A lunar micrometeorite preserves the beginnings of the solar system
About half a century ago, six Apollo missions and three Luna missions to the Moon collected rock samples. The former brought in some 380 kg, and the latter brought in less than ½ kg. The curators of these collections have wisely preserved much of this material over the years for future scientific study. More recently, the Chinese lunar exploration program Chang’e 5 returned from the Moon in December 2020 with 2 kg of material which will soon be available for analysis.
Advances in geological and chemical instrumentation and analysis have now allowed researchers to study tiny samples, some as small as 100 to 200 µm in diameter. Such a fragment of soil from the Moon 16 mission was recently studied by Svetlana Demidova of the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow and her colleagues. The team found that the sample is likely a stony micrometeorite that likely slammed into the Moon no earlier than 3.4 billion years ago and possibly around 1 billion years ago.
Demidova and her colleagues used a suite of geochemical tools to interrogate the tiny spot, shown below in a backscattered electron image. Plagioclase (Pl) mineral phases, highlighted by the yellow squares, were studied using Raman spectroscopy and show signs of shock waves, indicating an impact event. Additionally, the oxygen isotope measurements of olivine (Ol) and pyroxene (Px) in the sample are distinct from those of moon rock.
The oxygen isotopic composition of the sample most closely matches that of extralunar LL chondrites. This is a group of common stony meteorites with a relatively low iron and metal concentration and whose parent asteroids may have bombarded the Earth-Moon system early in its history.
The grain of merrillite (Mer) in the Moon 16 allowed researchers to age the fragment using uranium-lead radiometric dating. The resulting age of 4.5 billion years corresponds to the time when chondrites were thought to form in the early solar system. This age also means that subsequent impacts and other activities then heated the sample to a maximum of 400 °C, the temperature at which the U–Pb timer would have been reset.
The age of 4.5 billion years indicates when the merrillite mineral in the sample formed but not necessarily when the sample arrived on the Moon. The fragment could have hit the area near the Moon 16 landing site or were transported there as ejected from an impact elsewhere on the surface of the Moon. Given the age of the basalt that filled the huge impact crater where Moon 16 sample was found, Demidova and her colleagues suspect that the fragment arrived on the Moon about 3.4 billion years ago.
The fragment has a curiously similar mineralogical composition to the asteroid Itokawa, a near-Earth object that the Japanese Hayabusa mission dated to be 1 billion years old. This similarity leaves open the possibility that the fragment came from an object with a composition similar to the asteroid Itokawa.
The Itokawa-like object could have either impacted the Moon or lost material as it passed through the Earth-Moon system. With many lunar samples remaining in Apollo, Luna and Chang’e 5 archives, researchers may be able to find evidence to support these possibilities. (SI Demidova et al., Nat. Star.2022, doi:10.1038/s41550-022-01623-0.)