Spatial and Temporal Variability of the 365‐nm Albedo of Venus Observed by the Camera on Board Venus Express

• Published in: Journal of geophysical research. Planets Vol. 125; no. 6
• Main Authors: Lee, Y. J., Kopparla, P., Peralta, J., Schröder, S. E., Imamura, T., Kouyama, T., Watanabe, S.
• Format: Journal Article
• Language: English
• Published: 01.06.2020
• Subjects: clouds; image analysis; observations; Venus
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2019JE006271

We mapped the distribution of the 365‐nm albedo of the Venus atmosphere over the years 2006–2014, using images acquired by the Venus Monitoring Camera (VMC) on board Venus Express. We selected all images with a global view of Venus to investigate how the albedo depends on longitude. Bertaux et al. (2016, https://doi.org/10.1002/2015JE004958) reported a peak in albedo around 100° longitude and speculated on an association with the Aphrodite Terra mountains. We show that this peak is most likely an artifact, resulting from long‐term albedo variations coupled with considerable temporal gaps in data sampling over longitude. We also used a subset of images to investigate how the albedo depends on local time, selecting only south pole viewing images of the dayside (local times 7–17 hr). Akatsuki observed mountain‐induced waves in the late afternoon at 283 nm and 10 μm (Fukuhara et al., 2017, https://doi.org/10.1038/ngeo2873). We expect that the presence of such waves may introduce 365‐nm albedo variations with a periodicity of one solar day (116.75 Earth days). We searched for such a periodicity peak at 15:30–16:00 local time and low latitudes but did not find it. In conclusion, we find that temporal albedo variations, both short and long term, dominate any systematic variations with longitude and local time. The nature of VMC dayside observations limits regular data sampling along longitudes, so longitudinal variations, if they exist, are difficult to extract. We conclude that any influence by the Venus surface on 365‐nm albedo is negligible within this data set. Plain Language Summary Recently, it was reported that mountains on the surface of Venus can affect the atmosphere at the altitude of the cloud tops (70 km). For example, the brightness of the clouds (albedo) in images made by the Venus Express spacecraft at ultraviolet wavelengths (365 nm) was suspected to peak over a high mountain, Aphrodite Terra. We searched for such surface effects using the Venus Express images taken at 365 nm over the years 2006–2014. We found that the albedo was strongly variable over this period and that different longitudes were systematically imaged at different times. It is therefore not possible to uncover the influence of mountains on the albedo, and we believe that the reported albedo peak near Aphrodite Terra is most likely not real. Another spacecraft, Akatsuki, observed global‐scale atmospheric waves in the late afternoon that are originated by mountains. We also searched for albedo changes at the same latitude with a period of one solar day (116.75 Earth days) that might be linked with these atmospheric waves but did not find any period above the noise level. We conclude that the influence of mountains on the 365‐nm albedo is too weak to be recognized in Venus Express images. Key Points Temporal variations of the 365‐nm albedo of Venus dominate over any systematic variations along longitude or over local time We found no systematic influence by mountains on the 365‐nm albedo distribution, in contrast to a previous report