Red Planet

Background

The seasons of Mars

The rotational axis of Mars is tilted by 24 degrees to its orbital axis - very similar to the axial tilt of the Earth. Because of this Mars experiences seasons in much the same way as the Earth does. However, Mars's orbit of the Sun takes 687 Earth days (compared with Earth's 365) so its seasons are correspondingly longer than Earth's.

The highly elliptical orbit of Mars has a noticeable effect on its seasons. When Mars is closest to the Sun (and travelling more quickly) its southern hemisphere is tilted towards the Sun. As a result, southern spring lasts only 143 Martian days, and southern summer 154 Martian days - southern summer is hotter and shorter than northern summer. Northern spring lasts 194 Martian days and northern summer 178 Martian days. Northern winter coincides with southern summer and is correspondingly short, and the other seasons are similarly reversed. Southern winter, when Mars is furthest from the Sun, is colder and longer than northern winter. The southern hemisphere therefore experiences a wider range of temperatures than the northern hemisphere.

The mystery of Mars's missing water

As soon as measurements by the two US Viking landers had confirmed the low atmospheric pressure and temperatures on the surface of Mars, it became obvious that Mars must be extremely dry today: a 'freeze-dried desert'. Water can only exist as a liquid over a small range of pressures and temperatures, and the conditions on Mars today would result in any liquid water rapidly boiling and evaporating into the thin Martian atmosphere. Therefore liquid water cannot exist on the surface of Mars today.

However, when orbiting spacecraft obtained detailed photographs of the Martian surface, features resembling dried-up riverbeds became apparent. There were also erosional features which looked like flash-flood plains. This evidence supported the theory that liquid water once flowed on the planet's surface. Scientists were therefore faced with the problem of Mars's missing water. Part of this water undoubtedly remains as ice in the residual polar caps of Mars. With the arrival of summer, the frozen carbon dioxide (dry ice) layer in the polar ice caps rapidly evaporates, exposing a layer of water ice beneath which survives the chilly Martian summers. However, the thickness of this layer of water ice has not yet been determined.

The flash floods on Mars appear to have emerged from regions of collapsed, jumbled terrain. This would indicate that the water came from the melting of a layer of permafrost beneath the Martian surface (similar to that found in the tundra in far northern regions of the Earth). If heat from volcanic activity occasionally melted this sub-surface ice, the ground would collapse as vast quantities of water were forced to the surface, producing a brief flash flood before the water boiled away. From photographs, scientists have estimated the depths and widths of the Martian flash-flood channels. Their results suggest the magnitude of the Martian flash floods must have been at least 100 times greater than anything known on Earth. Scientists disagree about the amount of water lying frozen in the polar caps and as sub-surface ice and permafrost, but some believe that if all this water were to melted it would be sufficient to form a shallow ocean, 200 metres deep, over the surface of Mars.

The volcanoes of Mars

The Martian volcanoes were first photographed by the US spacecraft Mariner 9, which entered orbit around the planet in 1971. The largest Martian volcano, Olympus Mons, rises 27 km above the surrounding plains, making it the highest known volcano in the Solar System. Olympus Mons is three times higher than Earth's highest mountain, Mount Everest. The highest volcano on Earth is Mauna Loa in the Hawaiian islands: it is largely submerged by the waters of the Pacific Ocean; its summit rises 8 km above the ocean floor. The base of Olympus Mons is ringed with cliffs reaching 6 km in height, and it is nearly 600 km in diameter. Only two of the volcanoes on Venus - Rhea Mons and Theia Mons - cover a larger area, and they are only 6 km high. At the summit of Olympus Mons, there is a complex caldera, approximately 80 km across, containing a number of overlapping volcanic craters.

The colossal size of Olympus Mons strongly suggests that there are no active plate tectonics on Mars, as there are on Earth. A 'hot spot' in the mantle of Mars has pumps molten rock upwards through the same vent for millions of years; this produces a giant volcano, rather than a chain of volcanoes like the Hawaiian islands on Earth. Olympus Mons is one of four large volcanoes clustered together on the Tharsis bulge, a region 2500 km across lying just north of the Martian equator. The other three volcanoes of the Tharsis group lie roughly in a line: they are called Ascraeus Mons, Pavonis Mons, and Arsia Mons. A separate, smaller group of Martian volcanoes is located almost opposite these on the planet. None of the Martian volcanoes appears to be active today, but it is unknown whether they are truly extinct or merely dormant.