Mars is the outermost of the terrestrial planets. It shares a family resemblance with the other three -- Mercury, Earth and Venus -- in so far as it is small, rocky and dense, with a relatively thin, shallow atmosphere, and orbits relatively close to the Sun. It is made of similar materials to the Earth, and is about half the size. Its average density is about 3.9, compared to Earth's 5.5, and its gravity is 38% of our own.

However, like the Earth it is a unique planet, which has its own story to tell. And that story is being revealed by a series of space missions that have made contact with the planet since the 1960's. The present decade, though, has witnessed an increase in the number of Mars missions and new data is being returned to us every day. The information given here takes account of findings up to June 1999, but it may be out of date by the time you read this; the pace of progress is so fast.

In appearance, Mars is very different from the Earth. First there is its red colour, caused by oxidised iron-rich dust on its surface. Although there are clouds and winds, the atmosphere is searingly thin, being equal to our own at a height of 30,000 metres (100,000 feet), and made of 95% carbon dioxide. There is no liquid water available at the surface, and the atmosphere is extremely dry, containing only 0.03% water vapour. The poles are covered by small icecaps, which are different from each other: the North Polar Cap is largely water ice, the South Pole is so cold that its cap is has more solid carbon dioxide. The combined quantity of ice from both caps is equivalent to about one and a half times that covering Greenland. Both poles show laminated terrain, indicative of successive periods of accumulating dust. Mars only receives 44% of our amount of sunlight, and there is hardly any ozone, so UV light bathes the surface almost unhindered.

Mars is split into two very different areas by what is called the Crustal Dichotomy. This is a division, at roughly 35 degrees North of the Martian equator, between the southern and northern hemispheres. The southern hemisphere is characterised by older, heavily-cratered terrain, whilst the northern hemisphere is relatively smooth, and largely volcanic plains. Above the 40 degree latitude line, these volcanic plains give way to smooth plains of sediment, showing buried craters, heavily suggestive of once being a sea bed which has now dried out. These plains have recently been discovered by Mars Global Surveyor to be the smoothest areas in the Solar System. The southern cratered uplands also contain evidence of flowing water in the form of braided channels, which occur in inter-crater areas.

Although there are no mountain chains as on Earth, Mars has a greater range of elevation than Earth. As there is no "sea level", an arbitrary line or "datum" has been set. This is related to air pressure and gravity, at a level where the pressure is 6.1 millibars. The southern uplands lie between 1-3 km (0.6 to 2 miles) above this datum, and the northern plains are mostly below it. The real variations in topography are caused by the huge shield volcanoes, which are concentrated in an area known as the Tharsis Bulge. This feature covers around 20% of the planet's surface and bulks up to 9 km (6 miles) above the datum, with some volcanoes reaching even higher. At the other extreme is Valles Marineris, an immense rift valley originating in Tharsis, 7 km (4.5 miles) deeper than the surrounding plains!

It is obvious that volcanism has played an enormous role in the shaping of the Martian surface. The volcanoes of Tharsis tower up to 26 km (18 miles) in height and Olympus Mons, the largest, is about 1,000 km (600 miles) across at its base. Mars Global Surveyor has found extensive evidence of layering in valley walls, suggesting several periods of extreme and prolonged volcanic activity. Mars was a very active world in the past.

If Mars were to regain its seas, there would be two small continents north of the equator: Tharsis in the West and Elysium in the East. The southern uplands would be high and dry, and the Valles Marineris would fill with water.

Were you to walk south from the North pole, you would see strange layered terrain, with alternate layers of dust and ice up to 3 km (1.6 miles) thick. Next would come the Mare Boreum - a region of dark dunes surrounding the pole. Continuing south you enter the Vastitas Borealis, or "Northern Waste", a largely featureless flat plain with very few craters - indicating that it may be one of the youngest parts of Mars. The Vastitas merges into the Acidalia Planitia, which is very similar to the Vastitas, but darker and with more evidence of crater ejecta. West of here is the volcano Olympus Mons. We next move into the area called Xanthe Terra which contains the Chryse Planitia, or "Plains of Gold", where NASA's Viking 1 and Pathfinder missions touched down. Chryse appears to be an outflow point for water, but is still fairly featureless. Near here to the east is the area of Cydonia, where the famous "Face" was supposed to be. This is now known to be just an eroded mesa. Further on, you come to the eastern extremity of the Valles Marineris, or "Mariner Valley", an immense system of rift valleys originating in Tharsis that would span the entire continental USA!

Your journey south takes you into one of the darkest regions on Mars, the Margaritifer Terra, an area of heavily cratered southern highland containing the large impact basin of Argyre Planitia. The companion to Argyre is Hellas, produced by an even larger impactor. This lowland zone is relatively featureless and full of dust. It is now known to be the lowest place on Mars, nearly 9 km deep (6 miles). Many of Mars' dust-storms seem to originate here. Between Hellas and Argyre is Noachis Terra, which is heavily cratered. This area contains many water-carved run-off channels, probably caused by recurrent flooding. This landscape is probably the oldest on Mars, dating back perhaps as much as 3.9 billion years.

Finally, we reach the southern polar region. Because of the ellipticity of Mars' orbit, southern winters are longer than northern ones. Southern summers are also colder than northern ones, because of its obliquity (wobble).. During the southern winter Mars is farther from the Sun than at other times of its year, so the temperatures generally, all over the planet, are lower.

Mars Global Surveyor is currently in its mapping phase, and is returning huge amounts of high-resolution data, so the picture of Mars is changing all the time. For example, recent photos show areas of tectonic expansion similar to that of the mid-Atlantic ridges on Earth. This is strongly suggestive of tectonic plate movement, which previously had been doubted as regards Mars, because of its lesser size.

All the above text must be seen as open to change. Obviously, these comments are meant to be read in conjunction with visual materials. Please refer to the media list in this Module.

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