The Surface of Venus
The surface of Venus is a very active place. On large scales the surface features are similar to those of the Earth in that there are continents, basins, volcanoes, trenches and so on. As the surface is permanently obscured by cloud (see The Atmosphere), more indirect methods must be used to map the surface. One probe which did this was the Magellan spacecraft which used radar from orbit to scan the surface and build up a topographic map that resolved features down to scales of about a metre.
Despite the inhospitable nature of the surface, there have also been several attempts to land spacecraft on Venus. The Venera series of probes, launched by the former Soviet Union between 1967 and 1983, included both orbiters and landers. The orbiters mapped the surface using radar and the landers, while only able to function for a short time before being crushed by the intense pressure, took photographs and rock samples. In-situ analysis of the samples showed that the rock was a kind of lava similar to that from shield volcanoes on the Earth.
Volcanoes are not the only Earth-like features seen on the surface of Venus. There are also numerous large mountain ranges and faults like those seen near active plate boundaries on our planet - the Himalayas and the San Andreas fault are well-known terrestrial examples. There are also other features which, although present on the Earth, are often obscured by extensive weathering over large periods of time. This indicates perhaps that the surface of Venus is comparatively young. In contrast, the youngest parts of the Moon's surface are the darker mare (or "seas") which have ages ranging from 3 to 4 billion years, implying that there has been no major geological activity on the Moon for at least 3 billion years.
One piece of evidence for a young surface is the size distribution of craters. On the Earth, small craters are easily eroded by the effects of the atmosphere (wind, rain, etc.) over time. By comparing the number of craters of a certain size, Venus shows an apparent lack of small craters. You might think that this is due to similar weathering processes, but the larger craters that are detected appear to be very well defined - if weathering was obliterating small craters, then the effects on larger craters should also be visible. So what is happening? Once again the answer lies in the atmosphere. The thickness of the atmosphere causes smaller meteors to burn up long before they get the chance to impact on the surface and make a crater.
What else can we learn from the crater distribution? By counting the number of craters in a specific region you can get an estimate of the age of the surface. As you might expect, older regions have more craters while younger regions have fewer. This method of crater counting gives an estimate for the age of the Venusian surface of 300 million years; this is one of the youngest surfaces in the Solar System again implying that there has been recent major geological activity on the planet.
As Venus is so similar in composition to the Earth, there has been a lot of debate over whether plate tectonics may have caused some of the large scale surface features. However, on Venus there are no "plates" as there are on Earth, but there is still movement of hot sub-surface magma (molten rock) which can alter the surface in various ways. One example of this is the elevated area known as Ishtar Regio, the highest peak of which is Maxwell Montes. But how did this region come to be raised up? One idea is that it is the result of an upwelling of hot magma (a plume) from deep in the mantle. An alternative idea is that the region has been compressed and pushed up over a region of convective down flow where rock is sucked down into the mantle, similar to the Himalayas on Earth.