Mars is distinguishable by its intense reddish-orange hue, stronger than the colour of any star and the cause of its association with the god of war. At its best Mars can shine at magnitude -2.8, rivalling Jupiter. But the most favourable appearances of the planet are few and far between, the main reason being the marked ellipticity of its orbit, which takes it from 206 to 249 million km from the Sun (average distance 228 million km). Mars has a diameter of 6790 km, just over half that of the Earth. Its day is just over half an hour longer than our own-24 hours 37 minutes - but its year is nearly twice as long, 687 Earth days. Its orbit lies outside the Earth's, so Mars can never appear as a crescent. But at times it displays a distinctly gibbous phase, like the Moon a couple of days from full. The temptation to assume too many similarities between Mars and the Earth led early astronomers astray. The dark surface markings of Mars, which range in colour from brown to grey-green, were termed seas and lakes in the belief that they really were filled with water, while the orange areas which make up the rest of Mars's surface were named after places on Earth - there is an Arabia, Libya, Syria and Sinai on Mars. Towards the end of the 19th century astronomers realized that there were no oceans on Mars after all, but this opened the way for a much more intriguing explanation for the dark areas: that they were covered with primitive vegetation, such as moss or lichen. In support of this view, observers noted that when the polar caps melted in the Martian summer, the surface markings became larger and darker; this was interpreted as vegetation growing in the milder, wetter conditions. The most extreme proponent of the life-on-Mars idea was an American astronomer, Percival Lowell. His ideas were inspired by an Italian observer, Giovanni Schiaparelli, who in 1877 reported seeing long, straight lines that appeared to criss-cross the planet's surface. Schiaparelli called these lines canali, a word which in Italian means 'channels'; but inevitably it was translated as 'canals', implying that they were artificial, although Schiaparelli himself kept an open mind about their true nature. For Lowell, there was no doubt: the canals were evidence of an advanced civilization on Mars. His beliefs sparked off a whole generation of science fiction, including the famous War of the Worlds by H. G. Wells. Most other astronomers, though, failed to see the canals, or in their place could detect only broad, irregular smudges. After Lowell's death in 1916 a few devoted followers kept the canal theory alive, but the idea of Martian civilization was doomed to oblivion in the light of new knowledge about conditions on the planet. By the 1950s it was clear that the atmosphere on Mars was far too thin for a human to breathe. Under so thin an atmosphere, temperatures would be frigid and dangerous amounts of ultraviolet radiation would penetrate to the ground. To make matters worse, no oxygen could be detected in the planet's atmosphere, although carbon dioxide was known to be present. No advanced life form could exist under such conditions, although hardy vegetation was still not ruled out. Mars is the subject of an ongoing programme of exploration by space probes, which started in July 1965 when the American craft Mariner 4 swooped past at a distance of 10,000 km, sending back the first close-up photographs. Its most important revelation was that there are craters on Mars, looking similar to those on the Moon only somewhat more eroded because of the effects of the planet's atmosphere. Unfortunately, a lunar-like Mars seemed an unpromising abode for life of even the lowliest form. The image of Mars as a dead world, in both the geological and biological sense, was reinforced in 1969 when Mariners 6 and 7 photographed more craters, apparently all of impact origin. But attitudes changed when Mariner 9 went into orbit around Mars and made the first survey of the entire planet in 197102. It revealed major formations that previous Mariners had, by ill luck, completely missed. For a start, there was a chain of three volcanic mountains atop a highland area known as the Tharsis Bulge which straddles the equator; these volcanoes are named Arsia Mons, Pavonis Mons and Ascraeus Mons ('Mons' means mountain). Other types of feature on Mars and the names given to them include lowland plains (Planitia), highland plateaus (Planum), valleys (Vallis), canyons (Chasma), and eroded craters (Patera). Northwest of the chain of Tharsis volcanoes is an even bigger volcanic mountain, Olympus Mons, originally seen from Earth as a white ring and then known as Nix Olympica ('the snows of Olympus'). Olympus Mons, 600 km wide and 27 km high, is the largest volcano in the Solar System, larger even than the volcanic