Correction

Abstract

Correction for “The case and context for atmospheric methane as an exoplanet biosignature,” by Maggie A. Thompson, Joshua Krissansen-Totton, Nicholas Wogan, Myriam Telus, and Jonathan J. Fortney, which published March 30, 2022; 10.1073/ pnas.2117933119 (Proc. Natl. Acad. Sci. U.S.A. 119, e2117933119). The authors note that an older version of Fig. 4 was published in error. The corrected figure and its legend appear below. The online version has been corrected. The authors note that on page 5, right column, second full paragraph, line 27, “However, the amount of abiotic methane generated in continental settings is several orders of magnitude smaller than the biogenic flux (78–82).” should instead appear as “Current observations suggest that the largest abiotic CH4 fluxes come from continental settings, and in particular, continental ophiolites (79). Although estimates for the global abiotic CH4 fluxes from such sources are challenging to determine, at present, such fluxes are much smaller than the current biogenic flux (78–83).” The new reference appears below. The online version has been corrected. The authors note that on page 6, right column, first paragraph, line 13, “Another source of uncertainty is what catalysts might be available in natural settings.” should instead appear as “Another source of uncertainty is what catalysts might be available in natural settings. For example, Etiope et al. (89) analyzed gas from various rocks composing ophiolites and determined that chromitites, rocks rich in chromium and ruthenium, were the only rocks to contain significant abiotic methane, suggesting that chromium and ruthenium may be important metal catalysts for abiotic methane production (89).” The new reference appears below. The online version has been corrected. The authors note that, in the SI Appendix, page 6, line 240, “Portella et al. 2019: Their study of serpentinization of chromitites in ophiolites found that chromitites can contain CH4 gas concentrations up to 0.31 μg/grock. Taking this CH4 concentration and multiplying it by Earth’s melt production rate (3.2 × 10 g/s) results in a global CH4 flux estimate of 2 × 10 3 Tmol/year (38).” should instead appear as “Portella et al. 2019: Their study of serpentinization of chromitites in ophiolites found that chromitites can contain CH4 gas concentrations up to 0.31 μg/grock (39). Extrapolating this concentration by Earth's global melt production rate gives a negligible CH4 flux compared to the biogenic flux. However, due to uncertainties in how these gas concentrations vary in time and between different sites and whether such processes could take place on a global scale, we do not include a global abiotic flux extrapolation for this source.” The SI Appendix has been corrected online. The authors note that, in the SI Appendix, page 6, line 245, “They also estimated that the lower oceanic crust contains a total of ∼300 Tmol of CH4 gas (39). Given that the lifetime of the oceanic crust is ∼200 Myrs, the estimated global abiotic CH4 flux due to serpentinization is 1.5 × 10 6 Tmol/year.” should instead appear as “They also estimated that the lower oceanic crust contains a total of ∼300 Tmol of CH4 gas (40). Taking this amount of methane gas in the lower oceanic crust and dividing it by the lifetime of Earth's oceanic crust results in a negligible global abiotic CH4 flux. However, due to various uncertainties in extrapolating these findings to a global scale, we do not include a global abiotic flux extrapolation for this source.” The SI Appendix has been corrected online. The authors note that Supporting Table 2 in the SI Appendix appeared incorrectly. The SI Appendix has been corrected online.