Are Lunar Meteorites Rocks From The Moon?
Lunar meteorites are part of lunar soil blasted from the Moon as high-speed ejecta during impact events. Lunar meteorites are of great scientific importance because they come from areas of the Moon that were likely not sampled by the Apollo missions.
There are 84 stones described in the scientific literature that appear to be lunar meteorites (170 stones to date in 2013, some resulting of fragmentation of the same lunar meteorite). So the total weight of all recovered moon meteorites is less than 60 kg (compared to thousands of tons of high cutting quality diamonds are found each year). Lunar meteorites are highly unique and precious treasures from space.
What is a Lunar Meteorite?
Lunar meteorites, or lunaites, are meteorites from the Moon. In other words, they are rocks found on Earth that were ejected from the Moon by the impact of an asteroidal meteorite.
How Did They Get Here?
Lunar escape velocity is 2.38 km/s (1.48 miles per second), only a few times the muzzle velocity of a rifle (0.7–1.0 km/s). Any rock accelerated by an impact on the lunar surface to lunar escape velocity or greater will leave the Moon’s gravitational influence.
Some ejected material becomes captured by the Earth’s gravitational field and lands on Earth shortly after ejection from the Moon. Other ejected material, however, assumes an orbit around the Sun. Some of that material may eventually strike Earth. This can take a long time. The record holder, lunar meteorite Yamato-82192/82193/86032, remained in space for about 9 million years before landing in Antarctica.
How Do We Know They Are Meteorites?
Like asteroidal meteorites, lunar meteorites have fusion crusts from the melting that occurs as they enter the earth’s atmosphere (the olive-green crusts on the photos above). Also, they contain certain isotopes that can only be produced by reactions with cosmic rays while outside the Earth’s atmosphere.
How Do We Know They Are from the Moon?
Chemical compositions, isotope ratios, minerals, and textures of the lunar meteorites are all similar to those of samples collected on the Moon during the Apollo missions. Taken together, these various characteristics are different from those of any other type of meteorite or terrestrial rock. For example, all of those meteorites in the List that are classified as feldspathic breccias are rich in the mineral anorthite, which is a plagioclase feldspar, mineralogically, and a calcium aluminum silicate, chemically.
Consequently, these meteorites all have high concentrations of aluminum and calcium. Because of some unique aspects about how the Moon formed, the lunar highlands are composed predominantly of anorthite. Anorthite is much less common on asteroids and, to the best of our knowledge, on the surface of any other other planet or planetary satellite.
How Many Are There?
It depends on how one count. At this time, 170 separate rocks have been described that appear to be lunar meteorites. However, some of these are “paired,” that is, they are different fragments of a single larger rock that made the Moon-Earth trip. When confirmed or strongly suspected cases of terrestrial pairing are taken into account, the number of actual meteorites reduces to 83. Pairing has not yet been established or rejected for the most recently found meteorites, so the actual number is not known with certainty. In the List, known or strongly suspected paired specimens are listed on a single line separated by slashes.
In all cases, the meteorite pairs were found close together. It is likely that the meteorites broke upon hitting the earth or traveling within the ice in Antarctica (see ANSMET, Antarctic Search for Meteorites).
There are persuasive arguments that QUE94281 & Yamato-793274/981031 and that Yamato-793169 and Asuka-881757 are source-cratered paired, that is, in each case the two meteorites were ejected from the Moon as separate rocks by a single impact and that the separate rocks fell to Earth at different places (Warren, 1994; Arai and Warren, 1999). So, the lunar meteorites represent ~20 impact sites on the Moon.
The actual number may be less if there are other source-crater pairings, which is not easy to establish with certainty.
How Rare Are They?
Of the 22,507 meteorites listed in the Catalogue of Meteorites (Grady, 2000), only 0.08% are lunar. Meteorites are very rare rocks; lunar meteorites are exceedingly rare.
Where Exactly on the Moon Did They Come From?
We have not identified the source crater from which any of the lunar meteorites originated. Paul Warren (1994) makes a persuasive case that lunar meteorites come from relatively small craters – those of only a few kilometers in diameter.
The main thrust of his argument is that all the lunar meteorites were blasted off the Moon in the last ~10 million years (most in the last few hundred thousand years) and that there haven’t been enough “big” impacts in that time to account for all the different lunar meteorites. If lunar meteorites come from small craters, it would be especially difficult to locate the actual source crater of a particular lunar meteorite.
Where, How, and When Were They Found?
All lunar meteorites have been found in areas that are well known to be good places to find meteorites. All such places are dry deserts where there are geologic mechanisms (e.g., see ANSMET) for concentrating meteorites or where rocks of terrestrial origin are rare. Many lunar meteorites (and most meteorites in general) have been found in Antarctica by expeditions funded by the U.S. (ANSMET) or Japanese (NIPR) governments. The meteorites from Australia, Africa, and Oman were found by private collectors.
