Mars and Saturn reach opposition in May

Press, 26 April 1984, Page 23

Mars and Saturn reach opposition in May

Mars and Saturn reach opposition on Mgy 11 and May 3 respectively. This means that both planets will be visible all night. They will be high in the sky at midnight to the north of the zenith.

The opposition magnitude of Mars will be -1.7, making it only slightly inferior in brightness to Jupiter. It will be two magnitudes brighter than Saturn. Mars can be found in the constellation, Libra. It can be readily recognised not only by its brightness but also by its distinctive ruddy colour.

Mars, unlike the major planets, comes to opposition every second year. The distance of Mars from the Earth at opposition varies considerably from one opposition to another because of the eccentricity of its orbit. This has two notable effects: both the opposition brightness and the apparent diameter of the Martian disc show wide variations. The most favourable oppositions occur at intervals of 15 and 79 years. The last close opposition was in 1971, and the next will be in 1986 and 1988. The wide variations in the distance of Mars mean that its brightness changes to a greater extent than that of the other bright planets.

Mars moves at 24 kilometres a second. This rapid movement can be readily seen with the unaided eye

by noting its position against the background of stars. This observation can be repeated a week later and it will be seen that Mars has moved quite noticeably. Mars gives a disappointing view in a small telescope. A white polar cap will be seen, whilst the reddish colour of the disk may be broken by a dark marking. Generally little more can be seen except with larger instruments. Spacecraft have revealed Mars as a world full of interest. Its diameter is roughly half that of the Earth and the thin atmosphere enables the surface to be clearly seen. The southern hemisphere of the planet bears a strong resemblance to the lunar landscape. The surface consists mainly of heavily cratered highlands rising to a height of 11 kilometres. The largest of the craters is 1600 km across. The craters we r e formed by a meteorite bombardment and are probably very old. This area also contains some weathered volcanic structures.

The southern highlands extend, at a lower level, across the equatorial zone. Here they are cut by a huge rift valley, extending for 4800 km. Its greatest width is 70km, and at one point it reaches a depth of 6.4 km.

The surface of the northern hemisphere is entirely different. For the most part

it consists of comparatively smooth plains, broken by widely scattered impact craters. Four huge shield volcanoes lie just to the north of the equator. The largest is 515 km across and reaches a height of 29km. These volcanoes are considered to be comparatively young.

A volcanic plateau lies close to the north pole. This extends for 400 km and reaches a height of 3km. Dust is found in large dunes which resemble those of the Sahara. Dust storms on Mars have been observed from Earth. Such a storm in 1971 shrouded the entire planet in dust particles raised by strong winds. These particles reached heights of 29km before the winds abated and deposited the dust on the surface.

Saturn, by contrast with the weird Martian landscape, presents a serene and beautiful appearance. It is also much more interesting in a small telescope, which will show some of the concentric rings that encircle the planet. These appear to be broken by a dark gap, known as Cassini’s Division. This was considered to be a region within the rings that was devoid of the tiny particles that make up the rings.

However, spacecraft have shown that the ring system is much more complicated and that the gap is not devoid of small objects. The

rings are made up of tens of thousands of ringlets which form a delicate and very beautiful pattern

No surface markings are visible on Saturn, because it is covered by a dense layer of clouds. These, like those of Jupiter, are arranged in bands and belts but these are not as distinct as those on Jupiter. Occasionally activity in the cloud layer has been revealed by the formation of a large whitish oval. Such activity is infrequent owing to Saturn’s low temperature caused by its distance from the Sun.

Mars has two tiny moons, 13km and 23km in diameter. Both are heavily cratered. Saturn, by contrast, has a retinue of satellites, which range in size from the giant Titan with a diameter larger than that of the planet Mercury down to minute rocky fragments. The exact number of Saturn’s moons is uncertain but there are at least 20.

Titan is covered by a nitrogen-rich atmosphere which hid its surface from the Voyager spacecraft. Three of the other satellites — Rhea, Dione and Tethys — are all thickly studded with craters. Tethys is 'unique in having one crater that has a diameter a third of that of the satellite itself. It is also remarkable in having a huge rift running from pole to pole. This has an average width of 150 km and a depth of 7km. May will see both Jupiter and Mercury in good viewing positions. Jupiter, in Sagittarius, rises on May 1 about 9 p.m. and on May 31 at 7 p.m. Mercury in the north-eastern morning sky rises about an hour before dawn during the middle of the month. It can be found low down in the north east at dawn. Mercury brightens by almost two magnitudes during the month. On May 29 it can be seen close to the waning crescent of the Moon.

The Eta Aquarid annual meteor shower has its maximum on May 5, but meteors from this shower can be seen after midnight for about three weeks. This shower is associated with Halley’s Comet, due at perihelion in 1986. Particular attention is being made to this shower this year and in the following years to ascertain whether its activity increases with the approach of Halley’s Comet. A Japanese expedition is going to Western Australia specially to observe this year’s display-