Mars Mars is the fourth planet from the sun and orbits the sun at a distance of about 141 million mi. Mars is named for the Roman god of war because it appears fiery red in the earths night sky. Mars is a small planet that has about half the diameter of Earth and about one-tenth Earths mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earths moon.

The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. The Martian day, or the time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year, or the time it takes to revolve once around the sun, is about two Earth years long. Mars has two moons, Phobos and Deimos. THE INTERIOR OF MARS Scientists believe that Marss interior consists of a crust, mantle, and core like Earths interior, but they do not know the relative sizes of these components. Because no spacecraft has ever brought instruments that can study Marss interior to the planet, the only real data that scientists have about the planets structure are its mass, size, and the structure of the gravity field.

Compared to Earth, Mars probably has a relatively thick crust. Beneath the surface is an area of volcanic activity in the northern hemisphere, it may be as thick as 80 mi. Beneath the landing site of the United States spacecraft Viking 2, it may be as thin as 9 mi. The core is probably consists of mostly iron, with a small amount of nickel. Other light elements, mainly sulfur, could exist in the core also. If so, the core may be quite large. Mars does not have a significant magnetic field, so scientists believe that Marss core is probably solid.

Mars does not, and probably did not ever, have active plate tectonics. Because Mars is so much smaller than Earth, it must cooled quickly after formation and the crust thickened, forming one solid piece and eliminating any possibility of plate tectonics as it was on and still is on Earth. Though the Martian crust is not broken into separate plates, Marss liquid mantle has sculpted the planets surface. The molten rock has broken through the crust to form volcanoes and its motion has cracked the crust to form large rifts. THE SURFACE OF MARS 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 -140 C to 15 C (about -225 F to 60 F). During most of the year wind speeds are normally low around 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 ground 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 mi high (almost twice as high as the earths 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. These four volcanoes are the most noticeable features of a large bulge in the surface of Mars, called Tharsis. Another volcano, Alba Patera, is also part of the Tharsis bulge, but is quite different in appearance.

It is probably less than 4 mi high, but has a diameter of 1000 mi. None of Marss volcanoes appear to be active. The Tharsis bulge has had a large 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 mi and depths of 4 mi.

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 Marss 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. The Mars Pathfinder spacecraft found minerals in Ares Vallis that are similar to minerals that form near water on Earth, supporting the theory that Mars had liquid water at some point in its history. 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 hemis …