Good views of Mars' from now to November

Press, 27 July 1988, Page 42

Good views of Mars' from now to November

A huge dust storm was detected in June sweeping across Mars and obscuring a large part of its surface. It originated as a small cloud over Hellas, an impact crater skm deep and 1600 km across. That was in early June, after which the storm rapidly developed to cover much of the southern hemisphere of the planet.

Much of the Martian surface is thickly covered with dust. At times it is carried to heights of 19km by winds exceeding 300km/h. These huge clouds of swirling, yellow dust blot out the surface features until the winds subside. The dust then settles, forming large sand dunes. Great dust storms, such as those seen in 1956 and 1971, can obscure the surface for weeks. The Martian atmosphere is very thin, with much fine dust in suspension. This has the peculiar effect of making the sky from Mars appear pink, and not blue as the sky does from Earth. Our denser air scatters the blue in sunlight. The distance separating Earth and Mars varies widely so that oppositions of Mars are not all equally favourable. The best views of the red planet occur when its opposition date is close to the time at which it is at perihelion. This year perihelion occurs on August 13 and opposition on September 28, with Mars closest to Earth six days earlier, at a distance of 59 million kilometres. This means that the current apparition of Mars is very favourable, especially for those in mid-latitudes, as

the planet is within a few degrees of the celestial equator.

Good views of Mars can be obtained until November. It has continued to brighten and in August reaches a magnitude of —2.2, making it almost as bright as Jupiter. This, and its distinctive reddish colour, will make it easy to pick out from the stars in the constellation, Cetus. A number of American and Soviet spacecraft have either flown by Mars or gone into orbit around it. The most successful of these were Viking 1 and 2, which reached Mars in 1976: Both consisted of two halves, an orbiter which surveyed the planet and a lander which sampled the surface for any sign of life. No form of microscopic life or organic molecules were found, although the experiments were inconclusive. Most astronomers believe that no life form existed at the landing sites.

These spacecraft have returned wonderful views of the surface so that we now know a great deal about it. The most obvious feature seen from Earth is the polar cap. This year the Martian southern hemisphere is tilted towards Earth so that we see the cap around its south pole. The caps consist of a mixture of frozen water and frozen carbon dioxide. Summer begins on Mars on September 11 and the polar cap will be seen to shrink in size as it melts. The southern hemisphere of the planet consists of heavily cratered highlands — as high as 11km. The craters were formed by meteorites and

are believed to be more than three billion years old. The northern hemisphere consists of lowlands in the form of plains and low-lying basinwith scattered, young craters.

The southern highlands extend into the equatorial region in the form of the Tharsis Ridge, dominated by a string of four huge shield volcanoes. The largest is called Olympus Mons, 500 km. across and 27km high. Its summit is crowned by a crater 70km wide. Lava has flowed from the crater on to the surrounding plain. Another remarkable feature photographed by the Viking spacecraft is vailed Valles Marineris, a huge rift valley. It is 4800 km long and 70km wide, and conveys the impression that once water flowed there, but it was probably formed by crustal faulting and subsequent erosion. Viewed from Earth, Mars reveals its polar caps as well as bright reddish regions and dark markings. Long before the space age it was believed that the bright areas were continents and the dark areas oceans. Then a famous American astronomer, Percival Lowell, devoted his skills to intensive observation of Mars. He depicted the planet as crisscrossed by a network of fine lines, which he termed canals. These he contended were irrigation waterways by which the Martians were directing water from the melting polar caps in order to cultivate their crops in the arid regions. That was the origin of the little green men of Mars, which have since

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been the subject of many fine stories in space fiction. Of course, the closeup views from space probes have revealed that Lowell’s canals do not exist and were merely an optical illusion. But the idea that at least one other planet in the Solar System may have had some life-form has persisted.

The question of whether there was ever some low form of life on Mars comes down to whether it ever had water. Today there is a very small trace of water vapour in the atmosphere, while it is probable that there is ice below the surface and possibly liquid water at a greater depth.

The Martian atmosphere is so thin that its pressure is less than at the summit of Mount Everest. The velocity of escape is a mere skm a second, so that any surface water would leak away into space. Yet some of the surface features, such as the rift valley, give the impression that water may have flowed on Mars at some past age. That would imply that once Mars had a much denser atmosphere, allowing rain to fall and rivers to flow. At one time the Martian volcanoes may have been active, spewing forth enough gases and water to thicken the atmosphere. Such events could only last for a very brief period because with the low velocity of escape any atmosphere so formed would soon leak into space. The much more likely idea is that Mars never had the conditions for any form of life as we

F.M. Bateson

know it to exist. Perhaps the two Soviet probes, Phobos 1 and 2, recently sent on their long journey to the red planet, will provide the answers. There are, of course, people who visualise that in a century or two humans will establish colonies on the Moon and then on Mars. The need for that may be that Earth has become overpopulated or we have so messed it up that it is no longer habitable. The trouble with such ideas is that all materials would have to be taken from Earth on a voyage to Mars that would last several weeks or months. Once there humans would not be able to return to Earth as they could not cope with the change in gravity. Maybe that is how humans will start to colonise space, but it appears a far-fetched dream with our present technology.

Mars in August moves into a good viewing position, rising just after 10 p.m. on August 1 and at 8.45 p.m. on August 31. Saturn in Sagittarius is the other planet well placed for evening viewing, setting at 3 a.m. on August 31 and thus being visible almost all night. Mercury reaches superior conjunction on August 3 and then appears in the evening sky. By the end of the month it will be clear of evening twilight and setting at 8 p.m.

Venus continues to dominate the morning sky during August, rising close to 4.30 a.m. throughout the month. Jupiter, in Taurus, rises at 2.50 a.m. on August 1 and at 1 a.m. on August 31.