SOLAR SYSTEM/Venus 245 Figure Characteristics of the global altimetry of Venus (A) The Venus hypsogram (the fraction of surface area vs elevation) has one peak, implying the absence of the surface elevation dichotomy that characterizes the distribution of the surface elevation on Earth (high standing continents and low lying ocean floor) The mean planetary radius (MPR) of Venus is 6051.84 km (B) The map in simple cylindrical projection, showing the areal distribution of the three major topographic provinces of Venus Lowlands (light grey, below km), midlands (white, km), and highlands (dark grey, above km) The majority of the surface is within the midlands ($20-km footprint) Magellan also collected data on the Venus gravity field Surface Conditions and Rock Composition In contrast to Earth, a very dense atmosphere (the pressure at the surface is $95 bar) consisting primarily of CO2 (Table 1) blankets Venus The relative role of three major contributors to the atmosphere, primordial nebular material, volcanic outgassing, and influx of volatiles by comets, in the formation of the present atmosphere is an open question Although the current atmosphere is very dry, a minute quantity of water is still detectable An important feature of water in the Venusian atmosphere is that the deuterium/ hydrogen (D/H) ratio is 150 Ỉ 30 times higher than is found in terrestrial water If water on Earth represents a sample of primitive water on Venus, the Venusian D/H ratio suggests that, depending on the