The Moon’s surface is covered by a layer of soil, known as regolith. This is an important boundary between the Moon and the surrounding space environment, where there are records of the Moon’s interaction with the solar wind, cosmic rays (from galactic and solar sources), and collisions from impacting bodies (asteroids, comets and planetary materials).
Sometimes these processes are destructive – in particular impact collisions can cause heating, melting, fracturing of lunar rocks and soil. However, sometimes these impact events deliver volatile-rich materials to the Moon in the form of chemical signatures deposited by vapour-rich impact plumes, and (more rarely), from fragments of rock that survive the impact collision process. We have a growing record of such meteorite fragments that have been found within the lunar regolith (see more information here), although to date, all these fragments are very small less than a cm or so in size.
In a new study published in Earth and Planetary Science Letters, researchers in Manchester working with collaborators in the US and France, revisited a famous meteorite collected by the Apollo 12 astronauts. The meteorite sample is called Bench Crater, as it was found next to an impact crater of the same name (meteorites tend to be named after the closest geographic location to where they were found). The meteorite even has its own Wikipedia page and record in the Meteoritical Society’s meteorite database. It was found by researcher Dr Jeff Taylor when he was looking through grain mounts (polished soil fragments) of Apollo 12 soil sample 12037, and spotted a dark-coloured fragment a couple of mm in length that looked a bit different from all the other ‘normal’ lunar materials. Jeff had discovered a fragment of a carbonacoues chondrite which had survived the impact that delivered it to the lunar surface (what a great day that must have been when he realised he had found something new and unique in a lunar soil samples)!
The Bench Crater meteorite has previously been studied in the 1980s and 1990s by researchers before who have looked at what minerals it contains and how some of these clay-like minerals still hold water. Our new study looked more closely at the isotopic chemistry of different mineral phases to understand how similar/different the meteorite is compared with the meteorites that we have found here on Earth, and to understand how the impact process that delivered Bench Crater to the Moon affected its volatile element records.
We used a NanoSIMS mass spectrometer at NASA’s Johnson Space Centre in the USA, which allowed us to look at very small slices of the meteorite to chatacterise its light isotope (carbon, nitrogen and hydrogen) isotope records. We also used a “big” SIMS in Nancy France to determine the oxygen isotope abundance and did more work to analyse the mineral major element composition and mineral structure.
Together, these data tell us that Bench Crater is distinctive from other types of carbonaceous chondrite meteorites – it has chemical similarities to some groups (like the CM and CI types, which are water-rich), but it doesn’t match perfectly with these meteorite groups. This is interesting, as it could suggest that the meteorite was from a new type from a different asteroid parent body. The meteorite has also been partially thermally processed (heated), although this was not enough to completely bake out the volatiles held within minerals.
The measurements we made in the Bench Crater meteorite provides direct evidence of the survivability of volatile-rich asteroidal material delivered to the lunar surface, and shows that light-elements are not necessarily lost, nor significantly isotopically fractionated (i.e., chemically altered), during the impact process. The data provides a critical ground truth for investigations of lunar volatile resources by future missions, such as that planned by Russia (the Luna 27 mission with its European Space Agency provided PROSPECT instrument package). Our study has important implications for understanding the transfer of volatile material across the Solar System and pinpointing the sources of hydrated and organic-rich material delivered to the Earth-Moon system at different points in its past.
Find the new paper (open access):
K. H. Joy, R. Tartèse, S. Messenger, M. E. Zolensky, Y. Marrocchi, D. R. Frank, and D. A. Kring (2020) The isotopic composition of volatiles in the unique Bench Crater carbonaceous chondrite impactor found in the Apollo 12 regolith. Earth and Planetary Science Letters. Vol 540 doi.org/10.1016/j.epsl.2020.116265
Related reading about meteorites found on the Moon and their scientific importance:
More details about the history of how the Moon is a tape recorder of small body migration can we read about in this earlier E&SS blog https://earthandsolarsystem.wordpress.com/2016/10/26/new-group-paper-the-moon-an-archive-of-small-body-migration-in-the-solar-system/ and reserach paper itself: Joy K. H., Crawford I. A., Curran N. A., Zolensky M. E., Fagan A. L., and Kring D. A. (2016) The Moon As An Archive Of Small Body Migration In The Solar System. Earth Moon and Planets DOI: 10.1007/s11038-016-9495-0
A possible piece of the Earth has been located on the Moon – read more here
Leftovers from impactors on the Moon found within ancient soil samples – a nice PSRD article
How we might be able to explore the Moon to access ancient regolith records and the secrets that they hold – article by Prof. Ian Crawford