Astronomy Antarctica an important source of meteorites

Press, 31 May 1977, Page 34

Astronomy Antarctica an important source of meteorites

F.M. Bateson

An American-Japanese field party in Antarctica last summer found 11 meteorites. The largest was in 33 pieces, scattered over 0.8 hectares in the Transantarctic Mountains. It is estimated that this large rock weighs 407 kg.

Meteorites were first found in Antarctica in 1969, when a Japanese party found a concentration of these rocks. The Japanese, in successive seasons, have searched several hundred square hectares, from which 992 fragments have been recovered. These are thought to have resulted from more than 300 separate falls.

The mountains where the latest finds were made lie in Victoria Land, 190 km north-west of McMurdo. Apart from the large meteorite, in 33 pieces, 10 meteorites were found in the area. One was of a metallic type weighing I.4kg, and the other nine were of a stony type, and collectively weighed 51kg. They were lying exposed on the bare ice where the snow cover had been worn away, probably by high winds.

About 1600 meteorites had been recognised and studied before the discovery Of large concentrations in Antarctica. This shows the value of searches in the southern continent, which lends itself readily to their discovery. Meteorites vary in composition from the irons, which are almost pure nickel-iron, to stones of light silicate materials.

The silicates are compounds of silicon and oxygen, with some aluminium, calcium, magnesium, and iron.

It is difficult to tell stony meteorites from terrestrial stones. Fortunately, most of the former also contain a small amount of iron, which has a fine honeycomb appearance not found in stones on earth. When an iron meteorite is polished and etched the surface shows a large crystalline structure, called the Widmanstatten pattern, showing that they were formed by slow cooling under moderate pressures. The irons also show stresses such as would occur in collisions.

It is generally believed that many meteorites are

fragments of larger bodies that have suffered collisions. These are thought to have originated in the asteroid belt. Possibly some stony meteorites are from material that never held together in a larger body. If this is correct then such meteorites would represent the primordial matter left over when the larger bodies of the Solar System formed. The age of meteorites has been found to be 4.5 thousand million years; about the same as that of the Earth. Calculations of the rate- of cooling show that the bodies of which they are fragments could not have been much larger than 200 km. Their parent bodies were probably heated so that the nickeliron collected in pockets, which were broken off in collisions, producing the iron meteorites.

There are several types of stony meteorites. One type contains a considerable fraction of carbon, and other gases. These are called carbonaceous chondrites. It. is believed that meteorites of this type were responsible for two spectacular events. The first was the Tunguska meteor, which exploded over Siberia on June 30, 1908. The second exploded in 1965 over Revelstoke, in Western Canada. No craters, or fragments, resulted from either event. All that has been recovered was a minute amount of black dust found on the snow at the Canadian site. The explosions are thought to have resulted from the disintegration of meteors high in the atmosphere, with forces in the megaton range. The explosions were so terrific that no fragments resulted, and therefore no craters were formed. There are, of course, many craters on the Earth’s surface that are the result of meteorites striking the Earth. More than 100 craters up to 28m in diameter were formed in Siberia in 1947 when meteorites struck. The best known meteorite crater is that in Arizona. It averages 1200 m in diameter and is 200 m deep. More than 30 tonnes of meteoritic iron has been found in the area round the crater. Fossil meteorite craters

have been found in many countries. These are comparable to craters on the Moon. The largest is in South Africa, measuring 120 km across. Although there has been considerable erosion huge volumes of crushed rock are still in evidence. Whether meteorites come from larger bodies that broke up in collisions, or from primordial matter, they tell astronomers much" about the history of the solar system. Thus the finds in Antarctica are important in providing a large collection of fragments for study.

Venus and Mars continue to provide a spectacular sight in the morning sky, where both planets rise within a few minutes of 4 a.m. throughout June. After June 4, when they will be close together, they gradually draw apart although they remain in the same part of the sky. Venus is much the brighter of the two. On the morning of June 13 Venus will be two degrees south of the Moon, with Mars nearby. This will provide a very pretty sight. Mercury is also in the morning sky, very low in the north-east at the beginning of June. It swings back towards the Sun, and rapidly becomes lost in the dawn skv.

Jupiter reaches conjunction with the Sun on June 4, and will appear in the dawn sky towards the end of the month. Saturn is the only bright planet in the evening sky, where it can be found low in the north-west in the early evening. It sets about 9.40 p.m. today and at 7.50 p.m. on June 30. The more distant planets, Uranus and Neptune, are well placed in the evening. Uranus is bright enough to be easily picked out with binoculars and can be located a short distance south-east of the star Lambda Virginis. Neptune comes to opposition on June 6, when its magnitude is 7.7, sufficiently bright to be seen with binoculars. It is in the constellation Ophiuchus, close to the border with Scorpius, and is north-east of the bright star, Antares. At 11 p.m. on June 29 Neptune will lie two degrees south of the Moon.