magnetic field, about 2 per cent the strength of the Earth's. The crust is rich in olivine and ferrous oxide, which gives the rusty colour. The tenuous martian atmosphere is composed of 95.3 per cent carbon dioxide, 2.7 per cent molecular nitrogen and 1.6 per cent argon, with oxygen as a major trace constituent. The atmospheric pressure at the surface is only 0.7 per cent that at the surface of the Earth. However, strong winds in the atmosphere cause extensive dust storms, which occasionally engulf the entire planet. A variety of clouds and mists occur. Early-morning fog forms in valleys and orographic clouds, formed when air masses cool as winds drive them from low to high ground, appear over the high mountains of the Tharsis region. In winter, the north polar cap is swathed in a veil of icy mist and dust, known as the polar hood. A similar phenomenon is seen to a lesser extent in the south. The polar regions are covered with a thin layer of ice, thought to be a mixture of water ice and solid carbon dioxide. High-resolution images show a spiral formation and strata of wind-borne material. The north polar region is surrounded by stretches of dunes. The polar ice caps grow and recede with the seasons, which arise - as they do on Earth - because the planet's rotation axis is tilted (by 25А) to the orbital plane. The martian year is about twice the length of the Earth year, so the seasons are also longer. However, the relatively high eccentricity of Mars's orbit makes them of unequal duration: southern summers, which occur when Mars is near perihelion, are shorter and hotter than those in the north. Seasonal changes in the appearance of features as observed from Earth are explained as physical and chemical changes. There is a marked difference in the nature of the terrain between the two halves of Mars divided roughly by a great circle tilted at 35А to the equator. The more southerly part consists largely of ancient, heavily cratered terrain. The major impact basins - the Hellas, Argyre and Isidis planitiae - are located in this hemisphere. The north is dominated by younger, more sparsely cratered terrain, lying 2-3 kilometres lower. The highest areas are the large volcanic domes of the Tharsis and Elysium planitiae. Both areas are dominated by several huge extinct volcanoes, the largest of which is Olympus Mons. These volcanic areas are located at the east and west ends of an immense system of canyons, the Valles Marineris, which stretches for more than 5,000 kilometres (3,000 miles) around the equatorial region and has an average depth of 6 kilometres. It is believed to have been caused by faulting associated with the upthrust of the Tharsis dome. There is evidence, in the form of flow channels, that liquid water once existed on the surface of Mars. Channels from the Valles Marineris appear to have been created in some kind of sudden flood. There are also sinuous, dried-up river beds with many tributaries, found only in the heavily cratered terrain. Mars has two small natural satellites, Phobos and Deimos, which are in near-circular orbits in the equatorial plane, close to the planet. They are very difficult to see from Earth. They are so different from Mars that it seems likely they are captured asteroids.