Mars

Mars is the fourth planet from the Sun - at 227,940,000 kilometres - and the seventh largest in the solar system. It was for a long time believed to have life on it. But the best evidence we have so far is that there is, in fact, no complex life on Mars. It has a diameter of 6 ,794 kilometres and travels around the sun in an oval-shaped pattern. It takes about 687 earth days to make a full orbit rotating on its axis of about 24 degrees from a perpendicular position. Mars rotates once every 24 hours and 37 minutes, thus experiencing a day of almost the same duration as Earth's.

From the Earth, Mars is easily seen in the night sky as a red star-like object that moves through the sky with a period of just over two years. As the orbit of Mars is an eccentric ellipse, its distance from the Earth at opposition (closest approach to the Earth) varies between 1.38 and 1.67 astronomical units. Humans have known of Mars since before recorded history. Even 3600 years ago, the Babylonians wrote about Mars' looping motion across the sky and changing brightness. Mars was one of five "stars that wandered" among the fixed stars of the night, and was special because of its colour: red. In ancient India, Mars appeared like a fire in the sky--for many other cultures, its redness recalled the fire and blood of war. In ancient Greece, the red wanderer personified the god of war, "Ares." When the Romans conquered Greece, they adopted this symbolism and named the planet for their god of war.

At times, Mars is the 3rd brightest object in night sky after the Moon and Venus. Seen through a telescope it appears as a small reddish disk on whose surface dark markings can (with difficulty) be seen. Also visible is one or other of its polar caps. Even at its closest to the Earth Mars is seen as a disk with a diameter of only 25 arc seconds and so with a small telescope very little can be seen. Even with large telescopes it is very difficult to see detail and many experienced observers were deceived into thinking that they had glimpsed features, such as the infamous canals, that in fact were not there. Almost all that we know about the surface of Mars and its atmosphere were discovered by the various space probes that have orbited the planet and landed on its surface.

The surface of Mars shows impact craters, like the Moon, mountains, rift valleys, ridges, hills plains and extinct volcanoes. The largest craters or impact basins on Mars may be buried beneath the northern smooth plains. In the southern hemisphere, the bombardment history typical of all the inner planets is recorded by a few large basins, such as Argyre. Rather than the relatively blocky craters found on the Moon, the oldest terrain on Mars shows degradation of crater rims and evidence of erosion by running water. A comparison of the number of craters in the Martian highlands versus those on the Moon suggests that even the earliest crust of Mars may have long since been buried by more recent volcanic deposits.

There are signs that water existed on the surface at some earlier stage of the planet's history. Winds can be very severe and are responsible for extensive weathering of the rocks on the surface. The surface temperatures on Mars can rise to about 0 C in the summer but fall to near -100 C before sunrise.

The highest point on Mars is Olympus Mons, a huge shield volcano more than 52,000 feet (15,900 meters) high and 370 miles (600 kilometres) across; about the same area as Arizona. The canyon system of Valles Marineris is the largest and deepest known in the solar system extending more than 2,500 miles (4,000 kilometres) and has 3 to 6 miles (5 to 10 kilometres) relief from floors to tops of surrounding plateaus. Canals observed by Giovanni Schiaparelli and Percival Lowell about 100 years ago were a visual illusion in which dark areas appeared connected by lines. The Viking missions of the 1970s, however, established that Mars has channels probably cut by ancient rivers. Although it is only 6794 km in diameter it is probably the most Earth like of all the planets. There are some suggestions that it might be possible to alter the planet's atmosphere sufficiently to enable a permanent base to be set up there. Until that is done Mars will represent a very hostile environment for any human visitor.

Mars has two tiny satellites - Deimos and Phobos - which orbit very close to the surface.