Probable detection of hydrogen sulphide (H2S) in Neptune’s atmosphere

Abstract

Recent analysis of Gemini-North/NIFS H-band (1.45–1.8 µm) observations of Uranus, recorded in 2010, with recently updated line data has revealed the spectral signature of hydrogen sulphide (H2S) in Uranus’s atmosphere (Irwin et al., 2018). Here, we extend this analysis to Gemini-North/NIFS observations of Neptune recorded in 2009 and find a similar detection of H2S spectral absorption features in the 1.57–1.58 µm range, albeit slightly less evident, and retrieve a mole fraction of ∼1−3 ppm at the cloud tops. We find a much clearer detection (and much higher retrieved column abundance above the clouds) at southern polar latitudes compared with equatorial latitudes, which suggests a higher relative humidity of H2S here. We find our retrieved H2S abundances are most consistent with atmospheric models that have reduced methane abundance near Neptune’s south pole, consistent with HST/STIS determinations (Karkoschka and Tomasko, 2011). We also conducted a Principal Component Analysis (PCA) of the Neptune and Uranus data and found that in the 1.57–1.60 µm range, some of the Empirical Orthogonal Functions (EOFs) mapped closely to physically significant quantities, with one being strongly correlated with the modelled H2S signal and clearly mapping the spatial dependence of its spectral detectability. Just as for Uranus, the detection of H2S at the cloud tops constrains the deep bulk sulphur/nitrogen abundance to exceed unity (i.e. >4.4−5.0 times the solar value) in Neptune’s bulk atmosphere, provided that ammonia is not sequestered at great depths, and places a lower limit on its mole fraction below the observed cloud of (0.4–1.3) ×10−5. The detection of gaseous H2S at these pressure levels adds to the weight of evidence that the principal constituent of the 2.5–3.5 bar cloud is likely to be H2S ice.