The False Spin of an Exo-Venus

Abstract

Direct imaging of terrestrial exoplanets will enable rotational and atmospheric characterization through time-resolved photometry and high-dispersion spectroscopy. However, the velocity field inferred from reflected light does not necessarily correspond to the rotation of the solid planet, but rather to the motion of the layer from which the photons emerge. Venus provides a crucial Solar System example of this ambiguity: the solid planet rotates slowly, whereas the cloud-level atmosphere exhibits superrotation with a period of only several days. Here we investigate the observational degeneracy between rapid planetary rotation and atmospheric superrotation. We construct a disk-integrated reflected-light velocity model that includes solid-body rotation, zonal winds, and phase-dependent illumination. We show that, for a single spectral tracer probing a narrow range of pressures, a zonal wind field whose latitude dependence is similar to solid-body rotation can exactly mimic the line profile of a rapidly rotating planet. The degeneracy can be broken by measuring the apparent rotational velocity as a function of wavelength or line formation pressure. For a Venus-like wind profile, the apparent period can vary from hundreds of days in the lower atmosphere to $\sim$4--5~days at the cloud deck. We estimate the resolving power and signal-to-noise ratio required to measure this vertical shear. The most robust diagnostic of atmospheric superrotation is not a single value of $v \sin i$, but an altitude-dependent ``false spin'' signature across multiple spectral tracers. These results have direct implications for interpreting rotational measurements of Venus-like worlds with the Habitable Worlds Observatory and complementary high-dispersion facilities.