The geologic evolution of Venus: Insights into Earth history

Through the clever use of stereo radargrammetric techniques on images from the Magellan mission to Venus in the 1990s, combined with recent quantitative analyses of magma reservoirs on Earth, McGovern et al. (2014, p. 59 in this issue of Geology) resolve an important outstanding question dating from that era: What forces are responsible for the graben extending radially from large shield volcanoes? They show that, in addition to radial dike emplacement, the larger graben are readily accounted for by magma reservoir infl ation, providing new insight into processes of magma ascent and eruption on Venus, and their relation to globally interconnected rift systems on Earth’s sibling planet. This new contribution serves as a reminder of the importance of Venus to our understanding of geological processes and history on Earth, and the need for new data to resolve some of the most fundamental problems in comparative planetology. HISTORY OF VENUS EXPLORATION The liftoff of Sputnik in 1957 launched a vigorous international program of planetary exploration and the fi eld of comparative planetology. How do our neighboring terrestrial (Earth-like) planets look, and what insights can they provide in understanding our own planet? Early missions to the Moon, Mercury, and Mars showed that these terrestrial planets with one-half or less the diameter of Earth looked very different from Earth. Their high surface area/volume ratio lets them conduct internal heat to the surface effi ciently and cool rapidly, becoming “one-plate planets” with thick, stable lithospheres. Their surfaces preserve evidence of geological processes from the fi rst half of solar system history. Intensely pockmarked by impact craters and resurfaced by volcanism, they reveal the roles of external forces (early accretion and high impact fl ux) and internal thermal evolution (conduction, convection, hot spot formation, magmatism, advection, and surface volcanism). But what about Venus, similar to Earth in size, density, and position in the Solar System? Is it Earth-like, with a young surface, plate tectonics, and tracks of hot-spot volcanoes, or like the smaller planets, with a heavily cratered surface? When imagination outstripped data, Venus was thought to have an Earth-like hot swampy surface, but we now know that its CO 2 atmosphere, sulfuric acid clouds, surface temperatures hot enough to melt lead, and very slow and retrograde rotation, are nothing like Earth. What about its surface and interior geology? Venera (Soviet Union) atmospheric probes and landers showed that the surface was littered with generally basaltic rocks and patchy soils (Hunten et al., 1983). A radar altimeter on the Pioneer Venus (PV, United States) orbiter (1978) provided a global topographic map, with a resolution of ~100 km, low by today’s standards. Its analysis showed many Earth-like features: evidence for continent-like highlands with marginal linear mountain belts, broad Iceland-like rises and plateaus, circular basins, and very long, linear lowlands, but with a global hypsometry different from that of current Earth. Instead of the latter’s bimodal distribution of global topography, the hypsometry of Venus was unimodal, skewed to higher elevations. Increasingly higher-resolution radar data provided additional evidence for Earth-like features. One of the rises in the PV data (Beta Regio) was cleaved by a rift zone, surmounted by volcanoes. The mountain belts surrounding a large continent-like plateau (Ishtar Terra) were composed of multiple parallel linear features analogous to Earth’s folded mountain belts. Often-circular features (coronae) were not impact structures, but complex deformation zones, more consistent with internal deformation origins. Higher-resolution radar imaging and more coverage were needed to fi out whether Venus was Earth-like. In 1983, Soviet Union orbiters Venera 15 and 16 surveyed the northern hemisphere (~30% of Venus), with radar imaging resolution of 1‐2 km (Barsukov et al., 1992), showing that many highlands consist of very highly deformed terrain (tesserae), and that organized linear folded mountain belts surround highland plateaus. Large volcanoes exist in abundance, but show little evidence for hot spot traces or convergent plate boundaries. Coronae looked like the fi ngerprints of mantle processes. Most remarkably, there were few impact craters, indicating an age on the order of only hundreds of millions of years. These results raised signifi cant, fundamental questions: Where is the ancient billions-of-years-old continent-like terrain on Venus? What is the average age of the surface? Is Venus characterized by Earth-like plate tectonics, or do its thermal regimes suggest another manifestation of plate tectonics? Is internal heat transferred mainly by conduction, advection, or plate tectonics? Are the fold belts global and if so, where is the corresponding extension and divergence? These questions guided the 1989 United States Magellan mission to Venus. THE MAGELLAN MISSION PROVIDES GLOBAL COVERAGE