Mars is the fourth planet from the Sun and there has been much speculation over the years about the possibility of other life forms existing there.
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 (the mean distance of the Earth from the Sun is one Astronomical Unit). Seen through a telescope Mars 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 arcseconds 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.
We know that the atmosphere of Mars has a pressure less than one hundredth of the Earth's and that it is composed mainly of carbon dioxide with a little nitrogen and argon; there is almost no oxygen. There is a small amount of water vapour which condenses in some places to give thin clouds and fog. There are polar caps at each pole which grow in the Martian winters, with the addition of deposits of solid carbon dioxide, and decrease in the Martian summers to leave what is thought to be a small residue of ordinary water ice.
The surface of Mars shows impact craters, like the Moon, mountains, rift valleys, ridges, hills plains and extinct volcanoes. There are signs that water existed on the surface at some earlier stage of the planet. Winds can be very severe and are responsible for extensive weathering of the rocks on the surface. Sometimes the winds blow up enormous sandstorms that obscure the view of the planet's surface. The surface temperatures on Mars can rise to about 0C in the summer but fall to near 100C before sunrise.
Mars rotates in 24 hours and 37 minutes about an axis tilted by 24deg. to its orbital plane. Although it is only 6794 km in diameter it is probably the most Earthlike 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 moons, Phobos and Deimos. They are both very small, both being less than 30 km across. It is likely that they were both asteroids that have been captured by Mars. Phobos is very close to Mars and its orbital period is less than the rotational period of Mars. It thus would be seen to rise in the west and set in the east! Deimos is further away from Mars and would be seen to behave more conventionally.
Is there life on Mars? In August 1996 researchers reported finding evidence of fossils of very primitive organisms in meteorite samples believed to come from Mars. Even if these results are confirmed this is a long long way far from life as we know it. However it offers the exciting possibility that the Earth is not a unique place in the Universe where life exists.
