When a rock hurtles through space and makes landfall on another body, it is called a meteorite. Every meteorite holds a record of the earliest processes and phases of Solar System history. Stony (or rocky) meteorites are divided into two main classes. Chondrites are the most common type, so called because of the presence of small, rounded inclusions (chondrules). Rocky meteorites that contain no chondrules are called achondrites and are igneous rocks or complex mechanical mixtures of the same. These “differentiated” meteorites can come from small bodies, like the asteroid 4 Vesta (recently visited by the Dawn spacecraft) or apparently, from even larger objects. Some scientists speculated for years, that among our meteorite collections, we must have samples of other planetary surfaces. In the early 1980s, the lunar science community was shocked by the discovery of a small rock found on the ice sheets in Antarctica. Analysis of ALHA81005 demonstrated conclusively that it came from the Moon.
But just how do we determine whether a meteorite is of lunar origin or not? Lunar rocks have certain distinguishing characteristics. We’ve studied a large number of both highland and mare samples that were brought back to Earth during the Apollo missions. First, they are not primitive – they are all igneous (products of crystallization from a melt) or modified by impact processing (shock metamorphism, making complex, aggregate rocks called breccias). Second, they have a unique chemical signature, being enriched in elements that have high melting points (refractory elements) and depleted in volatiles (elements with low melting points). Third, they typically have ages that are very old, but younger than the most common meteorites; lunar highland samples tend to be around 4 billion years old, while the most common meteorites all formed about 4.6 billion years ago. Finally, all meteorites have unique oxygen isotope ratios (i.e., the ratios of 17O and 18O to 16O) that are distinct from samples of the Earth and Moon (which lie on the same line). Usually, a meteorite that is a suspected sample of the Moon can be definitively shown to be a lunar rock (or not) through analysis of its oxygen isotopes.
After the discovery of the first lunar meteorites, the floodgates opened and we now recognize more than 100 individual stones as being rocks from the Moon. Some are multiple pieces of a single object (shown by identical compositions and collection locales in close proximity). Others represent distinct meteor entry events. In the field of meteoritics, a “fall” is an object recovered from an identified fireball or meteor; a “find” is a meteorite collected from the ground without any observation of its arrival. All lunar meteorites collected to date are finds. Most of these objects come from Antarctica, where the dark rocks are highly visible on the continent’s wind-blown ice sheets. Others come from sparsely vegetated deserts in Africa, where dark rocks are also easily seen against vast expanses of light-toned sand deposits.
Are the lunar meteorites significantly different from the samples returned by the Apollo missions? Not really. Lunar meteorites are mostly samples of the highlands regolith, the fragmental layer that covers bedrock on the Moon. They contain many different fragments (clasts) of ground-up rocks, including fragments of other fragmental rocks. Some are impact melt breccias, rocks formed by shock melting and fusion of the crust during the impact of large objects on the Moon. These melt rocks are then later hurled into space by another subsequent impact. Rocks launched into space can then be swept up by the Earth and become incoming meteorites. Although most lunar meteorites are from the highlands, some represent the maria (the dark volcanic plains of the Moon), usually of composition different from those collected during Apollo.
Recently, I had the opportunity to observe in detail a spectacular meteorite, NWA5000 – with a mass of more than 11 kg, the largest meteorite attributed a lunar origin. Space artifacts collector Steve Jurvetson has obtained a large fragment of this rock, a sawn slab almost a foot across in maximum dimension. Looking at this cut slab closely, I noticed something very unusual. The sample is beautiful (see image above), a complex breccia with large (tens of cm across) white clasts, intruded by fine-grained, dark gray matrix. In an Apollo sample, we would call this a clast-laden impact melt; it closely resembles melt breccias from the lunar highlands returned by the Apollo 16 mission in 1972. What drew my attention was that within the crushed and brecciated large white clasts were bits of bright metal, all surrounded by rusty deposits.
The rust was not the surprise; almost all meteorite finds show rust where the chemically reduced, pure meteorite iron is exposed to the free oxygen and water vapor in the Earth’s atmosphere. The surprise was the metal inclusions. Most large lunar igneous rocks have very little, if any, meteoritic metal within them. Yet these large white clasts contain significant amounts of metal. Analysis of this material shows it to be kamacite, an alloy of iron and nickel that is common in meteorites, but not in lunar samples. Yet nearly all of the large igneous rock clasts in NWA5000 appear to contain these bits of kamacite.
How can meteoritic metal get into a lunar sample? As we analyzed the Apollo collections, we found that almost all of the impact breccias returned from the Moon contain meteoritic contamination. Typically, this manifests itself as a cryptic chemical component of siderophile (“iron-loving”) elements (such as iridium) within the shock-melted portion of the rock. This siderophile enrichment happens during the impact event when vaporized meteorite projectile is intimately mixed with the shock-melted and vaporized target under tremendous pressures and temperatures. The superheated melt overtakes and incorporates large quantities of cooler, fragmental debris (mineral grains and larger rock fragments from the crust). This debris quenches the melt to create a fragment-laden rock with a fine-grained melt matrix.
The puzzling feature in this case is that the metal in NWA5000 is in the clasts, not the melt matrix. It is virtually unheard of to find large metal grains within pristine igneous rocks from the Moon. In fact, the absence of meteoritic metal in such rocks is one of the most reliable indicators that the lunar rock is pristine (i.e., generated from internal, endogenic melting, a true igneous rock). I am not the only one to note the unusual nature of this sample. Tony Irving, the petrologist from the University of Washington who made the initial examination of this sample, noted that metal grains in such rocks are very uncommon. He suggested that these rocks were pieces of a giant sheet or pool of impact melt which segregated (differentiated) during slow cooling into a chemically stratified sequence of rocks. Such a process has been proposed for some of the largest impact craters (basins) on the Moon for many years, but has never been observed or proven to occur.
Is NWA5000 a fragment of a large, differentiated melt sheet on the Moon? Perhaps. But as the collection of meteorites from ice sheets and remote deserts proceeds, we continue to discover increasingly strange and new meteorite types, some of which have been proposed to come from planets (e.g., the planet Mercury) as yet unsampled, or at least, unrecognized. As the large igneous clasts in this meteorite are so unlike other lunar samples, I am forced to at least consider the possibility that NWA5000 may be some as-yet unrecognized type of achondrite meteorite, from an unknown object in space. Determination of the oxygen isotopes of this rock would be extremely valuable. If they are the same as other lunar meteorites and the Apollo samples, we would have direct evidence of a postulated but undocumented process having occurred on the Moon. If they are different, we would have discovered an entirely new class of meteorite. Either way, a significant new discovery awaits.