The Oldest Rocks on Venus: the Importance of Tessera Terrain for Venus Exploration (Invited)

Venus tessera terrain is a major, yet unsampled, tectonic unit on Venus characterized by multiple sets of intersecting compressional and extensional structures. Tessera terrain is temporally, morphologically, and perhaps also compositionally unique on Venus. Stratigraphic studies of tessera terrain establish that they consistently appear locally, and perhaps even globally, as the oldest material on a planet with an average surface crater retention age of ~500 million years. Thus, the tesserae provide the best chance to access rocks that are derived from the first 80% of the history of the planet, an era obscured by the emplacement of voluminous (presumably basaltic) plains. Analysis of Magellan imagery, topography and gravity data show that tessera terrain is characterized by higher strain rates and a thinner lithosphere than at present and thus records an extinct geodynamical era on Venus. Yet very little is understood about the number, morphology and stratigraphy of geologic units within tessera terrain, nor mass wasting processes operating on the surface. Improved radar imagery at the 5-25 m scale, and optical images below the clouds (<1 km) and at the surface will help assess the geologic processes operating in the pre-plains era. Such data products are also essential for judicious landing site selection, since tessera meter-scale roughness will limit landing site safety and sample access. Improved topography data are required to quantify the deformation recorded by ubiquitous tesserae structures that are finer than Magellan resolution. Tessera terrain is unsampled, but recent analyses of radiance from the surface at 1 micron using instruments on Venus Express and Galileo are consistent with felsic compositions for tesserae. Silicic compositions likely require both water and a plate recycling mechanism (e.g., subduction) for formation. The high D/H ratio of the Venus atmosphere is consistent with the loss of a significant inventory of water over the history of the planet. Felsic tesserae may herald from an ancient water-rich Venus, perhaps with an ocean and potentially habitable. Further assessment of tessera composition requires more comprehensive 1 micron radiance measurements from orbital, near-surface and surface platforms and in-situ measurement of mineralogy and chemistry. Radiance data need tobe supported by improved laboratory measurements of the emissivity of relevant rocks and weathering products in a Venus environment. Venus weathering experiments also support the interpretation of in situ analyses at the surface of Venus and may constrain sampling strategy. If confirmed, felsic tesserae would be critical targets for sample return due to their potential to include ancient rocks and/or minerals formed in the presence of water (e.g., zircons). In sum, the tesserae are the oldest materials exposed on the Venus surface and are the best candidates for containing ancient rocks and for comprising evolved compositions. They uniquely and critically constrain the geochemistry, geodynamics and history of water on Venus through time.