Mars Landing before 2000?

Press, 29 July 1989, Page 30

Mars Landing before 2000?

Twenty years have passed since men first walked on another celestial body — the Moon. The aim for the next decade is to duplicate this with a landing on Mars. This plan is based on the assumption that before the year 2000 humans will be capable of surviving the long return journey from Earth. They would return with samples of Martian rocks and dust. Mars, therefore, will be the main objective of planetary exploration in the 19905. The Americans plan unmanned space flights to the red planet in 1992 and 1994. The Russians have already shifted their emphasis from Venus to Mars. They launched two spacecraft, Phobos 1 and 2, towards Mars in July, 1988. Extensive details of the Martian surface were obtained in 1976 by the U.S. Viking 1 and 2 spacecraft. These followed a yearlong photographic survey of the planet in 1971 by Mariner 9. Several early Mariners flew by Mars as also did a number of Soviet Mars probes. Not all these were successful. There is a very marked difference in the landscape on different regions of Mars. The highlands are mainly confined to the southern hemisphere, reaching heights of 11km. These highlands are heavily covered with impact craters. The largest is skm deep and 1600 km across. Well-weathered volcanic areas are scattered amidst the impact craters. It is believed that the various formations in the southern hemisphere are all of great age, possibly formed about 3.5 billion years ago. The whole area has a strong resemblance

to the highlands of the Moon. Both bodies bear witness to the terrific hail of debris that fell on their surfaces not long after they were formed out of the solar nebula. A huge rift valley slashes across the equatorial regions. This extends for 4800 km with a depth of 6.4 km and a width at its widest of 70km. Four large shield volcanoes lie close to the equator. These reach heights of 29km. The largest, Olympus Mons, is 515 km across at its base. These volcanoes have complex craters on their summits. The northern hemisphere of Mars consists largely of plains and smooth, low-lying basins. Young impact craters are sparsely scattered over the plains. These lowlands form a wide belt around north latitude 65 degrees. Closer to the equator is a large area of jumbled rocks, while near the pole there is a volcanic plateau, 3km high and 400 km wide. Mars has a very tenuous atmosphere mainly composed of carbon dioxide. White clouds and mist form at times, mainly in the lee of the highlands. Blue clouds have ben recorded at heights of 13km.- These are caused by crystals forming around dust particles high in the atmosphere. An observer on the surface of Mars would look up to a pink sky. That is caused by the huge dust storms that on occasions have hidden the entire globe under a pall of dust carried aloft by hurri-cane-force winds. These winds are caused by the large temperature differences between the polar regions and the equatorial areas. Extensive sand

dunes are formed as the winds abate and the dust' falls on the surface. The prelude to the coming space missions to Mars came last year when Mars had a very favourable opposition in September. The distance between Mars and the Earth at oppositions varies within wide limits because of the eccentricity of the orbit of Mars. The previous favourable opposition was in 1971 and the next will occur in 2003 as these occur at intervals between 15 and 17 years. A world-wide Mars Watch was organised last year to monitor Mars for changes and to track the course of dust storms. The southern hemisphere was pointed towards the Earth. The south polar cap began to melt during the Martian spring and was reduced to two brilliant, white spots atop mountains near the South Pole. Then for a time the thawing of the polar cap stopped. That was because of dust storms which appeared to have reduced the incoming solar radiation. The polar cap continued to thaw after the dust cleared and was seen to split into a number of very small rifts. The south polar cap is composed of mainly frozen carbon dioxide whereas the north polar cap is of water ice. The latter has brought an increase in the number of water vapour ice clouds seen at recent oppositions. Five dust storms swept across the surface of Mars during last year’s opposition. Two of these were extensive and for several weeks blotted out large areas of the surface. The dust settled on the surface as the winds died away. Many changes in the sur-

face features were measured. At the same time the Soviet Phobos spacecraft obtained about 40,000 infrared spectra of the planet and several hundred of one of its two tiny moons. These craft were designed to study solar activity and the interplanetary medium during their long journey to Mars. However, their primary aim was to make a detailed study of Mars and in particular of Phobos, the larger of the two small Martian moons. Unfortunately, the first Soviet spacecraft failed a few days before it was due to reach Mars. The two moons of Mars are believed to be captured asteroids. The reason why it is desirable to study them closely is that they have insufficient radioactive elements to supply any internal heat, hence it is expected that tectonic activity has not occurred. These facts mean that they are largely today composed of the original material from the solar nebula out of which the solar system formed. Certainly they have been affected by external forces such as the solar wind and meteorites but these have not altered their basic composition, and they can therefore tell us much about the early history of the planets, including the Earth. Mars must have once been very much warmer than it is today. It also had at one time a denser atmosphere and running water. It is possible that under these conditions primitive life forms existed. They may even still exist today beneath the surface. Yet Mars has

a great deal in common with the Earth but has evolved in a different manner. Why did its climate change? Was this due, as has been suggested, to drastic changes in its axial tilt? Its evolution will also tell us much about the early history of our own planet. Finally Mars, and also our Moon, are possible sites for future human establishments. That may sound far-fetched but will come in the middle of the next century. The brilliant Venus is already a prominent object in the early evening sky. It will continue to draw away from the Sun during August, setting at 8 p.m. on August 1 and at 9 p.m. on August 31. It will be joined by Mercury, which will get clear of evening twilight ; from August 8 and will be close to Mars on August 5. These two planets might be difficult to see as they then set as evening twilight ends so to view them a very clear northwestern horizon is necessary. Mercury will reach its greatest elongation east of the Sun on August 29. Two nights later it will set about 8 p.m. Saturn remains in Sagittarius throughout August and is well placed for viewing. It will set at 5.30 a.m. on August 1 and at 3.30 a.m. on August 31. Jupiter is in the morning sky rising at 5.30 a.m. on August 1 and at 3.50 a.m. on August 31.