Volcanology of Venus
The surface of Venus is dominated by volcanic features and has more volcanoes than any other planet in the Solar System. It has a surface that is 90% basalt, and about 65% of the planet consists of a mosaic of volcanic lava plains, indicating that volcanism played a major role in shaping its surface. There are more than 1,000 volcanic structures and possible periodic resurfacing of Venus by floods of lava. The planet may have had a major global resurfacing event about 500 million years ago, from what scientists can tell from the density of impact craters on the surface. Venus has an atmosphere rich in carbon dioxide, with a density that is 90 times greater than Earth's atmosphere.
Atmosphere of Venus
The altitude of the troposphere most similar to Earth is near the tropopause—the boundary between troposphere and mesosphere. It is located slightly above 50 km. According to measurements by the "Magellan" and "Venus Express" probes, the altitude from 52.5 to 54 km has a temperature between 293 K (20 °C) and 310 K (37 °C), and the altitude at 49.5 km above the surface is where the pressure becomes the same as Earth at sea level. As manned ships sent to Venus would be able to compensate for differences in temperature to a certain extent, anywhere from about 50 to 54 km or so above the surface would be the easiest altitude in which to base an exploration or colony, where the temperature would be in the crucial "liquid water" range of 273 K (0 °C) to 323 K (50 °C) and the air pressure the same as habitable regions of Earth. As CO is heavier than air, the colony's air (nitrogen and oxygen) could keep the structure floating at that altitude like a dirigible.