Morphology and Dynamics of Venus's Middle Clouds With Akatsuki/IR1

Abstract

The Venusian atmosphere is covered by clouds with superrotating winds whose accelerating mechanism is still not well understood. The fastest winds, occurring at the cloud tops (∼70-km height), have been studied for decades, thanks to their visual contrast in dayside ultraviolet images. The middle clouds (∼50-55 km) can be observed at near-infrared wavelengths (800-950 nm), although with very low contrast. Here we present the first extensive analysis of their morphology and motions at lower latitudes along 2016 with 900-nm images from the IR1 camera onboard Akatsuki. The middle clouds exhibit hemispherical asymmetries every 4-5 days, sharp discontinuities in elongated "hook-like" stripes, and large contrasts (3-21%) probably associated with large changes in the optical thickness. Zonal winds obtained with IR1 images and with ground-based observations reveal mean zonal winds peaking at the equator, while their combination with Venus Express unveils long-term variations of 20 m/s along 10 years.