  An unknown absorber near the cloud top level of Venus generates a broad
absorption feature from the ultraviolet (UV) to visible, peaking around 360 nm,
and therefore plays a critical role in the solar energy absorption. We present
a quantitative study on the variability of the cloud albedo at 365 nm and its
impact on Venus' solar heating rates based on an analysis of Venus Express and
Akatsuki's UV images, and Hubble Space Telescope and MESSENGER's UV spectral
data; in this analysis the calibration correction factor of the UV images of
Venus Express (VMC) is updated relative to the Hubble and MESSENGER albedo
measurements. Our results indicate that the 365-nm albedo varied by a factor of
2 from 2006 to 2017 over the entire planet, producing a 25-40% change in the
low latitude solar heating rate according to our radiative transfer
calculations. Thus, the cloud top level atmosphere should have experienced
considerable solar heating variations over this period. Our global circulation
model calculations show that this variable solar heating rate may explain the
observed variations of zonal wind from 2006 to 2017. Overlaps in the timescale
of the long-term UV albedo and the solar activity variations make it plausible
that solar extreme UV intensity and cosmic-ray variations influenced the
observed albedo trends. The albedo variations might also be linked with
temporal variations of the upper cloud SO2 gas abundance, which affects the
H2SO4-H2O aerosol formation.
