Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments

Abstract

Terrestrial exoplanets likely form initial atmospheres through outgassing during and after accretion, although there is currently no first-principles understanding of how to connect a planet’s bulk composition to its early atmospheric properties. Important insights into this connection can be gained by assaying meteorites, representative samples of planetary building blocks. We perform laboratory outgassing experiments that use a mass spectrometer to measure the abundances of volatiles released when meteorite samples are heated to 1200 ◦C. We find that outgassing from three carbonaceous chondrite samples consistently produce H2O-rich (averaged ∼66 %) atmospheres but with significant amounts of CO (∼18 %) and CO2 (∼15 %) as well as smaller quantities of H2 and H2S (up to 1%). These results provide experimental constraints on the initial chemical composition in theoretical models of terrestrial planet atmospheres, supplying abundances for principal gas species as a function of temperature. We are at the dawn of an exciting technological era in astronomy with new large-aperture telescopes and advanced instrumentation, both in space and on the ground, leading to major advances in exoplanet characterization. To optimize the use of these new facilities, we need suitable theoretical models to obtain a better understanding of the diversity of exoplanet atmospheres. Statistical studies using NASA’s Kepler mission data suggest that terrestrial and other low-mass planets are common around G, K and M stars [1, 2, 3]. Given the large number of current and anticipated low-mass exoplanet discoveries, the next phase in exoplanet science is to characterize the physics and chemistry of their atmospheres. For the foreseeable future, Solar System meteorites provide the only direct samples that can be rigorously studied in the laboratory to gain insight into the initial atmospheric compositions of these planets. Although gas giant planets, like Jupiter and Saturn, form primary atmospheres by capturing gases from the stellar nebula, atmosphere formation for low-mass planets is more complicated. While nebular ingassing can contribute to early atmosphere formation if a protoplanet accretes enough mass before the gas disk dissipates, several factors can result in loss of nebular volatiles early in the planet’s history [4, 5, 6]. For instance, terrestrial planets’ inability to retain significant primary atmospheres can be due to low planetary mass, large impactors and high EUV and X-ray flux from young host stars [7]. Instead, low-mass planets likely have secondary atmospheres which form via outgassing of volatiles during and after planetary accretion [8]. The Solar System’s terrestrial planets are believed to have formed by accretion of planetesimals that have compositions similar to chondritic meteorites, which are a likely source of atmospheric volatiles for such planets [9, 7]. As a result, an important step towards establishing the connection between terrestrial planets’ bulk compositions and their atmospheres is to directly measure the outgassed volatiles from meteorites. While meteorites come in a wide variety with a range of volatile contents, they can be classified into three main types: chondrites, achondrites and irons. Chondrites are stony meteorites that come from undifferentiated planetesimals composed of aggregate material from the protoplanetary disk, while achondrites and iron meteorites have melted and derive from partially or fully differentiated planetesimals. Both chondrites and achondrites likely contributed to forming the Sun’s terrestrial planets [7]. Our study focuses on CM-type