Geological Evidence for the Great Climate Change Event on Venus Richard E. Ernst Carleton University (Ottawa, Canada) & Tomsk State University (Tomsk, Russia) On Venus, a major pulse of LIP-style volcanism occurred ~700 million years ago and released a massive volume of carbon dioxide CO2. This caused runaway greenhouse warming to a current surface temperature of 450C, and an atmosphere 90 times as dense as that of the Earth, composed of 96% CO2. As a consequence, there is no liquid water on the surface of Venus and therefore no significant water erosion. However, it has been proposed that prior to this hyper-warming event Venus had conditions that were more Earth-like, even including the presence of oceans and rivers, and the potential for life. We propose that this dramatic climate change model for Venus can be evaluated through geological mapping (using Magellan spacecraft radar data) of two types of units: older units called tesserae, and younger widespread basaltic lava flows that cover 90% of Venus. Tesserae would have been emplaced prior to global warming at a time when there could have been water erosion on Venus. Our hypothesis is also that the earliest basaltic lava flows on Venus were also emplaced during cooler (Earth-like) conditions, while the later basaltic flows would have been emplaced under the much hotter current conditions. Our team has identified evidence for water erosion in tesserae, consistent with the climate modelling. This discovery indicates that tesserae could have a much longer geological history than previously believed, maybe even extending back 4 billion years, like the earliest rocks on Earth. Detailed mapping of tesserae (of their erosional history, linear structures and superimposed lava ponds and sedimentary basin accumulations) is underway to help unravel their potentially protracted geological history. Detailed mapping of the oldest volcanic plains aims to recognize the transition in climate from Earth-like to inhospitable, by identifying flow units exhibiting erosion which are being cut by flow units exhibiting no erosion.