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 often called lunar “soil.”
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.
Two special kinds of lithologies occur in the lunar regolith. (Lithology means “rock type,” but it is 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 would call such material volcanic ash. On the Moon, it is usually called pyroclastic glass. In both cases, molten rock cools and solidifies above the Moon’s surface, leading to glassy spherules.
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 does not, 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.
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.
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.
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 Earth’s atmosphere, the solar-wind implanted gases are driven off. This leads to a vesicular fusion crusts – a fusion crust with gas bubbles. If a meteorite has a highly vesicular fusion crust, then it is likely to be a lunar meteorite.