Mars is the fourth planet from the Sun (between the Earth and Jupiter) and has long been referred to as the Red Planet, because it appears to shine with a reddish hue when viewed from Earth. However, recent NASA photographs indicate that the planet isn't really red at all - but is more of a tan or butterscotch color. The diameter of Mars is approximately 6794 kilometers (4219 miles), which is considerably smaller than the diameter of the Earth (12,763 km / 7926 mi). A Martian day is surprisingly similar in length to one on Earth (24 hours and 37 minutes), but a Martian year is nearly twice as long (687 days). Mars has two moons, Diemos and Phobos, both of which are irregularly shaped and pitted with craters. Diemos orbits Mars once every 31 hours; Phobos speeds through the Martian sky three times a day, circling the planet every seven hours.

Except for Earth, Mars has the most highly varied and interesting terrain of any of the terrestrial planets, some of it quite spectacular:
       - Olympus Mons: the largest mountain in the Solar System rising 24 km (78,000 ft.) above the surrounding plain. Its base is more than 500 km in diameter and is rimmed by a cliff 6 km (20,000 ft) high (right).
       - Tharsis: a huge bulge on the Martian surface that is about 4000 km across and 10 km high.
       - Valles Marineris: a system of canyons 4000 km long and from 2 to 7 km deep (top of page);
       - Hellas Planitia: an impact crater in the southern hemisphere over 6 km deep and 2000 km in diameter.
Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains.

The Martian atmosphere is very different from Earth; it is composed primarily of carbon dioxide (95.32%), with small amounts of other gases, such as Nitrogen (2.7%), Argon (1.6%), and Oxygen (0.13%). Martian air contains only 1/1000th as much water as Earth's air, but it can still condense to form clouds, fog, and even frost. The average recorded temperature on Mars is -63° C (-81°F), with a maximum temperature of 20° C (68° F) and a minimum of -140° C (-220° F). Evidence suggests that long ago, under a denser Martian atmosphere, water may have flowed on the planet. Many features strongly resemble shorelines, river beds, gorges, and islands. The recent Pathfinder mission indicated that the jumble of rocks near the landing site had been deposited by a great rushing flood.

         The Red Planet has two small moons that rotate very close to the surface. Phobos has a 22 kilometer diameter and is 9,378 kilometers from Mars. Deimos is 23,459 kilometers from the planet and has a 12 kilometer diameter. Both moons were discovered by a man named Hall in 1877.

Mars has long been the subject of much debate as to whether it does currently or once harbored life of any sort. Although the question of whether life once existed in Mars's distant past has yet to be answered, scientists generally conclude that life on the Mars of today is an impossibility. Mission biologists contend that Mars is now self-sterilizing: the combination of the solar ultraviolet radiation saturating the surface, the extreme dryness of the soil, and the oxidizing nature of the soil chemistry prevent the formation of living organisms on the Martian surface.

Perhaps the most enduring image from the surface of Mars (and certainly the most controversial) is that of "The Face," which was first photographed by Viking Orbiter 1 in 1976. Subsequent images from the Mars Global Surveyor spacecraft of this outropping in the Cydonia region show that it is nothing more than a bumpy hill.

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