Science: Icebound moonstones chart the journey to Earth

时间:2019-03-04 08:05:08166网络整理admin

METEORITES recovered from Antarctica have shown researchers how these rocks journeyed from the Moon to Earth. Otto Eugster, of the University of Bern in Switzerland, has outlined the evidence from five lunar meteorites found in the ice (Science, vol 245, p 1197). Eugster suggests that the meteorites came from two different impact craters on the Moon, far from the Apollo and Luna landing sites. Some of the material took about 16 million years to reach Earth and may then have lain in the Antarctic ice for 170 000 years before its discovery. If an object larger than about 100 metres in diameter strikes the surface of the Moon, it could eject chunks of rock at speeds greater than the lunar escape velocity – the speed an object must travel at to escape the Moon’s gravitational pull and begin its passage through space. The pull is so weak that an object has to reach a speed of only 2.4 kilometres a second to get away. Judging from the number and size of craters on the surface of the Moon, it has experienced about three such impacts every million years during its recent history. Computer simulations show that once material is ejected from the Moon, it may take as long as 100 million years to transfer to the Earth, although in most cases the journey takes less than a million years. The reason why the transfer takes so long is that rocks and other particles that escape from the Moon’s gravitational pull may shoot away from our planet, or form a temporary ring around it, similar to Saturn’s but on a more modest scale, before eventually falling to Earth. The noble gases, such as helium, argon and xenon, are produced in the rocks by natural radioactivity as time passes, so the amount present indicates the age of the rocks. Other isotopes are produced when cosmic rays interact with the material once it is exposed to space. The presence of isotopes reveals how long the meteorites were exposed before reaching Earth. Once the rocks are shielded by the Earth’s blanket of air, however, the interactions proceed in a different way. This makes it possible to estimate how long the meteorites have lain in the ice of Antarctica. Three of the five samples seem to have arrived on Earth at about the same time, roughly 75 000 years ago. One of the other two meteorites arrived some 170 000 years ago. The date for the time of arrival of the other sample is less precise, but it has been here less than 100 000 years. More intriguingly, there are much larger differences in the lengths of time that the different samples were exposed to interplanetary cosmic rays. For three of the samples, the best explanation for the present proportions of isotopes is that the material was first exposed on the Moon by an impact about 16 million years ago, then blasted into space by a second impact some 5 million years ago, taking a long time to reach Earth. The two other meteorites, however, seem to be formed from material that was exposed to cosmic rays on the surface of the Moon for hundreds of millions of years. Then, after an impact ejected the material, it made the journey to Earth very quickly, in a couple of hundred thousand years. All the figures fit with the estimate of three sufficiently large events per million years. The confirmation that the meteorites do come from the Moon adds greatly to the information researchers gathered from the rocks brought to Earth by the manned Apollo missions and the unmanned Luna probes. These craft sampled an area representative of only 5 per cent of the Moon’s surface. The rocks they brought back were all very similar in composition. The lunar meteorites clearly come from a different region of the Moon. This suggests that models of the composition of the Moon based on the Apollo and Luna samples may have to be revised. Eugster points out that because material ejected from the Moon can reach Earth, it is likely that meteorites have also come from Mars, which, like the Moon,