The surface of Mars would be a harsh place for humans, but it is more like the surface of Earth than any other planet. The temperature on Mars does not get much cooler than the temperature at Antarctica. At the surface it ranges from about -225° F to 60° F. During most of the year wind speeds are fairly low-about 4.5 mph-but during dust storms they can approach 40 to 50 mph. These winds often originate in large basins in the Southern Hemisphere and carry large volumes of dust from the basins to other regions, sometimes covering the entire planet in the storm. The dust is not sandy, as in a sandstorm on the earth, but has the consistency of flour.

The northern and southern hemispheres of Mars have different characteristics. The Southern Hemisphere has many impact craters and has a generally much higher elevation than the Northern Hemisphere. The southern highlands are probably the oldest terrain on Mars. The northern hemisphere of Mars contains a much wider variety of geologic features, including large volcanoes, a great rift valley, and a variety of channels. The Northern Hemisphere also contains large expanses of relatively featureless plains.

Mars has the largest volcano in the solar system, Olympus Mons. It is 16 miles high (almost twice as high as the Earth's Mount Everest) and covers an area comparable to the state of Arizona. Near it, three other volcanoes almost as large-Arsia Mons, Pavonis Mons, and Ascraeus Mons-form a line running from southwest to northeast.

The Tharsis bulge has had a profound effect on the appearance of the surface of Mars. The Tharsis bulge includes many smaller volcanoes and stress fractures, in addition to the large volcanoes. Its presence affects the weather on Mars and may have changed the climate by changing the rotation of the planet. Valles Marineris (named for the U.S. Mariner spacecraft that discovered it) is the most notable stress feature associated with the Tharsis bulge. It is a Great Rift Valley extending from the Tharsis region away to the east-southeast. It is about the same length as the distance from New York to California. This canyon system reaches widths of 440 miles and depths of 4 miles.

Hellas Planitia is a giant impact basin in the Southern Hemisphere. The impact of a large meteorite formed the basin long ago. With a diameter of about 1250 miles, it is the largest such basin on Mars.

Three types of channels on Mars were probably formed by the action of water. These channels are unrelated to the "canals" thought to be seen in early telescopic views of Mars. Channel networks are similar in appearance to streambeds on the earth and occur in the southern highlands. These channels may date from a time early in Mars's history when the atmosphere was thicker and liquid water could flow on the surface. Outflow channels, which giant floods may have formed, occur on the boundary between the southern highlands and the northern plains regions. Ares Vallis, where the Mars Pathfinder spacecraft landed, is one of these outflow channels. Landslides and other erosion probably formed fretted channels by enlarging preexisting channels.

Mars has small, permanent ice caps at its north and south poles. The caps increase in size in the winter of

Each hemisphere. The caps in the north and south are quite different from one another. The northern permanent cap is composed of water ice and is about 620 miles across. A seasonal cap of frozen carbon dioxide adds to the northern ice cap in the northern winter. The southern permanent cap is one-third the diameter of the northern cap because summer in the Southern Hemisphere is warmer than in the north. The southern seasonal cap is larger than the northern cap-more carbon dioxide is frozen out in the south than the north because Mars is farthest from the sun, and therefore coldest, in the southern winter. Carbon dioxide may also make up the southern permanent cap.