Large Brightness Variations of Uranus at Red and Near-IR Wavelengths

TL;DR

This study quantifies how Uranus's brightness varies significantly with viewing angle at red and near-IR wavelengths, revealing strong polar asymmetries and flux variations linked to atmospheric composition.

Contribution

It provides detailed measurements of Uranus's brightness dependence on sub-latitude across multiple wavelengths, highlighting the planet's polar brightness asymmetry and atmospheric effects.

Findings

Red flux varies by 30% with sub-latitude

Blue flux varies by 3% with sub-latitude

Brightness asymmetry linked to polar regions and methane distribution

Abstract

Uranus is fainter when the Sun and Earth are near its equatorial plane than when they are near the projection of its poles. The average of the absolute values of the sub-Earth and sub-Sun latitudes (referred to as the sub-latitude here) is used to quantify this dependency. The rates of change of magnitude with sub-latitude for four of the Johnson-Cousins band-passes are B-band, -0.48 +/- 0.11 milli-magnitudes per degree; V-band, -0.84 +/- 0.04 ; R-band, -5.33 +/- 0.30; and I-band -2.79 +/- 0.41. Evaluated over the range of observed sub-latitudes, the blue flux changes by a modest 3% while the red flux varies by a much more substantial 30%. These disk-integrated variations are consistent with the published brightness characteristics of the North and South Polar Regions, with the latitudinal distribution of methane and with a planetary hemispheric asymmetry. Reference magnitudes and colors are also reported along with geometric albedos for the seven Johnson-Cousins band-passes.