Periodic Variation of Mesoscale Ultraviolet Contrast at the Cloud Top of Venus

Abstract

Images of Venusian clouds taken at ultraviolet (UV) wavelengths frequently exhibit mesoscale cellular structures at low latitudes. To obtain clues regarding the origin of these structures, we studied the temporal variation of the mesoscale UV contrast using 283- and 365-nm images taken by the ultraviolet imager (UVI) onboard the Akatsuki spacecraft. The 283-nm channel is mostly affected by SO2 at the cloud top and the 365-nm channel is mostly affected by an unknown absorber, though both absorbers are thought to influence both wavelengths. We defined the mesoscale as a spatial scale roughly below 1,000 km, where the effect of large-scale streaky patterns is not significant according to the spatial spectra of 283-nm images taken by UVI and 2.02-mu m images taken by the near-infrared camera (IR2) onboard Akatsuki. We found that the temporal variation of the standard deviation of the mesoscale UV contrast is dominated by quasi-periodic oscillations with amplitudes of 30%-50% relative to the mean value and periods of around 4-5 Earth days. Oscillations of the planetary-scale UV contrast with similar periods were also observed, suggesting that the mesoscale structures are influenced by planetary-scale waves. The mesoscale contrast tends to be enhanced when the background albedo decreases in the course of the propagation of planetary-scale waves. This correlation suggests that the solar heating near the cloud top level feeds the mesoscale dynamics. An enhancement of the mesoscale contrast in the afternoon was seen at a wavelength of 365 nm. On Venus, the brightness pattern of the cloud top shows signs of convection, where heat is transported either upward or horizontally through the fluid motion. These processes could have a large impact on material and energy transportation on the planet. We examined ultraviolet (UV) cloud images captured by the Akatsuki Venus orbiter to track changes in contrast over time at both a planetary and mesoscale (below 1,000 km). Unlike visible wavelengths, Venus' clouds display changes in brightness over time and space when viewed at UV wavelengths. The UV contrast at the mesoscale was found to oscillate with periods of around 4-5 Earth days. Similar periods of oscillation are observed in the planetary-scale brightness variation, suggesting that planetary-scale waves influence the motion at a scale below 1,000 km. The contrast at this scale tends to be enhanced when the background becomes darker. This relation implies that the heating of clouds by the Sun enhances vertical or horizontal inhomogeneity of the temperature and then powers the observed convection. The temporal variation of the mesoscale ultraviolet (UV) contrast of Venusian clouds was studied using images taken by the UV camera on AkatsukiThe mesoscale contrast was found to oscillate with periods of 4-5 Earth days, which are also the periods of planetary-scale wavesThe mesoscale contrast tends to be enhanced when the background becomes darker, suggesting that solar heating feeds the mesoscale dynamics