What maintains the superrotation of Venus's atmosphere?
In plain words
The solid body of Venus turns once every 243 Earth days, but its clouds circle the planet in about 4 days, roughly sixty times faster. Which waves and eddies keep feeding spin into the atmosphere is unresolved.
Precise statement
At cloud-top altitude (about 70 km) zonal winds reach about $1e4\,\mathrm{cm}/\mathrm{s}$, roughly 60 times the equatorial surface rotation speed. Identify the angular-momentum balance that maintains this superrotation: thermal tides driven by solar heating of the cloud layer, the Gierasch-Rossow-Williams mechanism (mean meridional circulation plus equatorward eddy transport by planetary waves and barotropic instability), or other waves, with their contributions as a function of altitude. An answer is a quantitative angular-momentum budget, from observations or from a general circulation model, that reproduces the observed wind profile and its variability.
What would settle it
Cloud-tracked and radio-occultation wind and temperature fields that resolve tidal and eddy momentum fluxes at several altitudes, matched by a general circulation model.
Status in the literature
Akatsuki cloud tracking (Horinouchi and coauthors, Science 2020, https://doi.org/10.1126/science.aaz4439) found thermal tides accelerate the equatorial cloud-top flow while waves and turbulence decelerate it; the full vertical budget remains open.