Venus's surface features include level ground, mountains, canyons, and valleys. Plains make up 65 per cent of the surface, and six mountainous areas cover about 35 percent of the surface. Thousands of volcanoes, between and 150 miles (45 and 240 km) wide, dot the plains. The tallest feature of the planet is a mountain range called Maxwell, about 7 miles (11.3 km) high and about 540 miles (870 km) long. There is a canyon 0.6 mile (1.0 km) deep in a region called Beta Regio. An impact crater forms when a planet collides with an asteroid. Venus has impact craters, but far fewer than the moon, Mars, or Mercury. Scientists conclude from this that Venus's surface is less than a billion years old. Some of its features are not seen on Earth. One is coronae, or crowns, which form when hot material from the planet's interior reaches the surface. Another is tessarae (tiles), which are elevated regions full of furrows and depressions all around.
Scientists have used radio astronomy, space probes, and radar to study Venus's surface, which is permanently shrouded under thick clouds of sulfuric acid. Two Soviet spacecraft that touched down on Venus in 1982 performed chemical analyses of rock samples from their respective landing sites. The analyses indicated that the samples were similar to basalt, a type of volcanic rock found on Earth. The most detailed studies that have been made of the surface of Venus are those made in 1990 and onwards by the United States spacecraft Magellan. While the craft was in orbit around Venus, its radar mapped virtually the entire surface of the planet. These radar studies showed that most of the surface of Venus consists of relatively smooth plains. There are also extensive mountainous regions, roughly circular basins, and numerous large craters. Other studies, including measurements of changing sulfur dioxide concentrations in the atmosphere, suggested that Venus has active volcanoes.