Characterization of the Small‐Scale UV Contrasts at Low Latitudes of Venus as Seen by the UVI Camera Onboard Akatsuki

Abstract

We characterize ultraviolet (UV) contrasts of Venus at the cloud top level, ∼70 km altitude, at low latitudes. We define the contrast as the albedo difference between bright and dark points at low latitudes of a single image. Our target is small‐scale contrast features, spanning 60–80 km, as seen in the reflected UV images acquired at 283 and 365 nm in 2016–2021 by the UV Imager (UVI) aboard JAXA's Akatsuki Venus orbiter. The number of analyzed images is 4,288 at 283 nm and 4,286 at 365 nm. The mean solar phase angle dependence of UV contrasts shows peak values of 45.6 (12.5)% at 283 nm and 53.9 (14.1)% at 365 nm near 40° solar phase angle, while the data span 0°–80°. In general, 283‐nm contrasts are slightly smaller than those at 365 nm, by ∼7% on average. For the first time, we compare the phase angle dependence of contrasts at 283 and 365 nm, and show that the 365‐nm contrast decreases more rapidly with increasing phase angle from 40°: −0.44 (0.06)%/degree at 283 nm and −0.60 (0.07)%/degree at 365 nm. This slope difference may be attributed to differences in the vertical distribution of the UV absorbers; for example, the unknown absorber has a smaller scale height than that of the gas above 70 km (3 km). We confirm the dominant temporal variations in contrasts, with the well‐known 4–5‐day periodicity followed by month‐scale fluctuations. Because of strong temporal variations, we are unable to separate a possible local‐time dependence of contrasts in this study.