244 SOLAR SYSTEM/Venus Moore P (1961) Astronomy London: Oldbourne Peale SJ, Phillips RJ, Solomon SC, Smith DE, and Zuber MT (2002) A procedure for determining the nature of Mercury’s core Meteoritics and Planetary Science 37: 1269 1283 Potter AE, Killen RM, and Morgan TH (2002) The sodium tail of Mercury Meteoritics and Planetary Science 37: 1165 1172 Potts LV, von Freese RRB, and Shum CK (2002) Crustal properties of Mercury by morphometric analysis of multi ring basins on the Moon and Mars Meteoritics and Planetary Science 37: 1197 1207 Robinson MS and Taylor GJ (2001) Ferrous oxide in Mercury’s crust and mantle Meteoritics and Planetary Science 36: 842 847 Sprague AL, Emery JP, Donaldson KL, et al (2002) Mer cury: mid infra red (3 13.5 mm) observations show het erogeneous composition, presence of intermediate and basic soil types, and pyroxene Meteoritics and Planetary Science 37: 1255 1268 Venus M A Ivanov, Russian Academy of Sciences, Moscow, Russia J W Head, Brown University, Providence, RI, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Venus, similar to Earth in many ways, also shows many differences and provides insight into different paths of evolution that can be taken by Earth-like planets The atmosphere of Venus is predominantly carbon dioxide; surface temperatures exceed the melting point of lead and surface pressures are almost 100 times that of Earth’s atmosphere The crater retention age of the surface of Venus is very young geologically, similar to that of the Earth; however, plate tectonics does not seem to be recycling the crust and lithosphere at present The surface is dominated by regional volcanic activity and vertical crustal accretion, and regional tectonism appears to have been much more pervasive in the earliest part of the preserved stratigraphic record, dating from less than a billion years ago The characteristics and distribution of superposed impact craters suggest that a major resurfacing event, perhaps catastrophic in nature, occurred on Venus in its relatively recent geological history Venus may thus be characterized by relatively recent episodic heat loss, rather than the more monotonic loss thought to be typical of the other Earth-like planets Despite the fact that the majority of the preserved geological record on Venus dates from the last $20% of its history, Venus may provide insight into processes, such as the formation of continents, that operated in the first half of Earth history Venus is the second largest terrestrial planet by size after Earth, and in major characteristics is close to our planet: The radius of Venus is 6051.8 km (0.95 of Earth’s radius), its mass is 4.87 Â 1027 g (0.81 of Earth’s mass), bulk density is 5.24 g cm (0.95 of Earth’s density), and surface gravity is 8.87 m s (0.91 of Earth’s gravity) For decades, Venus was considered as a ‘twin’ planet to Earth Current knowledge of Venus geology is derived from several interplanetary missions, including landers and orbiters, as well as Earth-based observations In the mid-1970s, the former Soviet Union conducted a series of successful landings; the Soviet landers transmitted panoramas of the surface of Venus, in addition to data on the near-surface environment, on surface rocks, and the chemistry of the atmosphere The Pioneer Venus was the first American orbiter of Venus; launched by the United States in 1978, the Pioneer Venus collected data on global topography and gravity The fundamental findings of this mission were that the global Venus hypsogram, in contrast to that of Earth, is characterized by one peak corresponding to the mean planetary radius (MPR), about 6051 km (Figure 1A), and that the gravity and topography of Venus are highly correlated Three major topographic provinces characterize the surface of Venus (Figure 1B): lowlands (below MPR, $11% of the surface), midlands (0–2 km above MPR, $80% of the surface), and highlands (>2 km above MPR, $9% of the surface) The spatial resolution of the Pioneer Venus imaging radar was too low to describe morphology of the surface in detail The systematic photogeological study of Venus began when high-resolution radar images were collected by the Soviet Venera-15/16 orbiters and by Earth-based radar observations from Arecibo Observatory At a resolution of $1–2 km, Venera-15/16 mapped the surface in the northern hemisphere above $30o N; images from the Arecibo telescope covered a large area between 65o S–65o N and 270o E–30o E In the early 1990s, the United States Magellan orbiter provided almost complete coverage ($97% of the surface) of Venus, providing high-resolution images (100–200 m) and medium-resolution altimetry