Probable detection of hydrogen sulphide (H$_2$S) in Neptune’s atmosphere. (arXiv:1812.05382v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Irwin_P/0/1/0/all/0/1">Patrick G.J. Irwin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Toledo_D/0/1/0/all/0/1">Daniel Toledo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Garland_R/0/1/0/all/0/1">Ryan Garland</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Teanby_N/0/1/0/all/0/1">Nicholas A. Teanby</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fletcher_L/0/1/0/all/0/1">Leigh N. Fletcher</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Orton_G/0/1/0/all/0/1">Glenn S. Orton</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bezard_B/0/1/0/all/0/1">Bruno B&#xe9;zard</a>

Recent analysis of Gemini-North/NIFS H-band (1.45 – 1.8 $mu$m) observations
of Uranus, recorded in 2010, with recently updated line data has revealed the
spectral signature of hydrogen sulphide (H$_2$S) 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 H$_2$S spectral
absorption features in the 1.57 – 1.58 $mu$m range, albeit slightly less
evident, and retrieve a mole fraction of $sim1-3$ 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 H$_2$S here. We find our retrieved
H$_2$S 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 $mu$m range, some of the Empirical Orthogonal Functions (EOFs)
mapped closely to physically significant quantities, with one being strongly
correlated with the modelled H$_2$S signal and clearly mapping the spatial
dependence of its spectral detectability. Just as for Uranus, the detection of
H$_2$S 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) $times 10^{-5}$. The detection of gaseous H$_2$S 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 H$_2$S ice.

Recent analysis of Gemini-North/NIFS H-band (1.45 – 1.8 $mu$m) observations
of Uranus, recorded in 2010, with recently updated line data has revealed the
spectral signature of hydrogen sulphide (H$_2$S) 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 H$_2$S spectral
absorption features in the 1.57 – 1.58 $mu$m range, albeit slightly less
evident, and retrieve a mole fraction of $sim1-3$ 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 H$_2$S here. We find our retrieved
H$_2$S 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 $mu$m range, some of the Empirical Orthogonal Functions (EOFs)
mapped closely to physically significant quantities, with one being strongly
correlated with the modelled H$_2$S signal and clearly mapping the spatial
dependence of its spectral detectability. Just as for Uranus, the detection of
H$_2$S 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) $times 10^{-5}$. The detection of gaseous H$_2$S 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 H$_2$S ice.

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