Last week we received the news that one of our group members and collaborators have been funded to go meteorite hunting in Antarctica (read more about it here). The expedition is focused around the missing iron meteorites; compared to the other classes of meteorites, there are far fewer iron meteorites than expected in the Antarctic collection. For this blog post, I’ll be talking about the missing lunar meteorites – why haven’t we found any in the Americas or Europe?
Before we answer that question, it’s first best to introduce lunar meteorites – what are they? Lunar meteorites are an important class of stony achondritic meteorites that are found in both hot- and cold-desert environments. They complement the Apollo sample collection in that they represent random samples of the whole lunar surface (as opposed to the restricted locations of the Apollo landing sites). Therefore, they provide information about areas not visited by astronauts, such as the lunar farside and the regions closer to the poles. So far, around 137 kg of meteorites are classified as lunar (originating from the Moon’s surface) with around 240 individual samples in the worldwide meteorite collection to date. However, some of these have been grouped based on similarities in composition, mineralogy, and spatial location on Earth. Overall this means there are around 118 individual meteoroids from the Moon that have landed on Earth and been found to date, some of which broke up during passage through the Earth’s atmosphere to give a larger number of individual samples.But how did they get here? Well the Solar System still has lots of loose material orbiting the sun in the form of asteroids, comets, and dust. This material is constantly bombarding all of the planets and moons in the Solar System, including Earth and the Moon. On bodies such as the Moon that have no atmosphere, the smallest of particles rain down on the surface and break up (or comminute) the lunar surface material. Sometimes a larger piece of asteroid or comet will strike the Moon’s surface and form an impact crater along with a curtain of ejected material, some of which has enough energy to escape the Moon’s gravitational pull. Some of this material then goes into orbit around the sun for up to 10 million years before colliding with Earth.
Currently the majority of lunar meteorites found are from dry hot- and cold-deserts such as Oman and Antarctica. This is because meteorites are easily seen on the sand or ice and cannot be confused with terrestrial rocks in these locations. So, returning to the original question, why haven’t any lunar meteorites been found in the Americas, Asia and Europe? Part of the reason is due to their composition and mineralogy. They contain the same common minerals as found on Earth, namely plagioclase, olivine, and pyroxene. Depending on the relative proportions of these minerals, the lunar meteorites can be grouped into anorthosites or basalts (indicating they came from the light, highland regions or dark, mare regions of the Moon respectively) – some of the same rock types that we encounter on Earth. As such, they can appear very similar to and be easily confused with Earth rocks. The only methods of proving they came from the Moon is chemical analysis – the subtle differences in isotope ratios (i.e. the abundance of one isotope compared to another of the same element), the age of the sample, and trace elements (Mn, Th, K) and the similarity to rocks returned by the Apollo sample return missions. So there are almost definitely some lunar meteorites in Europe and the Americas but they are so similar to Earth rocks that none yet have ever been found.
Another curiosity is that no lunar meteorites have been seen to fall to Earth (known as ‘fall’ events). All other meteorite types, including meteorites from Mars, have been seen to fall as a shooting stars and then been recovered by scientists. It is unknown why a lunar meteorite has never been seen to fall – it is most probably just down to probability. At some point in the future a lunar meteorite will be seen to fall and the more time that passes, the more likely this will be. Hopefully one day a lunar meteorite will be seen to fall in the UK!
So, while the Apollo samples are highly important for lunar science, lunar meteorites are equally important for different reasons, namely that they provide samples from other areas of the Moon. To find a lunar meteorite in Europe, the Americas or Asia would be an exciting discovery, not in the least because it would be the first lunar meteorite found on these continents, but also because it would add another lunar sample to the collection to be analysed and provide another window in the geology and history of our nearest celestial neighbour!
For more information on the subject of lunar meteorites see:
See also the recent blog by our collaborator Abigail Calzada-Diaz from Birkbeck College about what lunar meteorites reveal about the Moon’s crust https://earthandsolarsystem.wordpress.com/2016/11/04/new-group-paper-new-lunar-meteorites-revealing-the-histroy-of-the-moons-crust/