HST/WFC3 Observations of Uranus' 2014 storm clouds and comparison with VLT/SINFONI and IRTF/SpeX observations

TL;DR

This study uses multi-instrument observations and radiative transfer modeling to analyze a major storm cloud complex on Uranus, revealing its layered cloud structure, particle sizes, and scattering properties, and challenging the idea of vigorous convection as its origin.

Contribution

It provides a detailed three-component cloud model for Uranus' storm clouds, integrating data from HST, VLT, and IRTF, and constrains particle sizes and cloud properties with radiative transfer analysis.

Findings

Three-layer cloud structure identified: deep tropospheric cloud, methane-ice cloud, and tropospheric haze.

Particle sizes in haze and cloud are around 0.1 micron, with storm cloud particles about 0.5 micron.

Clouds have low opacity and are unlikely to be formed by vigorous moist convection.

Abstract

Observations of Uranus were made on the 8/9th November with HST/WFC3 at a time when a huge cloud complex was present at 30 - 40N. We imaged Uranus in seven filters spanning 467 - 924 nm, and analysed these observations with the NEMESIS radiative-transfer and retrieval code. The same system was also observed in the H-band with VLT/SINFONI ON 31st October and 11th November (Irwin et al., 2016). To constrain the background cloud particle sizes and scattering properties we conducted a limb-darkening analysis of the background cloud structure at 30-40N by simultaneously fitting: a) these HST/OPAL observations at a range of zenith angles; b) the VLT/SINFONI observations at a range of zenith angles; and c) IRTF/SpeX observations made in 2009 (Irwin et al., 2015). We find that the observations are well modelled with a three-component cloud comprised of: 1) a vertically thin, but optically thick 'deep' tropospheric cloud at a pressure of ~2 bars; 2) a methane-ice cloud at the methane-condensation level with variable vertical extent; and 3) a vertically extended tropospheric haze. We find that the particles sizes in both haze and tropospheric cloud have an effective radius of ~0.1 micron, although we cannot rule out larger particle sizes in the tropospheric cloud. We find that the particles in both the tropospheric cloud and haze are more scattering at short wavelengths, giving them a blue colour, but are more absorbing at longer wavelengths, especially for the tropospheric haze. For the particles in the storm clouds, which we assume to be composed of methane ice particles, we find that their mean radii must be ~0.5 micron. We find that the high clouds have low integrated opacity, and that "streamers" reminiscent of thunderstorm anvils are confined to levels deeper than 1 bar. These results argue against vigorous moist convective origins for the cloud features.