greenhouse effect. Venus was the target of a large number of Soviet and American probes in the 1970s and 1980s, notably the Soviet Venera and Vega series and the American Pioneer Venus. The extremely high temperature and pressure present considerable difficulties and many of the probes were destroyed either before returning data or after a relatively short period of operation. Nevertheless, it proved possible to analyse the chemical composition of some surface rocks and to return limited panoramic views of the surface terrain, showing rocky desert landscapes. The first radar maps produced by spacecraft orbiting the planet showed that most of the surface consists of vast plains, above which several large plateaux rise to heights of several kilometres. The two main highland areas are Ishtar Terra in the northern hemisphere and Aphrodite Terra in the equatorial region. The Maxwell Montes are the highest feature, rising to 11 kilometres above the mean level of the planetary surface. In 1990, the US Magellan probe arrived in orbit around Venus and commenced a programme of mapping the surface in much greater detail than had been achieved previously, by means of sophisticated radar techniques. Ample evidence has been found of both impact features and volcanic activity in the relatively recent past. By solar system standards, the surface of Venus is young: the oldest craters appear to date from 800 million years ago. However, no evidence has yet been found of current volcanism. The thick atmosphere and high surface temperature mean that impact craters take forms rather different from those on other planets and satellites. Smaller meteorites burn up readily on passing through the atmosphere so there is an absence of smaller craters. The material thrown out in the powerful impacts of larger meteorites did not travel far and tended to spread round the craters in molten form. Large numbers of volcanic features have been identified: lava flows, small domes 2-3 kilometres across, larger volcanic cones hundreds of kilometres across, "coronae" and so-called "arachnoids". The coronae of Venus are circular or oval volcanic structures surrounded by ridges, grooves and radial lines. They appear to be collapsed volcanic domes and are different from any features seen on other planets or satellites. The "arachnoids", which get their informal name from their spider-like appearance, are similar in form to coronae, but generally smaller. According to one theory, arachnoids may be precursors to coronae. The bright lines extending outwards for many kilometres indicate formations that may have been created when magma upwelled from the planet's interior, causing the surface to crack.