The Origin and Evolution of the Solar System

36 The structure of the Solar SystemTable 1.11. Numbers of falls and finds up to the end of 1975.Falls Finds TotalsStones 791 593 1384Irons 46 610 656Stony-irons 11 67 78The largest meteorites found have masses up to about 30 tonne, most of thembeing of iron. The largest known stone meteorite that fell in Jilin, China, in 1976had a mass of 1.76 tonne. Judging by what is observed on the Moon, much largerobjects must have struck the Earth from time to time. As previously mentioned insection 1.5.1, a few-kilometre-size object may have fallen to Earth at the end ofthe Cretaceous period, some 65 million years ago. Marine clays deposited at thattime have a high iridium content and iridium is a much more common element inmeteorites than it is on Earth. More positive evidence for the fall of larger bodiescan be seen in the craters that exist in various parts of the Earth. The largest ofthese is the Barringer crater in Arizona which is more than 1000 m in diameterand 170 m deep. Small amounts of meteoritic iron have been found in the vicinityand it is estimated that the crater was formed by the fall of an iron meteorite witha mass of approximately 50 000 tonne—that is with a diameter about 25 m if itwas a sphere.In 1908 there was a huge explosion in the Tunguska River region of centralSiberia. The noise was heard at a distance of 1000 km and a fireball, brighter thanthe Sun, crossing the sky. The event was recorded on seismometers all over theworld. In 1927 an expedition discovered a region of about 2000 km ¾ of uprootedtrees, with the direction of fall indicating that the explosion was at the centre ofthe region. However, neither a crater nor fragments were found that could beidentified as of meteoritic origin. Fine fragments of meteoritic dust have beenfound embedded in local soils and the current belief is that the event was causedby the impact of a small comet with the explosion centre produced some 10 kmabove the surface.1.6.1 Falls and findsRecovered meteorites may be classed as either falls or finds. The former categoryconsists of those objects that are seen to fall and are recovered shortly afterwards.Table 1.11 shows the proportions of falls and finds for the three major types ofmeteorite—stones, irons and stony-irons.The distribution of the different kinds of meteorites differs for falls and findsand, in particular, the proportion of irons in the finds is much larger. The proportionof falls may be taken as representing the relative numbers of the different