Publication Date:
2021-07-21
Description:
The so‐called unknown absorber in the clouds of Venus is an important absorber of solar energy, but its vertical distribution remains poorly quantified. We analyze the 283 and 365 nm phase curves of the disk‐integrated albedo measured by Akatsuki. Based on our models, we find that the unknown absorber can exist either well mixed over the entire upper cloud or within a thin layer. The necessary condition to explain the 365 nm phase curve is that the unknown absorber must absorb efficiently within the cloud scale height immediately below the cloud top. Using this constraint, we attempt to extract the SO2 abundance from the 283 nm phase curve. However, we cannot disentangle the absorption by SO2 and by the unknown absorber. Considering previous SO2 abundance measurements at midinfrared wavelengths, the required absorption coefficient of the unknown absorber at 283 nm must be more than twice that at 365 nm.
Description:
Plain Language Summary:
There is an unknown absorber in the clouds of Venus. It absorbs solar energy effectively at ultraviolet (UV) and blue wavelengths, but its vertical location, either above or below the cloud top level (about 70 km altitude), remains unclear. This uncertainty affects our understanding of the vertical deposition of solar energy in the atmosphere. We investigate the vertical distribution of the unknown absorber using the dependence of the full‐disk brightness on the scattering direction (the Sun‐Venus‐spacecraft angle) at 365 nm, with data from JAXA's Akatsuki spacecraft over 3 years. We find that the unknown absorber could exist in the entire cloud, or as a thin layer near but below the cloud top. Using these constraints on the vertical distribution of the unknown absorber, we analyze the 283 nm full‐disk brightness. At this shorter wavelength, the SO2 gas and the unknown absorber are both effective absorbers. We attempt to quantify the SO2 abundance, and find that the brightness dependence on the scattering direction alone is insufficient to separate the contribution from the two absorbers at 283 nm. Further analysis with spectral phase curve observations will better define the SO2 abundance.
Description:
Key Points:
The vertical distribution of the unknown absorber is investigated with the aid of full‐disk phase curves at wavelengths of 283 and 365 nm
First time the 283 nm full‐disk brightness phase curve of Venus is analyzed over a broad phase angle range
The unknown absorber must result in sufficient absorption within the cloud scale height immediately below the cloud top
Description:
EC, H2020, H2020 Priority Excellent Science, H2020 Marie Skłodowska‐Curie Actions (MSCA)
http://dx.doi.org/10.13039/100010665
Keywords:
523
;
atmosphere
;
phase curve
;
simulation
;
UV image
;
Venus
Type:
article
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