The first 80% of the History of Venus?

1. The first 80% of the History of Venus?

2. Some Geological Conclusions from Magellan Analysis -Volcanism and tectonism are the most abundant geological processes. -The styles and abundance of volcanism and tectonism combine attributes of both the Earth (e.g., very heavily tectonically deformed regions such as tessera) and the smaller terrestrial planetary bodies (e.g., vast volcanic plains deformed by wrinkle ridges). -The distribution of impact craters precludes recently active plate tectonics despite many Earth-like tectonic features (e.g., folded mountain belts). -Some features (e.g., coronae) are somewhat unique to Venus and may provide important information on mantle convection, lithospheric evolution processes. -The distribution and state of preservation of existing impact craters is consistent with a range of catastrophic resurfacing models. -The geological record and sequence of events can be correlated with geophysical data to assess crustal thickness variations and mantle convection patterns. -The number of impact craters is very small, indicating that the surface geological record is very young, less than 20% of the history of the planet itself. -80% of the geological record is no longer obviously preserved in the surface morphology, but may be in the surface rocks.

3. Some Post-Magellan Controversies - 1 • What is the origin of layered rocks seen in Venera panoramas? • Is the geological history ‘directional’ or ‘non-directional’? • Do the tessera form from upwelling or downwelling? • Was tessera formation catastrophic? • Are tessera and volcanic rises genetically related? • How do the folded mountain ranges of Western Ishtar form? • What is the origin of Western Ishtar, upwelling or downwelling? • Are the tessera composed of thickened basaltic crust or a different low-density composition (Airy, Pratt, both)? • Coronae: Are they mantle plumes? Why up to 2500 km wide? Which are still active?

4. Some Post-Magellan Controversies - 2 • Is the impact crater distribution random? • Can crater distribution and degradation state be used to date geological units and events? • Has global resurfacing been catastrophic, nonlinear, or linear? • What is (was) the role of water in geodynamics, petrogenesis? • What geodynamical model(s) best account for the observed geology? • How does the observed geological history mesh with mobile-lid -> stagnant lid models? • What has been the degassing history and the interaction of the surface and atmosphere over geologic time? • Does widespread volcanic degassing influence tectonics? • What happened to the first 80% of the geologic record?

5. Stages in the Evolution of Venus: Speculations Grinspoon and Bullock • Stage 1: Accretion. • Stage 2: Initial degassing, atmosphere formation. • Stage 3: Differentiation, crustal formation, solidification. • Stage 4: Atmospheric stabilization, ocean formation. • Stage 5: Evolutionary degassing and volatile exchange. • Stage 6: Atmosphere-Surface-Ocean Exchange phase. • Stage 7: Evolution to Dry Venus: Oceans dry up. • Stage 8: The Turning Point: Tessera form at ~80% point. • Stage 9: The Early Aftermath: Vast volcanic plains. • Stage 10: The Late Aftermath: Local large shields, rifts, etc. • Stage 11: The Future: (?)

6. What Do We Need to Know? • Detailed structure and history of the atmosphere. • Current and past volcanic outgassing rates. • Improved understanding of atmosphere-surface interactions. • Improved observed geological history and time scale. • Geochemistry, mineralogy and petrology of surface features/terrains, especially tessera. • Clues to the first 80% of the history of Venus. • Improved knowledge of thicknesses, structure of crust and lithosphere. • Current seismicity and global variations. • Missions and Experiments Needed: Long term seismic monitoring, heat flow, multiple landings, in-situ geochemistry, isotopic signatures, sample return.