[11] MARS, the outermost of the four terrestrial planets (Mercury, Venus, Earth, and Mars), is the second closest to Earth after Venus. It is slightly more than half the size of Earth and almost twice the size of the Moon. The atmosphere is very thin, less than one one-hundredth that of Earth, and composed primarily of carbon dioxide.

Temperatures are cold, the mean annual surface temperature being approximately -50°C at the equator and close to -130°C at the poles. Because of the thin atmosphere, the diurnal temperature range is large, greater than 100°C at the equator. Summer temperatures rise above 0°C at midday despite the low diurnal mean.

The rotational axis is inclined to the ecliptic, like Earth's, so that Mars experiences distinct seasonal weather patterns. A particularly striking seasonal event is the annual dust storm. During summer in the southern hemisphere, large dust storms develop and obscure much of the planet's surface from view. Such storms, long known from telescopic observations, were observed from orbit by Mariner 9 and both Viking orbiters. During the height of the 1977 dust storm season, wind speeds up to 25 meters/sec were recorded by the Viking landers, although they were far from the center of dust storm activity.

Another regular seasonal event is the formation of clouds of carbon dioxide ice particles in the polar regions during the fall as gas starts to condense out of the atmosphere onto the growing cap. So much of the atmosphere condenses out in this process that atmospheric pressure decreases more than 30 percent from fall to winter. The pressure decrease is smaller in northern winter because of the smaller northern cap.

Other cloud activity is related to water in the atmosphere. Although very small amounts of water are present, the atmosphere is close to saturation much of the time, and a wide variety of water ice clouds have been observed.

The Martian surface has some characteristics of Earth's surface, some of the Moon's, and some unique features. The planet is very asymmetric in appearance. Most of the southern hemisphere is densely cratered and superficially resembles the lunar highlands. In contrast, the northern hemisphere is relatively sparsely cratered d and has many large volcanoes that have no lunar counterparts.

The most prominent volcanic region is Tharsis, where, there are several very large volcanoes that resemble terrestrial shield volcanoes, such as those in Hawaii, except that those on Mars are many times larger. The different features of the volcanoes, such as calderas, lava flows, and lava channels are also many times larger than their terrestrial counterparts.

[12] Tharsis is close to the center of a 6000-km diameter, 7-km-high bulge in the Martian crust Numerous fractures radiate from the center of the bulge and extend out as far as 4000 km. The fractures are arrayed unevenly and are concentrated in intensely fractured zones called fossae. The radial fractures are so extensive that they are the dominant structural element over half the planet's surface.

Close to the equator, east of Tharsis, a series of vast interconnected canyons constitutes the Valles Marineris. These canyons also are enormous by terrestrial standards, being almost 4000 km long, up to 250 km across, and up to 9 km deep. The canyons are aligned radial to the center of the Tharsis bulge, and appear related in some way to the radial fractures.

To the east, the canyons become shallower and merge into a type of terrain peculiar to Mars: Large areas of the surface apparently collapsed to form arrays of jostled blocks that are at a lower elevation than the surrounding terrain. Because of the jumbled nature of the surface, this terrain has been termed chaotic.

From many of the regions of chaotic terrain, large, dry river beds emerge. The channels generally start full size and extend down the regional slope for several hundred kilometers. Most large channels emerge from the chaotic terrain just east of Valles Marineris and flow into the Chryse basin to the north, but several occur elsewhere. In addition to these very large channels, numerous smaller tributary systems and dendritic drainage networks are present throughout the equatorial regions.

The origin of the channels is not known, and they have been the subject of a lively debate since their discovery in 1972. The main issue is whether or not they were formed by running water. If they were, then different climatic conditions in the past may be implied. There are also some intriguing biological implications if "wet" periods have occurred in the planet's history.

The effects of wind are evident over almost all the Martian surface. Many of the classic dark markings apparently are associated with wind activity. They can commonly be resolved into arrays of streaks that start at craters and are aligned parallel to the predominant winds. The streaks may be lighter or darker than the surroundings.

Wind action is also evident from the streamlined form of many features. High resolution pictures show that the north pole is almost entirely surrounded by dune fields that form a dark collar around the pole. Dunes also occur elsewhere, such as in the canyons and within craters, especially in high southern latitudes. Thick sequences of layered deposits of unknown origin are found at both poles. They lie unconformably on the terrain and appear to be very young compared with most other features on the planet. The deposits possibly are accumulations of windblown debris mixed with condensed volatiles like water.

The geologic histories of Mars and Earth are quite different, partly because of the internal dynamics of the planets and partly because of the differing effects of the atmospheres and oceans. Earth's geology is dominated by the effects of plate tectonics. The rigid outer shell of the [13] Earth (the lithosphere) is divided into plates that move laterally with respect to one another. Where plates diverge, as at midoceanic ridges, new crust forms; where r' they converge, one plate generally rides under the other to form a subduction zone. Thick sediments may accumulate in a subduction zone; these may ultimately be compressed, partially melted, and uplifted to form linear mountain chains of folded and partly metamorphosed rocks, such as the Andes and the Himalayas. Melting of the subducted plate as it moves clown into the mantle may also given rise to volcanism in the subduction zone. Where plates move laterally with respect to one another, they form transcurrent faults such as the San Andreas. The present configuration of the Earth's surface is thus a partial record of the motion of different plates with respect to one another.

Mars displays little, if any, evidence of plate motion. The crust appears very stable . Long linear mountain chains and subduction zones are absent, and transcurrent faults and compressional features of any kind are rare. Its geologic history is thus very different from that of the Earth.

Crustal stability on Mars results in the preservation of much older features. On Earth, surface materials are recycled at a relatively rapid rate by erosional processes and subduction. The two processes are commonly interdependent; for example, erosion is greatly increased in mountainous regions along subduction zones. On Mars, however, recycling of crustal materials is extremely slow, as evidenced by the preservation of large areas of old, densely cratered terrain that probably dates back approximately four billion years.

Crustal stability may also be the cause of the large size of the Martian volcanoes. On Earth, volcanoes are limited in size because plate motion usually carries them away from the magma source. On Mars, however, a volcano remains over its source and can continue to grow as long as magma is available.

The preservation of features billions of years old on the Martian surface indicates extremely low erosion rates. On Earth, most erosion results from running water. Small channels in the old cratered terrain of Mars are evidence of an early period of fluvial action, but survival of the old craters indicates that the period was short. For most of the planet's history, wind has probably been the main erosive agent. Despite giant dust storms, however, the wind clearly has not been very efficient in eroding the surface, because so much old terrain survives. Most of the wind's action probably involves reworking previously eroded debris.

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