Perhaps the most remarkable aspect of the Martian surface is the distribution of craters larger than 10km in diameter, which are much commoner in the southern hemisphere than hi the northern. Much of the northern hemisphere has been flooded with lava and this has evidently obliterated the craters that must have existed at one time. Using estimates of the present rate of crater formation on Mars, we can deduce that most of the large craters in the southern hemisphere were formed early in the planet’s history, probably during the first billion years. The lava plains that have covered the northern hemisphere are very similar to the lunar maria. There is plenty of variation in the density of craters on these plains with the most heavily marked regions having ten times as many craters as the least marked. This variation shows that the lava flooding must have extended over a considerable period from more than a billion years ago until almost the present. Why the flooding should have occurred preferentially in the northern hemisphere is not known, but the reason is probably connected with the fact that much of this hemisphere lies several kilometers below the average level of the southern hemisphere. The lava plains are not the only volcanic features on Mars; there are also the volcanic craters already described in the section on plate tectonics.

Martian craters show a wide variation in the extent to which they have been degraded. On the volcanic plains of the northern hemisphere nearly all the craters have a fresh appearance, showing that since the plains were formed there has been little significant erosion. Earlier in the history of Mars, the erosion must have been much more rapid and detailed studies of the forms of craters have shown that the time of most rapid erosion occurred near the end of the initial cratering episode that produced the major southern features.

Long before the advent of spacecraft observations, it was recognized that there could be no liquid water on the surface of Mars. It came as a surprise when sinuous channels up to 1500 km long and 200 km wide were discovered. They look just as if they were formed by erosion due to a running liquid; they have tributary systems and almost without exception the direction of flow is downhill. The channels are quite unlike the lava channels (rilles) found on the Earth and the Moon and it is clear that a liquid much less viscous than lava was responsible. The most likely candidate is water, and if this is correct then Mars, or at least the equatorial regions where the channels mainly occur, must once have been warmer and wetter than it is today. The source of the water is not clear for most of the channels, but in some cases it appears to have been a region of what is called CHAOTIC TERRAIN. Such terrain is a depressed region characterized by a disorderly array of broken slabs of rock. It is thought to be due to the withdrawal of subsurface material and the subsequent melting of surface ice. FRETTED TERRAIN is another landform that may have resulted from the melting of ice below the surface causing landslides. This terrain is a flat lowland, bordered by steep cliffs with an intricate geometry. Close to the cliffs there are numerous small elevated plateaus.

It must be pointed out that these sinuous channels have no connection with the canals reported by observers in the late nineteenth and early twentieth centurion. These canals were supposed to be long, accurately straight features on the limits of telescopic observations and some people fancifully took them to be a network of irrigation canals constructed by inhabitants of Mars to carry water from the polar caps to the rest of the planet. Spacecraft photographs have shown conclusively that the canals do not exist and that most of them are totally spurious features. In a few cases it appears that the observer’s eye mentally joined together several smaller features that happened to lie in a more or less straight line.

The tilt of the Martian rotation axis is very nearly the same as the Earth’s and as a result there are seasons on Mars . The orbit of Mars around the Sun is appreciably elliptical and the planet is at perihelion during the southern summer. As a result the southern summers are hotter and shorter than those in the north. Mars has polar ice caps which contract each summer and grow again during the following winter. At maximum extent they can reach down to a latitude of 60°. The southern cap disappears almost completely during some summers, but because of the cooler summers in the northern hemisphere the polar cap there is never reduced to such a small size. The caps are made partly of carbon dioxide ice which vaporizes directly and condenses again without passing through the liquid state. A large fraction, possibly as much as 20 per cent, of the atmospheric carbon dioxide is held in the polar caps. There is also probably some water ice in the caps. This never melts and may form the permanent cap left each summer. At latitudes above 45° the ground below the martian surface is always below the freezing point of water and a layer of permafrost may have formed there similar to that in the Earth’s polar regions. There may also be more water trapped in the REGOLITH, the granular material that lies above the bedrock. Altogether about one per cent of the surface material could be water, enough to form a layer a few tens of meters deep if it were all melted and spread evenly over the surface.

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