Lunar micrometeorite preserves solar system’s early history

About half a century ago, six Apollo and three Luna missions to the Moon collected rock samples. The former returned some 380 kg, and the latter brought back less than a ½ kg. Curators of those collections have wisely preserved much of that material over the years for future scientific studies. More recently, the Chinese Lunar Exploration Program’s Chang’e 5 returned from the Moon in December 2020 with 2 kg of material that will soon be available for analysis.

Advances in geological and chemical instrumentation and analysis have now enabled researchers to study tiny samples, some just 100–200 µm in diameter. One such fragment of soil from the Luna 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 probably impacted the Moon no earlier than 3.4 billion years ago and perhaps around 1 billion years ago.

Demidova and her colleagues used a suite of geochemical tools to interrogate the tiny speck, shown below in a backscatter-electron image. The plagioclase (Pl) mineral phases, highlighted by the yellow squares, were studied using Raman spectroscopy and show evidence of shock waves, which is indicative of an impact event. Furthermore, oxygen-isotope measurements of the olivine (Ol) and pyroxene (Px) in the sample are distinct from those of lunar rock.

The sample’s oxygen-isotope composition most closely matches that of extralunar LL chondrites. They’re a group of common stony meteorites with a relatively low iron and low metal concentration and whose parent asteroids may have bombarded the Earth–Moon system early in its history.

The merrillite (Mer) grain in the Luna 16 sample afforded the researchers the opportunity to age the fragment using uranium–lead radiometric dating. The resulting age of 4.5 billion years corresponds with the time that chondrites were thought to be forming in the early solar system. That age also means that later impacts and other activity subsequently heated the sample to no more than 400 °C, the temperature at which the U–Pb chronometer would have been reset.

The 4.5-billion-year age indicates when the merrillite mineral in the sample formed but not necessarily when the sample arrived on the Moon. The fragment could have impacted the area in the vicinity of the Luna 16 landing site or have been transported there as ejecta from an impact elsewhere on the Moon’s surface. Given the age of the basalt that filled the huge impact crater where the Luna 16 sample was found, Demidova and her colleagues suspect the fragment arrived on the Moon no earlier than around 3.4 billion years ago.

The fragment has a curiously similar mineralogical composition to the Itokawa asteroid, a near-Earth object that the Japanese Hayabusa mission dated to be 1 billion years old. That similarity leaves open the possibility that the fragment originated from an object with a composition similar to the Itokawa asteroid.

The Itokawa-like object could have either impacted the Moon or shed some material as it passed through the Earth–Moon system. With ample lunar samples remaining in the Apollo, Luna, and Chang’e 5 archives, researchers may be able to find evidence to support those possibilities. (S. I. Demidova et al., Nat. Astron., 2022, doi:10.1038/s41550-022-01623-0 .)