Geological Evidence for Petrogenetic Diversity on Venus: Implications for Future Exploration Strategies

Introduction: A long-standing, fundamental question in planetary geoscience is: "How similar are the geological histories of Earth and Venus, and when and how did their evolution diverge?" Did Venus once have oceans and a more Earth-like climate, as suggested by Pioneer-Venus data [1]? If so, when, how, and why did it transition to current conditions, and are traces of this early period, and the transition, manifested in the currently exposed geological record? What is the evidence as seen in the surface products of mantle partial melts (the petrogenetic record)? Does Venus, like the Earth, have both mafic secondary crust (derived from mantle partial melts) and felsic tertiary crust (derived from reworking of secondary crust and potentially involving water) [2]. In this analysis we review evidence for the presence of mafic and felsic crust in the currently observed geological record of Venus in order to set a framework for addressing these questions. Magellan radar image data revealed that the surface of Venus was composed primarily of plains units containing geomorphic features consistent with a volcanic origin [3]. Indeed, over 80% of the surface appears to have been resurfaced by effusive volcanic activity [4], primarily interpreted to be basaltic (mafic) in nature, an observation consistent with the geochemical results from the Venera/Vega landers [5]. A small number of features initially observed in the Magellan data (pancake domes [3,6-7], festoons [8]) differed from the widespread effusive, apparently low-viscosity lavas interpreted to be of basaltic origin, and were interpreted to be either of a more felsic nature [6-8], or to be basal-tic magmas that attained higher viscosity through increased bubble content [6]. Subsequent to these initial global descriptions, numerous studies began placing geologic features in the context of local, regional and global stratigraphic relationships [e.g., 9]. The highly deformed tesserae were seen to be the earliest strati-graphic unit and were thought to represent either deformed volcanic plateaus or more ancient tectonically thickened crust that could be either mafic or felsic in nature [10]. Following tessera formation, a sequence of volcanic units with differing characteristics were observed , all apparently forming in the last 20% of the history of Venus [11]. Among the fundamental unre-solved questions concerning the history of Venus are: 1) What is the petrogenetic diversity displayed by the array of volcanic features? 2) Is there evidence for significant compositional variation, perhaps ranging from ultra-mafic to felsic? 3) What are the geological envi