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

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 (similar to 70-km height), have been studied for decades, thanks to their visual contrast in dayside ultraviolet images. The middle clouds (similar to 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 4-5days, 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 20m/s along 10 years. Plain Language Summary The atmosphere Venus is surprisingly fast with velocities 60 times faster than the solid globe of Venus. This atmospheric phenomenon is called superrotation and its mechanisms are yet unexplained for the scientists. The Japanese space mission Akatsuki from the Japan Aerospace Exploration Agency arrived at Venus in December 2015 to try to unveil this mystery. Among its instruments, the camera IR1 was prepared to observe the middle clouds of Venus (50-55 km over the surface), which are the most unknown and hardest to observe since they normally exhibit very low contrast in the images. Thanks to the images from the camera IR1, we have observed with high spatial resolution the middle clouds of Venus along the first year of observations of Akatsuki, discovering that they exhibit higher contrasts than expected and a wide variety of cloud patterns unrelated to what we observe at the top of the clouds (70 km above the surface). Finally, the motions of the middle clouds obtained through the combination of images from Akatsuki, amateur observers and the past mission Venus Express, have allowed to reconstruct a composite of the winds of Venus along 10 years, unveiling that the superrotation may be subject to long-term variabilities unreported before.