Did Venus ever have surface oceans?
In plain words
Venus's atmosphere has far more heavy hydrogen than Earth's, a sign that it once held more water that escaped to space. Whether that water was ever a liquid ocean or stayed as steam is unknown.
Precise statement
The Venus atmospheric D/H ratio is about 100 to 150 times Earth's ocean value. Climate models allow a temperate surface to about 0.7 Ga if water condensed early, while other models keep water in a steam atmosphere that never condensed after the magma ocean. Determine whether liquid water existed on the surface and for how long. An answer is a measurement or decisive constraint (noble-gas and D/H profiles, tessera composition, interior water content).
What would settle it
Descent-probe noble gas and isotope measurements plus orbital infrared mapping of tessera rock composition from planned Venus missions.
Status in the literature
Unverified note
A 2024 analysis of volcanic gas chemistry (arXiv:2412.01879) argued for a dry Venus interior, disfavoring a long ocean epoch.