s41550-021-01338-8.