Allan Hills 81005 (ALHA81005), the first meteorite to be recognized as originating from the Moon, was found during the 1981–82 ANSMET collection season, on 18 January 1982. The three Yamato 79xxx meteorites were collected earlier, but not recognized to be of lunar origin until after 1982. The first lunar meteorite to be found appears to be Yamato 791197, on 20 November 1979.
Although the discovery that there are rocks on earth that originated on the Moon is relatively new, lunar rocks have surely been dropping from the sky throughout geologic history. That fact does not make them necessarily easy to find or recognize, however. In the absence of a fusion crust, a lunar (or martian) meteorite is less likely to be recognized as a meteorite than is an asteroidal meteorite because it more closely resembles terrestrial rocks in mineralogy and density.
A weathered lunar meteorite would not be an impressive or suspicious looking rock if found in a cornfield or streambed (see Dar al Gani 400 or QUE94281) and a brecciated lunar meteorite could easily be overlooked in the field as a terrestrial sedimentary rock. Also, lunar meteorites contain a much smaller amount of metal than ordinary chondrites, so they aren’t particularly magnetic.
Although he has studied Apollo lunar rocks for many years, the writer of this article did not recognize the MAC88105 lunar meteorite as a Moon rock when another member of the 1988 ANSMET team handed it to him in the field and asked, “Do you think this is a meteorite?”
How Are They Named?
By long-standing convention, meteorites are named after the location where they fall or are found. For example, Calcalong Creek is a place in Australia. Somewhat contrary to the convention, the Antarctic meteorites in the U.S. collection often go by abbreviated names, where ALHA = Allan Hills, EET = Elephant Moraine, MAC = MacAlpine Hills, and QUE = Queen Alexandra Range. Similarly, the Northwest Africa and Dar al Gani meteorites are sometimes abbreviated NWA and DaG. Because hundreds to thousands of meteorites have been found in Antarctica and hot deserts, serial numbers are used in addition to names.
For the Antarctic meteorites, the first two digits of the numeric part of the name represents the collection year. (See map of Antarctic meteorite locations for the U.S. collection.)
How Are They Classified?
Historically, the lunar surface has been divided into two types of terrane, the mare (pronounced mar-ay, which is the Latin word for sea) and the terra (land). The maria are giant impact basins that have been filled with basalt – solidified lava from ancient volcanoes.
Lunar rocks are classified by what minerals they contain (mineralogy), how the mineral grains are put together (texture), how the rock formed (petrology), and chemical composition (chemistry).
Feldspars are some of the most common minerals of the crust of the Earth and Moon. Rocks of the lunar highlands contain a high proportion (70–90%) of a type of feldspar known as plagioclase. In particular, most plagioclase of the lunar highlands is the Ca-rich variety known as anorthite. Rocks composed mainly of the anorthite are known as anorthosites. Lunar scientists often refer to the highlands crust as “feldspathic” or (more specifically) “anorthositic” to indicate that it is rich in anorthite feldspar. Anorthite, like all forms of feldspar, is rich in aluminum and poor iron.
Rocks from the maria are classified as basalts because they are crystalline, igneous rocks (texture) consisting mainly of pyroxene and plagioclase (mineralogy). Specifically, they are called mare basalts because they formed when magmas from inside the Moon erupted into low spots on the surface to form the geologic features called maria (petrology).
Mare basalts are subclassified by chemical composition (chemistry), for example, “low-titanium (Ti) mare basalt.” All mare basalts are rich in iron because they contain mainly pyroxene, olivine, and ilmenite, all of which are iron-rich minerals.
Impacts of asteroidal meteorites onto the Moon both break rocks of the lunar crust apart and glue them back together.
Most rocks from the highlands are breccias (pronounced brech´-chee-uz), a textural term for a rock that is composed of fragments of other rocks and that is held together by shock compaction or by material that was partially or totally molten. An impact can melt rock, forming impact melt. The melt usually collects fragments of rock called clasts as it is forced away from the point of impact.
When the melt cools, it forms an impact-melt breccia – clasts suspended in a solidified (glass or crystalline) matrix of impact melt. The lunar surface is covered with fine-grained material called soil or regolith.
The shock wave associated with an impact can lithify the regolith – it can turn the fine, powdery material into a coherent rock called a regolith breccia. At depth, coarser fragments can be lithified to form a fragmental breccia. Breccia is a textural term that applies to rocks of both the maria and highlands. A most lunar meteorites are feldspathic regolith breccias, that is, rocks consisting of lithified soil from the lunar highlands.
Meteoritical Society Member