Lunar meteorite
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Image:Lunar Meteorite.jpg A Lunar meteorite is a meteorite that is known to have originated on the Moon.
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Discovery
The first lunar meteorite, Yamato 791197, was discovered in 1979 in Antarctica; but at that time its origins were unknown. The first meteorite of definite lunar origin was Allan Hills 81005 which was found in 1981 [1]. Meanwhile more than two dozen other different meteorites from the Moon are known, with a total mass of about 8kg.
Lunar origin may be established by comparing the mineralogy, the chemical composition, and the isotopic composition between meteorites and samples from the Moon collected by Apollo missions. The first Lunar meteorite to have its origin pinpointed was Sayh al Uhaymir (SaU) 169 discovered in Oman. It is believed to have been ejected from the skirt of ejecta surrounding the Lalande crater within the last 340,000 years.
Transfer to Earth
Lunar meteorites are launched from the Moon by large impacts. Until the discovery of Sayh al Uhaymir, no single crater could be identified as the origin of lunar meteorites. Noble gas measurements have shown that the typical time for a lunar meteorites to arrive at the earth is in the range of hundred thousands to one million years. This shows that after break-up from moon, lunar meteorites do not directly travel towards earth. They rather spend a long time orbiting the earth moon system.
Scientific relevance
For statistical reasons, the composition of lunar meteorites is closer to the average composition of the lunar surface material than the composition of the samples from Apollo and Luna missions. Apollo and Luna samples are from a relatively small area on the nearside of the Moon, whereas lunar meteorites possibly also samples material from the farside of the Moon.
When the first meteorite from moon was discovered in 1982 this led to new speculations that there could also exist meteorites from Mars. In fact, the first Martian meteorite was identified in the same year. There are also speculations about the possibility of finding "Earth meteorites" on the surface of the Moon. This would be very interesting because in this case stones from earth older than 3.9 billion years, which are destroyed on earth by various geological processes, may have survived on the Moon. Thus some scientists propose new missions to the Moon to search for ancient rocks from Earth.
History
Today, about one in every thousand newly discovered meteorites is a lunar meteorite, whereas the vast majority of meteorites are from the asteroid belt. In the early 19th century most scientists believed that all meteorites were from the moon. Although today supported only by a minority of researchers, there are also theories that Tektites are from moon, and should therefore also be regarded as lunar meteorites. However, most scientists regard such theories as outdated.
Own your own moonrock
Lunar meteorites are the only source of moon rocks which can be owned by the public. All rocks collected during the Apollo moon-landing program are considered to be the property of humanity as a whole, held for the benefit of scientific study, and it is illegal to own, buy or sell them. (A recent attempt to sell stolen moonrocks, originally given as gifts to heads-of-state by the American Government, was foiled by an undercover "sting" after a tip-off from a meteorite expert). The small amount of moonrock returned by the Russian Luna 16 probe is also unavailable.
Of those lunar meteorites which are found on Earth, those found in Antarctica are similarly considered to be the property of mankind. Only those found elsewhere - chiefly in deserts in areas such as Oman - are traded on the open market, like other meteorites.
References
- [1] Warren P. H., Taylor G. J., Keil K. (1983) Regolith breccia Allan Hills A81005: Evidence of lunar origin and petrography of pristine and nonpristine clasts, Geophys. Res. Lett. 10, 779-782.