Rare Fragment of Stony Asteroid Found in Lunar Soil

The Luna 16 landing craft. Credit: NASA.

The lunar surface is covered by a layer of impact-generated regolith (soil). Other than the occasional lava flow, meteorite impact (impact gardening) is the main source of modification to the lunar surface. As such, the lunar regolith should be rich in impactor material and could provide a record of the evolution of impactor types throughout the Moon’s long history. However, recognizable fragments of extralunar material are rare in lunar soil samples, with only two small fragments previously identified: a carbonaceous chondrite found at Bench Crater and an enstatite chondrite found at Hadley Rille. New work by S. I. Demidova (Vernadsky Institute) and colleagues now brings this number to three with the discovery of a micrometeorite fragment in the Luna 16 soils.

Fragment #443 is an approximately 200-micrometer-sized rock fragment recovered by the Luna 16 robotic mission from the eastern part of Mare Fecunditatis. Chemical analysis of the minerals olivine and pyroxene in this fragment showed compositions (e.g., Mg abundance in olivine and Ca abundance in pyroxene) that are atypical for most lunar samples, but similar to those in LL ordinary chondrites, as well as samples of asteroid Itokawa collected by the Japanese Hayabusa mission. Oxygen and sulfur isotope analyses also showed an affinity with Itokawa and LL chondrites. Radiometric age dating of the fragment revealed an age of 4548 Ma (million years), indicating that this sample is one of the most ancient objects in the solar system and has not been significantly reheated since its formation. All analyses combined suggest that fragment #433 is an LL chondrite.

Results from the Hayabusa mission have suggested that Itokawa and the LL chondrites are derived from the Flora asteroid family. Demidova and coauthors suggest that this family may be the source for fragment #433 and thus a source of meteoritic material to not only Earth but also the Moon. READ MORE