The lost oceans of Venus

Whether Venus once had oceans and, if so, when it lost them are critical issues in the understanding of habitability on exoplanets, as well as for the future of life on Earth. Analysis of distinctive, lowland polygonal terrains on Venus reveals 6 main types. Many are several kilometres across, suggesting a mode of formation by contraction of confined marine clays, supporting the hypothesis that Venus had oceans within the last billion years. By analogy with the Messinian desiccation of the Mediterranean Sea on Earth, we identify other features of possible marine origin, including canali, which we conclude formed as submarine channels. The distribution of wrinkle ridges is consistent with them overlying salt layers, calculated to be at least 64 m thick, but probably significantly more, precipitated from evaporating oceans. While the interpretation of these features is not unambiguous, there is a compelling case that Venus once supported oceans, and perhaps life, across most of its surface.