A lunar micrometeorite preserves the solar system’s early history

Meteorites and other hunks of space rock have been colliding with the Earth and Moon for their entire history. Counting impact craters on both bodies provides some estimate of the flux of material bombarding the inner solar system. And if it can be measured, a meteorite’s composition yields information about the geological processes that were responsible for its formation.

Discoveries of meteorites on Earth, though, are rare. Many burn up in the atmosphere before reaching Earth’s surface. For the ones that do make it to the ground, flowing water or other erosive processes often break them down. Slower-moving plate-tectonic activities also hide evidence of impacts.

The cold, dry Antarctic desert offers conditions more favorable for the preservation of meteorites, and, consequently, more are found there than anywhere else on Earth. But the Moon is an even better hunting ground for finding meteorites because of its lack of atmosphere, limited erosion, and absence of plate tectonics. ¹

Half a century ago, six US Apollo missions and three Soviet Luna missions to the Moon collected rock samples of roughly 380 kg and less than ½ kg, respectively. Curators of those collections have wisely preserved much of that material over the years for future studies (see the article by Brad Jolliff and Mark Robinson, Physics Today, July 2019, page 44 ). 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 made available to the international community for analysis.

Luna 16, which in September 1970 was the first robotic sampling probe to land on the Moon, returned to Earth with lunar soil—the sandy fraction of the Moon’s rock-strewn surface. The region where the sample was taken, the Mare Fecunditatis basin, features a huge impact crater that was subsequently filled with basaltic magma. The surface there consists of that solidified basalt pockmarked with the remains of many small impact events.

One recovered 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. ² The fragment had already been identified in 1990 as a micrometeorite. ³ Demidova and her team found that the tiny micrometeorite dates from the early solar system and that it probably struck the Moon no earlier than 3.4 billion years ago and perhaps around 1 billion years ago.

Extralunar

Researchers have already analyzed and categorized large volumes of material collected from the Moon’s surface. Geochemical analyses offer precise measurements of the isotopic composition of minerals and other properties. That information is useful for determining whether a sample originated from the Moon.

Extralunar and local materials can easily mix, though, especially after an impactor pulverizes the surface. Isotope measurements, therefore, aren’t always reflective of a material’s origins. One strategy to overcome that limitation is to look for relatively pristine material with exotic components that could come only from asteroids or meteorites.

The soil fragment was first identified as a micrometeorite after it was magnetically separated from other material recovered by Luna 16. Mineral chemistry analyses classified the sample as an ordinary chondrite, a category that includes about 77% of meteorites. It’s a class of stony meteorites made of material accreted from a parent asteroid and characteristic chondrules—small, round silicate grains that formed from molten droplets during planetary formation (see Physics Today, March 2015, page 14 ).

With a diameter of just 200 μm, the soil sample defied most measurement techniques, which in the 1990s required milligrams of material and yielded large uncertainties. But geological and chemical instrumentation and analysis have been steadily advancing for the past 10–15 years. Now researchers can study a sample with a length scale as small as the soil fragment and a mass as small as a nanogram.

In evidence

To learn more about the sample’s history, Demidova and her colleagues used a suite of geochemical tools to interrogate the speck of soil, shown in figure 1. One exotic component that they found is a metal sulfide vein, which isn’t produced by lunar processes. Illia Dobryden, of the RISE Research Institutes of Sweden, used Raman spectroscopy to analyze the plagioclase (Pl) mineral phases, highlighted by the yellow squares. The results showed evidence of shock metamorphism, a pressure-induced change to the mineral’s crystal structure that’s indicative of an impact event.

Figure 1.

Martin Whitehouse of the Swedish Museum of Natural History, Renaud Merle of Uppsala University, and Alexander Nemchin of Curtin University completed the isotopic analyses. The secondary-ion mass spectrometry method they used was in its infancy in the 1970s when the sample was recovered and is now a standard analysis approach for rare specimens. The minimally destructive analytical technique directs a focused ion beam on a sample to eject ions from the surface and direct them to the mass analyzer.

The oxygen-isotope composition of the olivine (Ol) and pyroxene (Px) in the sample, in particular, is distinct from that of lunar rock. The sample’s composition most closely matches that of ordinary LL chondrites—the subtype that originates from an asteroid parent body with low metal concentrations.

Additional evidence for the chondritic origin comes from the proportion of the iron-rich components fayalite and ferrosilite that are found in olivine and low-calcium pyroxene, respectively. Figure 2 shows the composition of those minerals in the sample and in other meteorites. The geochemical evidence agrees with the isotope results and seems to suggest that the sample comes from LL chondrites, although it does somewhat overlap the population of L chondrites.

Figure 2.

Date of entry

The soil sample also contains merrillite— a phosphate mineral with trace amounts of uranium and lead. That mineral grain in the sample, therefore, afforded the researchers the opportunity to determine the fragment’s age using uranium–lead radiometric dating. The resulting age of 4.5 billion years is consistent with the time that chondrites were thought to be forming in the early solar system.

Finding such an old meteorite fragment that struck the Moon surprised Whitehouse. “It means that the grain didn’t experience particularly high temperatures above 400 or 500 degrees,” says Whitehouse. Above that threshold, lead atoms generated by the decay of uranium would diffuse out of the crystal, which would reset the U–Pb chronometer.

The fragment could have crashed 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. The landing site is characterized predominantly by basaltic rocks that were present before the sample slammed into the surface. The timing of the basalt deposition, therefore, limits the age of the impact event. Based on the basalt’s age, Demidova and her colleagues suspect that the fragment arrived on the Moon no earlier than around 3.4 billion years ago.

Itokawa-like

Given the number of inferences needed to estimate when the micrometeorite may have landed on the Moon’s surface, the 3.4-billion-year timing is far from guaranteed. Curiously, the soil sample has a similar mineralogical composition to a sample of the near-Earth Itokawa asteroid. An Itokawa sample was retrieved by Japan’s Hayabusa mission in 2010 and was determined to be 1.3 billion–1.5 billion years old. ⁴

That similarity leaves open the possibility that the fragment hit the Moon more recently than 3.4 billion years ago, after the merrillite grain crystallized. The fragment may have originated from an object with a composition similar to that of Itokawa. Demidova and her colleagues refer to that possible object as “Itokawa-like” and suggest that it could have impacted the Moon or shed some material as it passed through the Earth– Moon system.

“We still don’t know why it is so similar to Itokawa samples. But we cannot prove the Itokawa origin,” says Demidova. “The next steps are to search for some other meteorite debris.” With ample lunar samples remaining in the Apollo, Luna, and Chang’e 5 archives, more micrometeorites should be recoverable to help learn more about the Moon’s impact history.