What is a meteorite?
A meteorite is a rock that was formed elsewhere in the Solar System, was orbiting the sun or a planet for a long time, was eventually captured by Earth’s gravitational field, and fell to Earth as a solid object. A meteoroid is what we call the rock while it is in orbit and before it is decelerated by the Earth’s atmosphere. A meteor is the visible streak of light that occurs as the rock passes through the atmosphere and exterior of the rock is heated to incandescence. Most (99%) recovered meteorites are pieces of asteroids. A few rare meteorites come from the Moon (0.7%) and Mars (0.5%).
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 meteoroid or possibly a comet.
How did lunar meteorites get here?
Because the Moon has no atmosphere to stop them, meteoroids strike the Moon every day. Lunar escape velocity averages 2.38 km/s (1.48 miles per second), only a few times the muzzle velocity of a common rifle (0.7-1.0 km/s). Any rock on the lunar surface that is accelerated by the impact of a meteoroid to lunar escape velocity or greater will leave the Moon’s gravitational influence. Most rocks ejected from the Moon become captured by the gravitational field of either the Earth or the Sun and go into orbit around those bodies. Over a period of a few years to tens of thousands of years, those orbiting the Earth eventually fall to Earth. Those in orbit around the Sun may also eventually strike the Earth up to a few tens of millions of years after they were launched from the Moon.
Words that confuse people
How do we know that they are meteorites?
On a broken or sawn face, all lunar meteorites look like some kinds of Earth rocks, even to an experienced meteorite scientist. We can often tell that they came from space, however, because many lunar meteorites have fusion crusts from the melting of the exterior that occurs during their passage through Earth’s atmosphere. On meteorites found in hot deserts, the fusion crusts sometimes have weathered away. As explained in more detail below, however, all meteorites contain certain isotopes (nuclides) that can only be produced by reactions with penetrating cosmic rays while outside the Earth’s atmosphere. The presence of cosmogenic nuclides is the ultimate test of whether or not a rock is a meteorite. All lunar meteorites that have been tested show evidence of cosmic-ray exposure.
How many lunar meteorites are there?
It depends upon how one counts. At this writing (December 31, 2021), 502 named lunar meteorites are recognized. Other rocks that have not yet been classified or described in the scientific literature but which may be lunar meteorites are being sold by reputable dealers. The complication is that some to many of these stones are “paired,” that is, two or more of the stones are different fragments of a single meteoroid that made the Moon-Earth trip. When confirmed or strongly suspected cases of pairing are considered, the number of actual meteoroids reduces to about 150. Pairing has not yet been established or rejected for the many recently found meteorites, so the actual number is not known with certainty. In the List, known or strongly suspected paired stones are listed on a single line separated by slashes. In most cases, the stones were found close together because a meteoroid broke apart upon encountering the Earth’s atmosphere, hitting the ground or ice, or while traveling within the ice in Antarctica. In the other cases, all from northern Africa, we do not know for sure where the meteorite were found so we cannot establish pairing by find-location proximity. The six LaPaz Icefield stones all have fusion crusts and the broken edges do not fit together, thus the LAP meteoroid likely broke up in the atmosphere. Among the ~42 lunar meteorite stones from Oman, 16 appear to all be pieces of a single meteorite.
Pairing and naming
Although it is confusing, meteorite scientists refer to all found pieces of a meteoroid as a single meteorite, ideally with a single name. Thus, Allende refers to hundreds of fragments of a single 2-ton meteoroid that broke apart over Mexico in 1969. All the pieces are paired stones of a fall and they are all called Allende. With finds (meteorites not observed to fall) different stones are often given different names because they are found at different times. If later studies show two stones to be paired, then one of the names is officially discarded. With the Antarctic and hot-desert meteorites, however, all the stones are originally given different designations because so many meteorites are found in a small area. This problem leads to the awkward combination names like Yamato 82192/82193/86032 when one is referring to “the meteorite,” in the accepted sense, as opposed to the individual stones. If the 16 stones of the Dhofar 303 clan of lunar meteorite had been found, for example, in the U.S., they would likely all been given the same name.
For several reasons, we know that the lunar meteorites derive from many different impacts on the Moon. The textural and compositional variety spans, and in some ways exceeds, that of rocks collected on the six Apollo landing missions, so the meteorites must come from many locations. More importantly, it is possible to determine how long ago a rock left the Moon using cosmic-ray exposure ages. Small rocks on the surface of the Moon and in orbit around the Sun or Earth are exposed to cosmic rays. The cosmic rays are so energetic that they cause nuclear reactions in the meteoroids that change one nuclide (isotope) into another. Some of those nuclides produced are radioactive. As soon as they fall to Earth, production stops because the Earth’s atmosphere absorbs nearly all cosmic rays. The cosmic radionuclides in the meteorite decay on Earth with no further production. The most well-known such isotope is 14C (carbon 14), which is produced from oxygen atoms in the meteoroid. Other important radionuclides produced by cosmic-ray exposure are 10Be, 26Al, 36Cl, and 41Ca. Because the various radionuclides all have different half-lives it is often possible to tell how long a rock was exposed on or near the surface of the Moon, how long it took to travel to Earth, and how long ago it fell. For example, cosmic-ray exposure data for Kalahari 008/009 suggest that the meteorite left the Moon at most a few hundred years ago. At the other extreme, Dhofar 025 took 13-20 million years to get here from the Moon (Nishiizumi and Caffee, 2001). Because there is a wide range in the Earth-Moon transit times, we know that many impacts on the Moon were required to launch all the lunar meteorites.
