Astronomy

Press, 4 February 1980, Page 12

Astronomy

F.M. Bateson

Mars and Jupiter come t® opposition within 12 hours of each other on February 25. They will afford viewers a spectacular view in the constellation, Leo, where they will be close together to the east of the bright star Regulus. The ringed planet, Saturn, will be further to the south-east on the border between Leo and Virgo. The distance to Mars varies greatly at opposition. If the planet comes to opposition when it is close to perihelion it can be as close as 56 million kilometres: Such close approaches occur at intervals of 15 to 17 years. The most recent was in 1971. This year opposition occurs when Mars is at aphelion (farthest from the Sun) so that it will be 101 million km away. This year the opposition is as unfavourable as possible. The landscape of Mars offers striking contrasts. Huge areas are basically deserts,.strewn with rocks ranging. up to several metres wide. These rocks appear to be fragments of lava flows, resembling the basaltic lavas of the Earth and the Moon. It is these desert areas that give Mars its reddish hue.

In the remote past Mars was bombarded by huge meteoric bodies. The evidence for this is found on the highland plateaux, where impact craters form a jumbled terrain. One impact crater measures 1800 km in diameter and 3km deep. This is the biggest impact crater known in the solar system.

Volcanoes were present on Mars, at an early stage of its history. This is shown by . chains of craters . &pd Extensive lava flows evident on the surface. . The/biggest volcanic crater, Olympus Mons, is 600 km across at its base and. 21km high. It is .roughly 20 times bigger -than the biggest terrestrial Volcano. At its summit ' there are , several craters measuring 90km across. The main body of Olympus Mons was formed by basalt lava flows. At ?the,base of this mountain ample evidence of erosion is present. There is sesne disagreement about wh?;n volcanism stopped on Mars, but probably its volcanoes have been dor’mant for 100 M years. The Martian atmosphere is very tenuous. It has a surface pressure less than 1 per cent that of the Earth. Ninety-five per cent of the atmosphere is carbon dioxide, with small

amounts ■ of nitrogen, argon, carbon monoxide, oxygen, and a trace of water vapour. Winds in the Martian atmosphere can reach velocities of 300 km per hour. They raise huge clouds of dust, which has. on occasions, totally obscured the surface. The particles are very fine, about a micron in diameter. This dust is probably volcanic ash . and sand from the deserts. - Evidence exists that water, or some other liquid, was once present. A huge canyon extends for 4000 km and, in places, is 6km deep and wide. Its appearance resembles a rift valley with a collapsed floor. A network of tributaries join the canyon and possibly 7, in a remote age, water flowed here, although it. was probably not abundant.

Mars has two polar caps, most of which are carbon dioxide — dry ice. They wax . and wane according to the seasons of the hemisphere in which they are found. In the summer; they melt .and shrink, even becoming non-existent occasionally. In winter they cover a much bigger area. Ground fogs have been observed over the polar regions in autumn.

Thin clouds, resembling our cirrus, form at times over the mountains., Wave clouds have also been detected, formed as the wind passes over a crater and then falling down, the lee .side of the crater only to rise again.. Such clouds appear to form on the crests of the wind and disappear again in the troughs. Mars has two tiny moons. The inner, Phobos, is an irregular shape measuring 27km by 21km by 19km. Its surface is heavily cratered from numerous impacts with other bodies. The most interesting features on its surface are long striations, from 100 m to 200 m long and 5m to 10m deep. A possible explanation of these striations is that Phobos, being very close to Mars, has a great deal more stress placed on its inward facing side than on the other side.- This makes a tendency for the inward face to orbit faster and so the resultant. stress has caused fractures in the rocks, visible as striations.Phobos may be an asteroid that passed too close to Mars and was captured by that planets In its ir-

regular shape and heavily cratered surface, - showing the effects of numerous collisions with other bodies, it resembles many asteroids. It is so close to Mars that it whizzes around in its orbit so quickly that each circuit requires only 7 7.6 hours. The outer satellite is even smaller than Phobos, and measures 15km by 12km by 11km. It is called Deimos,- and orbits 23,490 km above the surface. Its surface is heavily cratered but there are no striations. These would not be expected, since.it is tod far from Mars to be subjected to the same stresses as Phobos. The surface temperature on Mars rarely rises much above freezing point. It may have been somewhat warmer in the distant past. If that was so, possibly conditions • were such that some forms of primitive life became possible. Higher- life forms would not /have existed. All searches to datfe for some form of primitive; life have failed, which does ■ not mean that one day evidence of such forms .may not be found. However, it is more likely that any life that existed has not survived the barren, conditions that have prevailed for about one hundred million years. During February Mars and Jupiter, .will . be visible throughout the hours of darkness, They will be in . their best viewing positions; around 1 a.m., when .they are highest in the ky as they cross the meridian. Saturn, rising about an hour after Jupiter, is also now in a better viewing position.

Venus remains visible in the western evening sky as a brilliant object. Mercury makes a brief visit to the evening sky in mid-February, It will be well north of the celestial equator so this will not be a favourable elongation from our altitudes. There will be some interesting conjunctions in February. Jupiter will be less than one degree fom the' Moon, just after it rises, on February 3. Also, at 4 a.m. on the same date, L the Moon will be within half a degree of Regulus. On the evening of February 19 Venus and the young Moon will be a pretty sight, being separated by four degrees.- At: midnight on February 24 Alderbaran the bright red star in Taurus, will be right alongside the Moon.