Regolith is the name for the layer of unconsolidated material at the surface of a planet – the loose stuff that overlies the solid rock. On Earth, soil is part of the regolith, so lunar regolith is consequently often called “soil.”
Apollo 11 astronaut Buzz Aldrin took this photo of his footprint in the lunar regolith on July 20. Image credit NASA: AS11-40-5878
Lunar regolith is composed in part of rock and mineral fragments that were broken apart from underlying bedrock by the impact of meteorites. A rock composed of bits and pieces of older rocks is called a breccia.
These are small rock fragments from the Apollo 11 regolith, which is dominated by mare basalt. All are from the 2-4-mm grain-size fraction. There are two impact-glass spherules in the image. Image credit: Randy Korotev
These are small rock fragments from the 1-2-mm grain-size fraction of Apollo 16 regolith sample 65902. The Apollo 16 regolith is dominated by anorthositic lithologies (the lightest-colored fragments in the image) and noritic impact-melt breccias (non-glassy gray). Notice in both photos that lunar rocks are not particularly colorful and that there are no reddish lunar rocks. Image credit: Randy Korotev
Two special kinds of lithologies occur in the lunar regolith. (Lithology means “rock type,” but it’s a more general term. Neither lithology discussed here is really a type of rock.)
One of the special lithologies in the lunar regolith is glass spherules. Glass spherules are formed in two ways. Some are formed when a meteorite impact melts material, the melt is ejected from a crater, and small globs of the melt solidify before they land. Such melt bombs are usually spherical and range in size from much less than a millimeter to about a centimeter. Several are evident as black, glassy spherules in the photos above.
Other glassy spherules derive from magma that is violently erupted from volcanoes – a “fire fountain.” On Earth, we’d call such material volcanic ash. On the Moon, it’s usually called pyroclastic glass. In both cases, molten rock cools and solidifies above the Moon’s surface, leading to glassy spherules.
Photomicrograph of two areas in a thin section of the Apollo 17 regolith (sample 71061 ) containing volcanic spherules (“orange glass”) . The dark area is where ilmenite has crystallized from the glass . The spherules (elongated on the right) are about 0.1 mm in diameter. Image credit Brad Jolliff
The other special lithology of the lunar regolith is called an agglutinate. Agglutinates are small glassy breccias formed when a micrometeorite strikes the lunar regolith. Micrometeorites are a millimeter or less in size. Millions of micrometeorites strike the Moon every day. (Millions strike Earth’s atmosphere every day, too.) When a micrometeorite strikes the lunar surface, some of the impacted regolith melts and some doesn’t, so the product is a glass with mineral and rock fragments entrained. The glass often shows flow features. Agglutinates are typically tens of micrometers to a few millimeters in size.
Photomicrograph of a thin section of a large agglutinate fragment from the Apollo 17 regolith. The fragment is about 3 mm across. Image credit Brad Jolliff
Agglutinates contain holes called vesicles – frozen gas bubbles in the glass. The bubbles occur for the following reason. Rock and mineral fragments at the lunar surface are exposed to the solar wind, ions of light chemical elements like hydrogen and helium that are emitted from the sun at exceedingly high speeds, several hundred kilometers per second. Because the Moon has no atmosphere and the solar wind ions are moving fast, they are imbedded or implanted into the surface material of the Moon. They do not penetrate very deep into a rock or mineral grain, only a few hundredths of a micrometer, so all the solar-wind- implanted atoms are at the very surface of lunar regolith grains. Meteorite impacts stir the surface regolith so that the upper few meters of regolith are rich in implanted ions of hydrogen and helium. The amount of solar-wind implanted ions is greater in the very finest material because the fine material has more surface area than the coarser material.
When a micrometeorite strikes fine-grained material at the surface, some of the material gets hot enough to melt and form the glass of an agglutinate. It also gets hot enough to liberate the solar-wind-implanted hydrogen and helium, causing bubbles in the glass.
The reason agglutinates and glass spherules are special is that both lithologies can only be produced at or above the Moon’s surface, proving that the breccia material was at or very near the lunar surface. This is how we know that regolith breccias are composed of regolith.