There are persuasive arguments (cosmic-ray exposure ages, chemical and mineral compositions) that the “YAMM” meteorites found in Antarctica, Yamato 793169, Asuka 881757, MIL 05035, and MET 01210 are source-cratered paired or launch paired, that is, the four meteorites were ejected from the Moon as separate rocks by a single impact, the rocks traveled to Earth separately, and that they fell to Earth at different places at different times (Warren, 1994; Arai et al., 2005; Zeigler et al., 2007). [August, 2021: Ramlat Fasad 532 from Oman may be another launch pair of the YAMM meteorites.] Other likely cases of launch pairing are (1) the “YQEN” meteorites, Yamato 793274/981031, QUE 94281, EET 87521/96008 (Arai and Warren, 1999, Korotev et al., 2003), NWA 4884 (Korotev et al., 2009), and the NWA 7611 clan (Korotev and Irving, 2021) and (2) the “NNL” meteorites, NWA 032/479, NWA 4734/10597, NWA 8632, and the 6 LAP stones (Zeigler et al., 2005; Korotev and Zeigler, 2014; Korotev and Irving, 2021). Almost certainly, some to many other launch pairings occur among the numerous, but not well studied, feldspathic lunar meteorites from hot deserts. So, the lunar meteorites represent fewer impact sites on the Moon than the number of meteorites.
Does it take a big impact to launch a lunar meteoroid?
Vogt et al. (1991) estimated that the frequency of impacts on the Moon large enough to eject lunar meteorites is greater than 5 per million years. On the basis of impact probability and the known size distribution of lunar craters, 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 ~20 million years (most in the last few hundred thousand years) and that there have not been enough “big” impacts on the Moon in that time to account for all the different lunar meteorites. As new lunar meteorites are found each year, Warren’s argument becomes more valid. James Head (2001) calculates on a theoretical basis that impacts causing craters as small as 450 m (about a quarter of a mile) in diameter can launch lunar meteorites.
More recently, Basilevsky et al. (2010) argue on the basis of the known number of lunar meteorites and the frequency of impacts on the Moon that “a significant part of the lunar meteorite source craters are not larger than a few hundreds of meters in diameter.” (That is big if it happens in your backyard, but it is not so big for the whole Moon.) If lunar meteorites come from such small craters, it would be especially difficult to locate the actual source crater of any particular lunar meteorite.
Vladimir Shuvalov and Natalia Artemieva of the Institute for Dynamics of Geospheres in Moscow reach the following conclusion on the basis of numerical impact modeling: “An interesting consequence may be connected with 83-km-diameter crater Tycho. ~100 Myr ago [the terresrial Cretaceous Period], the crater was created by 6-7 km-diameter projectile in an oblique (30-45°) impact. This impact event delivered 25–100 km3 of lunar material to the Earth, i.e., our planet was uniformly covered by ‘Tycho’ meteorites with average density 0.1-0.3 kg/m2 (assuming 30% losses in the atmosphere). These massive deposits may be found in proper stratigraphic layers similar to the Ordovician meteorites .”
Prediction 19 years before the first lunar meteorite was recognized
“The occurrence of secondary craters in the rays extending as much as 500 km from some large craters on the moon shows that fragments of considerable size are ejected at speeds nearly half the escape velocity from the moon (2.4 km/sec). At least a small amount of material from the lunar surface and perhaps as much or more than the impacting mass is probably ejected at speeds exceeding the escape velocity by impacting objects moving in asteroidal orbits. Some small part of this material may follow direct trajectories to the earth, some will go into orbit around the earth, and the rest will go into independent orbit around the sun. Much of it is probably ultimately swept up by earth.” Shoemaker E. M., Hackman R. J., and Eggleton R. E. (1963) Interplanetary correlation of geologic time. Advances in Astronautical Sciences, vol. 8, p. 70-89.
Where on the Moon did they come from? Are any from the far side of the Moon?
Although scientists like to speculate that a certain lunar meteorite came from a certain crater or region of the Moon, no one has identified with certainty the source crater from which any of the lunar meteorites originated.
There is some evidence and model results indicating that asteroidal meteoroids strike the western (leading) hemisphere of the Moon (that is, the “side” with Mare Orientale, which means east because astronomical telescopes see the Moon upside down!) a bit more frequently than the eastern hemisphere (the Mare Marginis “side”). On the other hand, lunar meteoroids leaving the eastern hemisphere may have a slightly better chance of reaching Earth. Overall, however, there is probably little East-West bias in our lunar meteorite collection. There are reasons to expect that asteroidal meteoroids strike the equatorial areas of the Moon a bit (1.28 times) more frequently that the polar regions.