islands of Hawaii on Earth. The whole Tharsis area is noted for the frequent appearance of white clouds, often described as being shaped like a letter W. The presence of mountains explains the preference of clouds for this region. Another dramatic and unexpected feature was an immense system of canyons, up to 500 km wide and 4 km deep, extending east from the Tharsis Bulge. This massive rift in the surface of Mars, now called the Valles Marineris (Mariner Valleys, named in the plural because of their multiplicity), not merely dwarfs the Earth's Grand Canyon, but at 4000 km long could span the entire United States. Valles Marineris corresponds to a broad, fuzzy 'canal'seen from Earth, called Coprates; it is visible from Earth mainly because dark dust collects on its floor. Coprates is one of the few supposed canals that has any corresponding surface feature. A few other canals, notably one called Cerberus, do coincide with dark streaks on the Martian surface, caused by darker dust or rock; but all of these are broad and irregular markings, unlike the thin, straight canals that Lowell drew. Close comparison of Lowell's canal maps with space probe pictures reveals few correlations at all. There seems no real explanation for the canal network that Lowell and his followers drew, other than the fallibility of human observers straining at, or past, the limits of visibility. Although the Lowellian canals were not in evidence, Mariner 9 found convincing signs that water had once flowed on Mars, an impression amply confirmed by subsequent probes. Sinuous channels, looking like dried-up river beds, snake across parts of the planet's surface. Some of the lowlands appear to have once been inundated by flash floods. Liquid water cannot exist on Mars today because the atmospheric pressure is too low. The existence of ancient watercourses imply that in the past the atmosphere was denser - and a denser atmosphere would keep the planet warmer. Perhaps the volcanoes of Mars gushed out enough gas to change the climate temporarily. If that were so, life might have had a chance to arise on Mars after all, and microscopic organisms such as bacteria might still cling to existence among the red sands of the planet. It comes as a surprise to learn that Mars today is not short of water, although most of that water is in frozen form, both in the polar caps and in a subsurface permafrost layer polewards of about 30° latitude north and south. This offers another explanation for the ancient water channels of Mars: the melting of subsurface ice by volcanic heating or meteorite impacts. Whether this, rather than a more Earth-like climate, was the real source of the ancient floods, or whether a combination of both factors was responsible, remains unresolved. The polar caps of Mars are made of water ice a few metres thick, augmented each winter by carbon dioxide. Carbon dioxide freezes out of the atmosphere to produce a thin sprinkling of frost that can extend more than halfway to the equator. During each Martian year the atmospheric pressure changes by 20 per cent or more as carbon dioxide evaporates from one polar cap with the coming of spring, migrates to the opposite hemisphere and then freezes out again as winter arrives at the other pole. One final puzzle from the pre-space probe era of observation remained to be explained: what causes the seasonal changes in the dark areas if there is no vegetation on Mars? Wind-blown dust provides the answer. Orbiting probes have recorded many examples of changes in surface markings caused by light and dark dust being blown by seasonal winds. Syrtis Major, for example, is a gently sloping area of darker volcanic rock which periodically becomes partly covered by paler dust and is later swept clear again. Dust blown by the winds on Mars, which can reach 200 km per hour, is also a powerful erosive agent, sandblasting the surface features of the planet. No discussion of Mars would be complete without reference to its two tiny moons, Phobos and Deimos. They were discovered in 1877 by Asaph Hall using the 66-cm (26-inch) refractor at the US Naval Observatory in Washington, DC, and both are beyond the range of normal amateur telescopes. Space probe photographs show that both are cratered chunks of rock shaped like lumpy potatoes. Phobos, larger and closer to Mars, measures about 27 ´ 18 km; Deimos is about 15 ´ 10 km. Phobos is remarkable in that it orbits Mars three times a day. It also lies closer than any other moon to its parent - a mere 6000 km above the surface of Mars. Probably they are asteroids which strayed too close to Mars and were captured by the planet's gravity. They will make a fascinating sight in the sky for the first astronauts to set foot on Mars, the planet of red, frozen deserts which was so nearly suitable for life of its own.