Methane depletion in both polar regions of Uranus inferred from HST/STIS and Keck/NIRC2 observations

TL;DR

This study reveals persistent methane depletion in both polar regions of Uranus, with implications for understanding its atmospheric circulation and cloud formation processes, based on observations from HST/STIS and Keck/NIRC2.

Contribution

It provides the first simultaneous evidence of methane depletion in both polar regions of Uranus and links these features to meridional circulation models and cloud formation.

Findings

Methane depletion is persistent and occurs mainly in the upper troposphere.

Latitudinal variations in methane correlate with cloud reflectivity.

Depletion depth increases poleward, reaching several bars at high latitudes.

Abstract

From STIS observations of Uranus in 2012, we found that the methane volume mixing ratio declined from about 4% at low latitudes to about 2% at 60 deg N and beyond. This is similar to that found in the south polar regions in 2002, in spite of what appears to be strikingly different convective activity in the two regions. Keck and HST imaging observations close to equinox imply that the depletions were simultaneously present in 2007, suggesting they are persistent features. The depletions appear to be mainly restricted to the upper troposphere, with depth increasing poleward from about 30 deg N, reaching ~4 bars at 45 deg N and perhaps much deeper at 70 deg N. The latitudinal variations in degree and depth of the depletions are important constraints on models of meridional circulation. Our observations are qualitatively consistent with previously suggested circulation cells in which rising methane-rich gas at low latitudes is dried out by condensation and sedimentation of methane ice particles as the gas ascends to altitudes above the methane condensation level, then is transported to high latitudes, where it descends and brings down methane depleted gas. Since this cell would seem to inhibit formation of condensation clouds in regions where clouds are actually inferred from spectral modelling, it suggests that sparse localized convective events may be important in cloud formation. The small-scale latitudinal variations we found in the effective methane mixing ratio between 55 deg N and 82 deg N have significant inverse correlations with zonal mean latitudinal variations in cloud reflectivity in near-IR Keck images taken before and after the HST observations. If the CH4/H2 absorption ratio variations are interpreted as local variations in para fraction instead of methane mixing ratio, we find that downwelling correlates with reduced cloud reflectivity.