There are no reasons to suspect that lunar meteorites come from the nearside of the Moon preferentially to the farside, or vice versa. So, half of the lunar meteorites come from the far side of the Moon. It is that simple. We just do not know which ones those are. There is no scientific basis for a statement in an advertisement on ebay: “The ONLY LUNAR meteorite from the dark side of the moon.” (Also, of course, the “dark side” of the Moon keeps changing with lunar phase! Except for some locations at the poles, any place in the dark will be sunlit 14 days later.)
Some great technical reading: Gladman et al. (1995), Le Feuvre and Wieczorek (2008), and Gallant et al. (2009).
For any given lunar meteorite, the probability is not exactly 50-50 that it came from either the near side or the far side. There is more mare basalt on the near side than the far side (FeO map; Fig. 18), so the chance is better than 50-50 that an iron-rich meteorite (mare basalt or basaltic breccia) is from the near side and that an iron-poor meteorite (feldspathic) is from the far side. As explained below, Sayh al Uhaymir 169, Dhofar 1442, Northwest Africa 4472/4485 and Northwest Africa 6687 must derive from the near side because of their high concentrations of thorium.
How big are lunar meteorites?
The largest single stone appears to be Northwest Africa 12760 at 58.1 kg (128 lbs). This stone is one of many of the NWA 8046 clan (paired meteorite stones) of which there is more than 200 kg. At the other extreme, several of the lunar meteorite fragments found in Antarctica and Oman only weigh a few grams (a U.S. nickel weighs 5 grams). The smallest named stones are Graves Nunataks 06157 at 0.788 g and Dar al Gani 1048 at 0.801 grams. The largest meteorite is NWA 12691, which consists of “many” pieces totaling 104 kg in mass.
Mass distributions for lunar meteorites are presented in Fig. 6. For at least 2 reasons, however, the figure is a bit misleading.
1) From Antarctica, for example, the 7 Dominion Range meteorites and the 6 LaPaz Icefield meteorites are each pairs (pieces) of a single meteoroid that made the Moon-Earth trip. The 42 lunar meteorites from Antarctica represent about 23 meteoroids and the 73 from the Arabian Peninsula represent about 30 meteoroids (Korotev and Irving, 2021), when likely pairings are considered.
2) Pairing relationships are not well established among the NWA (Northwest Africa) meteorites; in some cases, a pile of many rocks has been given a single name (e.g., NWA 12691) whereas in others (most notably the 40+ named stones of the NWA 8046 clan) different rocks from a single fragmented meteoroid are given separate names, as for the Antarctic meteorites. The 281 northern Africa lunar meteorite stones for which there are compositional data probably represent 77-99 different meteoroids (Korotev and Irving, 2021).
Regardless of these complications, it is clear that lunar meteorites from Antarctica and the Arabian Peninsula are about the same size whereas, for some reason unknown, those from northern Africa are considerably larger.
How rare are lunar meteorites?
Meteorites are very rare rocks; lunar meteorites are exceedingly rare. It difficult to assess how rare they really are. Of the ~42,000 named meteorite stones found in Antarctica, where record keeping has been superb, (1976-2018), 1 in 1000 meteorite stones is from the Moon (42 stones representing 22-23 meteorites; for martian meteorites, it is 1 in 1400).
Another measure of rarity is mass. The total mass of all known lunar meteorites is about 781 kg (1722 lbs). By comparison, the total mass of all stony meteorites is 92,200 kg.
The mass of all known lunar meteorites is now (December 31, 2021) about 2.94 times the mass of the rocks >1 cm in size in the Apollo lunar sample collection.
Lunar meteorites for sale
Meteorites, including lunar and martian meteorites, are easily available for purchase. Samples (end cuts, slices, chips, crumbs, dust) of the lunar meteorites sell on the internet for between about $50 and $4,000 per gram, depending upon rarity (perceived or real!) and demand. By comparison, the price of 24-carat gold is about $40 per gram and gem-quality diamonds start at $1000-2000/gram. Prices have declined with time as the number of lunar meteorites has increased.
Most rocks advertised on the Internet as lunar meteorites are, in fact, meteorites from the Moon sold by reputable dealers. Some are not, however. Also, on more than one occasion, I have seen samples advertised as one particular lunar meteorite (e.g., Dhofar 081) when the sample in the photo is clearly from a different lunar meteorite (e.g., Dhofar 911). Caveat emptor.
I have been contacted by eight men who have wanted to buy a lunar meteorite to mount in a piece of jewelry for their girlfriend, fiancée, or wife. Be aware that compared to many gemstones, lunar meteorites are not “hard” rocks and most have fractures from meteorite impacts on the Moon. And although I love lunar meteorites, they are not all that attractive compared to gemstones. Get her a diamond, emerald, opal, or agate!