Highlands on Venus

Most of the highlands on Venus are concentrated into two main continental sized areas: Ishtar Terra, the size of Australia, in the northern hemisphere, and Aphrodite Terra, about the size of Africa, near the equator. Aphrodite Terra, with a length of some 1500 km, is reminiscent of major continental collision zones on Earth, such as the mountain belt that extends from the Alps to the Himalayas. Ishtar Terra is a region of elevated topography with a horizontal scale of 2000–3000 km. A major feature is Lakshmi Planum which is an elevated plateau similar to Tibet with a mean elevation of about 4 km. This plateau is surrounded by linear moun-1.21 The Galilean Satellites 107 tain belts. Akna, Danu, Freyja, and Maxwell Montes, reaching elevations of 10 km, are similar in scale and elevation to the Himalayas. The gravitational anomalies associated with topographic planetary features further constrain their origin. Gravity anomalies obtained from tracking Pioneer Venus provided further major surprises. Unlike on the Earth, gravity anomalies correlate with high topography on Venus. Large positive gravity anomalies are directly associated with Beta Regio and eastern Aphrodite Terra. One of the most important observational constraints on the geodynamics of Venus comes from studies of impact cratering on the surface. Some 840 impact craters have been identified from Magellan images with diameters ranging from 2 to 280 km. The distribution of craters on Venus cannot be distinguished from a random distribution. Unlike the Moon and Mars, older and younger terrains cannot be identified. The surface of Venus appears to be of a near-uniform age. Correlations of this impact flux with craters on the Moon, the Earth, and Mars indicate a mean surface age of 0.5 ± 0.3 Ga. Another important observation is that 52% of the craters are slightly fractured and only 4.5% are embayed by lava flows. These observations led Schaber et al. (1992) to hypothesize that a global volcanic resurfacing event had occurred at about 500 Ma and that relatively little surface volcanism has occurred since. Further statistical tests have shown that a large fraction of the surface of Venus (≈80–90%) was covered by freshvolcanic flows during a period of 10–50 Myr. It is well established that the geologic evolution of Venus is far more catastrophic than the Earth’s.Other major features unique to Venus are the coronae. These are quasi-circular features, 100–2600 km in diameter, with raised interiors and elevated rims, often with annular troughs. It has been suggested that the perimeters of several large coronae on Venus, specifically Artemis, Latona, and Eithinoha, resemble terrestrial subduction zones in both planform and topography. Artemis chasma has a radius of curvature similar to that ofthe South Sandwich subduction zone on the Earth. Large coronae may be incipient circular subduction zones. The foundering lithosphere is replaced by ascending hot mantle in a manner similar to back-arc spreading on the Earth.