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Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments


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, which are 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 1,200 °C. We find that outgassing from three carbonaceous chondrite samples consistently produce H2O-rich (average of ~66%) atmospheres but with substantial 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, and supply abundances for principal gas species as a function of temperature.

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Fig. 1: Mole fractions of the measured species outgassed as a function of temperature for each chondrite sample.
Fig. 2: Ratios of mole fractions of outgassed bulk elements hydrogen, carbon, oxygen and sulfur as a function of temperature for the three chondrite samples.
Fig. 3: Comparison between equilibrium calculations and experimental results under the same pressure and temperature conditions.
Fig. 4: Oxygen fugacities relative to the quartz–fayalite–magnetite (QFM) buffer from theory and experiments.

Data availability

Source data are provided with this paper. The data that support the findings of this study and corresponding plots in the paper are available from or from the corresponding author upon request. Figures 14, Extended Data Figs. 15 and Supplementary Figs. 24 have associated raw data that are available from or from the corresponding author. The thermochemical equilibrium models used in Figs. 3 and 4 and Extended Data Fig. 5 are available from L.S. ( upon request.

Code availability

The code used to calibrate and analyse the data used in this study is also available from


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We thank A. K. Skemer (UCSC) for his helpful insights, K. Kim (UCSC) for performing the preliminary XRD experiments, and L. Nittler (Carnegie Institution for Science) for helpful discussions about bulk chondrite compositions and their uncertainties. M.A.T. acknowledges support from a Achievement Rewards for College Scientists Foundation scholarship. M.T. is supported by NASA Emerging Words grant no. 80NSSC18K0498 and NASA Planetary Science Early Career Award grant no. 80NSSC20K1078. T.J. was supported by University of California Santa Cruz start-up funds.

Author information




M.A.T. performed the outgassing experiments and data analysis and wrote the manuscript. M.A.T. and M.T. conceived the research. D.L., T.J., M.T. and M.A.T. collaborated to configure the experimental set-up. M.T. provided the meteorite samples used in the experiments and imparted essential guidance on the data analysis and interpretation of the results. D.L. provided the laboratory equipment for the experiments and helpful suggestions for the data analysis. T.J. helped prepare the experiments and maintain the instruments. L.S. provided the chemical equilibrium models and greatly contributed to interpreting the results and their implications. J.J.F. gave important insight into the scope of this work and its implications for exoplanet atmospheres. All authors contributed to the editing of the manuscript.

Corresponding author

Correspondence to Maggie A. Thompson.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Paolo Sossi, Aaron Wolf and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Data calibration steps.

Each figure illustrates the partial pressures (bars) for the molecular species measured from 200C to 1200C. Each sample’s data is calibrated by first correcting for ion fragments and atmospheric adsorption and then background subtracting. (a) is the raw mass specrometry data for the Murchison sample, (b) is the raw mass spectrometry data for the background measurement, (c) is the Murchison measurement corrected for ion fragments and atmospheric adsorption, (d) is the background measurement corrected for ion fragments and atmospheric adsorption, (e) is the Murchison measurement corrected for ion fragments and atmospheric adsorption and background-subtracted (i.e., data from (d) subtracted from data from (c)).

Source data

Extended Data Fig. 2 Results of analyzing ion fragments using a non-linear least squares regression.

The outgassing abundances in (a) are for the Murchison sample with the panel on the right side showing the average standard deviation determined from the Monte Carlo simulation for each of the species measured. The abundances in (b) are the average of the three CM chondrites.

Source data

Extended Data Fig. 3 Comparison between the yields of major volatiles released from Jbilet Winselwan samples during two identical experiments.

The partial pressure summed over temperature for each volatile species is normalized to the total pressure of released gases summed over temperature and expressed as a percentage. The uncertainty on the mean relative abundance for each volatile species is the 95 % confidence interval of the mean. The volatile yields are fairly reproducible between the two experiments, especially for the most dominant outgassed species (H2O, CO, CO2).

Source data

Extended Data Fig. 4 Comparison between the yields of major volatiles released from the samples.

The partial pressure summed over temperature for each volatile species is normalized to the total pressure of released gases summed over temperature and expressed as a percentage. The data for Winselwan is the mean of the two individual experiments conducted with the uncertainty reported as the 95 % confidence interval of the mean (see Methods and Extended Data Fig. 3). The mean relative abundance of all three samples for each volatile species is also shown with the uncertainty reported as the 95 % confidence interval of the mean. All three samples have similar outgassing abundances for the most dominant outgassing species (H2O, CO, and CO2) While H2 and H2S have larger variations up to an order of magnitude, the relative abundances for each species across the three samples are within 3σ of each other.

Source data

Extended Data Fig. 5 Additional Outgassing Species from Chemical Equilibrium Calculations.

Outgassing abundances for additional species not measured in the experiments calculated assuming chemical equilibrium for Murchison (a) and an average CM chondrite bulk composition (b) at 1E-3 Pa. The outgassing of H2O is also shown as a reference.

Source data

Supplementary information

Supplementary Information

Supplementary Figs. 1–4, Tables 1–3 and references.

Source data

Source Data Fig. 1

Excel workbook with the datasets used to generate all of the panels in Fig. 1.

Source Data Fig. 2

Excel workbook containing the data required to generate Fig. 2.

Source Data Fig. 3

Excel workbook containing the data to generate all of the panels in Fig. 3.

Source Data Fig. 4

Excel workbook containing the data to generate Fig. 4.

Source Data Extended Data Fig. 1

Excel workbook containing the data to generate all of the panels in Extended Data Fig. 1.

Source Data Extended Data Fig. 2

Excel workbook containing the data to generate Extended Data Fig. 2.

Source Data Extended Data Fig. 3

Excel workbook containing the values and uncertainties used to generate Extended Data Fig. 3.

Source Data Extended Data Fig. 4

Excel workbook containing the values and uncertainties used to generate Extended Data Fig. 4.

Source Data Extended Data Fig. 5

Excel workbook containing the data to generate Extended Data Fig. 5.

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Thompson, M.A., Telus, M., Schaefer, L. et al. Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments. Nat Astron 5, 575–585 (2021).

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