Photograph of a petrographic thin section of Apollo 16 regolith breccia 60016. The longest dimension is 16 mm. A few glass spherules and are evident. Agglutinates are hard to find in regolith breccias and there may be none in this section. Image credit: Randy Korotev
Meteorite impacts both break rocks apart and glue rock fragments back together again. During impacts of meteorites that form craters of hundreds of meters in diameter or larger, the material just below the point of impact melts and some even vaporizes. Material that is deeper may just shatter in place, creating rock fragments. When the shock wave associated with the impact passes through fine grained surface material, the material can be compressed into a rock, something like making a snowball by squeezing snow in your hands. If the resulting rock contains glass spherules or agglutinates, it is called a regolith breccia. If it consistent only of fragmental material with no glass spherules or agglutinates, it is called a fragmental breccia. Regolith breccias consist of fine-grained material from the upper few meters of the Moon; fragmental breccias consist of material that was deeper.
Lunar meteorite NWA 2995 is a fragmental breccia. (We are looking at a sawn face.) Again, note that the clasts are shades of gray, not colored. Also, there are several different kinds (colors and textures) of clasts, which we would expect in a lunar regolith or fragmental breccia considering the diversity of rocks in the Apollo 11 and 16 regolith samples, above. In contrast, many terrestrial sedimentary rocks and basalts contain of clasts or phenocrysts of a single rock or mineral type. In lunar breccias, it is common to see clasts that are themselves breccias – “breccias within breccias.” In this photo, there is a gray breccia containing white clasts on the right. Notice that there is no preferred orientation of the clasts. In terrestrial sedimentary rocks, clasts often are aligned in the same direction because the Earth has more gravity than the Moon. In lunar breccias, the clasts are usually not rounded. In terrestrial sedimentary rocks, clasts are often rounded because the pebbles from which they form were rounded by abrasion against each other in water or ice before they were cemented into a new rock. Note also that aspect ratio (length to width) is short, almost always less than 3. Elongated clasts and phenocrysts are more typical of terrestrial rocks. In most, but not all, lunar regolith and fragmental breccias, the matrix is darker than the clasts. Photo credit: Randy Korotev
This is a thin section of Apollo 17 sample 72275, another fragmental breccia (fov: 38 mm. At the far left is a breccia in a breccia in a breccia. The dark clasts are glassy breccias, the white clasdts are anorthosites. Photo credit: Randy Korotev
It is a fascinating and curious observation that many lunar meteorites are regolith breccias. Regolith breccias were collected by the Apollo astronauts, but another type of breccia – impact-melt breccia – is more common in the Apollo collection, however. Thin regolith occur on the surface of asteroids, and some “regular” (asteroidal) meteorites are regolith breccias. Because the Moon is closer to the sun than are the asteroids and because the lunar regolith is thicker than asteroidal regolith, lunar regolith breccias are much richer in solar-wind implanted gases than are asteroidal regolith breccias.
When a lunar meteoroid that is a regolith breccia is heated and the surface melts as it passes through the Earth’s atmosphere, the solar-wind implanted gases are driven off. This leads to a vesicular fusion crusts – a fusion crusts with bubbles. If a meteorite has a highly vesicular fusion crust, then it’s likely to be a lunar meteorite.
Vesicular fusion crust on lunar meteorite QUE 94281.
Below are photos of regolith breccias from the Apollo collection. Sample 15565 was collected at station 9a, a location at the edge of Hadley Rille where the soil consists mainly of mare basalt. However, the sample is a regolith breccia consisting mostly of nonmare material, but with some mare basalt. The large clasts in the photos are KREEP or mare basalts. The composition of the matrix does correspond to that of any Apollo 15 soil in that it is KREEPier (Sm = 19 ppm, compared to 12 ppm in the station 6).
This sample of 15565 consists of three pieces. The mass depicted here is 126 g.
The largest piece is about 5 cm in longest dimension. The large clast at the top is a mare basalt.
Smallest piece, ~2 cm.
Medium-sized piece, 13 cm, with a large mare basalt clast at the right.
Thin section of Apollo 15 sample 15205 (fov = 3.3 cm).
In the Apollo 16 regolith breccias below, the dark clasts are usually KREEPy impact-melt breccias and the light clasts are anorthosites, usually brecciated. There are no mare basalts clasts.
Fragment (~3 cm) of Apollo 16 regolith breccia 66035.
Two views of fragments from Apollo 16 regolith breccia 66055.
Fragment (~3 cm) of Apollo 16 regolith breccia 66075.
Left: NASA photo of a sawn face of Apollo 16 regolith breccia 60019. This rock is coherent (difficult to break). Right: NASA photo of a sawn face of lunar meteorite MacAlpine Hills 88105, also a coherent regolith breccia. (Both rocks were deliberately sawn and broken in the NASA lab.) Note the similarities: shades of gray, clasts of different sizes, no rounded clasts, no preferred orientation of clasts, clasts and matrix are the same hardness – the fractures run through the clasts as if they are not there.