Lunar rocks and soil
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General characteristics
As noted above, the lunar regolith comprises rock fragments in a continuous distribution of particle sizes. It includes a fine fraction—dirtlike in character—that, for convenience, is called soil. The term, however, does not imply a biological contribution to its origin as it does on Earth.
Almost all the rocks at the lunar surface are igneous—they formed from the cooling of lava. (By contrast, the most prevalent rocks exposed on Earth’s surface are sedimentary, which required the action of water or wind for their formation.) The two most common kinds are basalts and anorthosites. The lunar basalts, relatively rich in iron and many also in titanium, are found in the maria. In the highlands the rocks are largely anorthosites, which are relatively rich in aluminum, calcium, and silicon. Some of the rocks in both the maria and the highlands are breccias—i.e., they are composed of fragments produced by an initial impact and then reagglomerated by later impacts. The physical compositions of lunar breccias range from broken and shock-altered fragments, called clasts, to a matrix of completely impact-melted material that has lost its original mineral character. The repeated impact history of a particular rock can result in a breccia welded either into a strong, coherent mass or into a weak, crumbly mixture in which the matrix consists of poorly aggregated or metamorphosed fragments. Massive bedrock—that is, bedrock not excavated by natural processes—is absent from the lunar samples so far collected.
Lunar soils are derived from lunar rocks, but they have a distinctive character. They represent the end result of micrometeoroid bombardment and of the Moon’s thermal, particulate, and radiation environments. In the ancient past the stream of impacting bodies, some of which were quite large, turned over—or “gardened”—the lunar surface to a depth that is unknown but may have been as much as tens of kilometers. As the frequency of large impacts decreased, the gardening depth became shallower. It is estimated that the top centimeter of the surface at a particular site presently has a 50 percent chance of being turned over every million years, while during the same period the top millimeter is turned over a few dozen times and the outermost tenth of a millimeter is gardened hundreds of times. One result of this process is the presence in the soil of a large fraction of glassy particles forming agglutinates, aggregates of lunar soil fragments set in a glassy cement. The agglutinate fraction is a measure of soil maturity—i.e., of how long a particular sample has been exposed to the continuing rain of tiny impacts.
Although the chemical and mineralogical properties of soil particles show that they were derived from native lunar rocks, they also contain small amounts of meteoritic iron and other materials from impacting bodies. Volatile substances from comets, such as carbon compounds and water, would be expected to be mostly driven off by the heat generated by the impact, but the small amounts of carbon found in lunar soils may include atoms of cometary origin.
A fascinating and scientifically important property of lunar soils is the implantation of solar wind particles. Unimpeded by atmospheric or electromagnetic effects, protons, electrons, and atoms arrive at speeds of hundreds of kilometers per second and are driven into the outermost surfaces of soil grains. Lunar soils thus contain a collection of material from the Sun. Because of their gardening history, soils obtained from different depths have been exposed to the solar wind at the surface at different times and therefore can reveal some aspects of ancient solar behavior. In addition to its scientific interest, this implantation phenomenon may have implications for long-term human habitation of the Moon in the future, as discussed in the section Lunar resources below.
The chemical and mineral properties of lunar rocks and soils hold clues to the Moon’s history, and the study of lunar samples has become an extensive field of science. To date, scientists have obtained lunar material from three sources: six U.S. Apollo Moon-landing missions (1969–72), which collectively brought back almost 382 kg (842 pounds) of samples; three Soviet Luna automated sampling missions (1970–76), which returned about 300 grams (0.66 pound) of material; and scientific expeditions to Antarctica, which have collected meteorites on the ice fields since 1969. Some of these meteorites are rocks that were blasted out of the Moon by impacts, found their way to Earth, and have been confirmed as lunar in origin by comparison with the samples returned by spacecraft.
The mineral constituents of a rock reflect its chemical composition and thermal history. Rock textures—i.e., the shapes and sizes of mineral grains and the nature of their interfaces—provide clues as to the conditions under which the rock cooled and solidified from a melt. The most common minerals in lunar rocks are silicates (including pyroxene, olivine, and feldspar) and oxides (including ilmenite, spinel, and a mineral discovered in rocks collected by Apollo 11 astronauts and named armalcolite, a word made from the first letters of the astronauts’ surnames—Armstrong, Aldrin, and Collins). The properties of lunar minerals reflect the many differences between the history of the Moon and that of Earth. Lunar rocks appear to have formed in the near-total absence of water. Many minor mineral constituents in lunar rocks reflect the history of formation of the lunar mantle and crust (see the section Origin and evolution below), and they confirm the hypothesis that most rocks now found at the lunar surface formed under reducing conditions—i.e., those in which oxygen was scarce.
Main groupings
The materials formed of these minerals are classified into four main groups: (1) basaltic volcanics, the rocks forming the maria, (2) pristine highland rocks uncontaminated by impact mixing, (3) breccias and impact melts, formed by impacts that disassembled and reassembled mixtures of rocks, and (4) soils, defined as unconsolidated aggregates of particles less than 1 cm (0.4 inch) in size, derived from all the rock types. All these materials are of igneous origin, but their melting and crystallization history is complex.
The mare basalts, when in liquid form, were much less viscous than typical lavas on Earth; they flowed like heavy oil. This was because of the low availability of oxygen and the absence of water in the regions where they formed. The melting temperature of the parent rock was higher than in Earth’s volcanic source regions. As the lunar lavas rose to the surface and poured out in thin layers, they filled the basins of the Moon’s near side and flowed out over plains, drowning older craters and embaying the basin margins. Some of the lavas contained dissolved gases, as shown by the presence of vesicles (bubbles) in certain rock samples and by the existence of pyroclastic glass (essentially volcanic ash) at some locations. There are also rimless craters, surrounded by dark halos, which do not have the characteristic shape of an impact scar but instead appear to have been formed by eruptions.
Most mare basalts differ from Earthly lavas in the depletion of volatile substances such as potassium, sodium, and carbon compounds. They also are depleted of elements classified geochemically as siderophiles—elements that tend to affiliate with iron when rocks cool from a melt. (This siderophile depletion is an important clue to the history of the Earth-Moon system, as discussed in the section Origin and evolution, below.) Some lavas were relatively rich in elements whose atoms do not readily fit into the crystal lattice sites of the common lunar minerals and are thus called incompatible elements. They tend to remain uncombined in a melt—of either mare or highland composition—and to become concentrated in the last portions to solidify upon cooling. Lunar scientists gave these lavas the name KREEP, an acronym for potassium (chemical symbol K), rare-earth elements, and phosphorus (P). These rocks give information as to the history of partial melting in the lunar mantle and the subsequent rise of lavas through the crust. Radiometric age dating (see below Mission results) reveals that the great eruptions that formed the maria occurred hundreds of millions of years later than the more extensive heating that produced the lunar highlands.
Ancient highland material that is considered pristine is relatively rare because most highland rocks have been subjected to repeated smashing and reagglomeration by impacts and are therefore in brecciated form. A few of the collected lunar samples, however, appear to have been essentially unaltered since they solidified in the primeval lunar crust. These rocks, some rich in aluminum and calcium or magnesium and others showing the KREEP chemical signature, suggest that late in its formation the Moon was covered by a deep magma ocean. The slow cooling of this enormous molten body, in which lighter minerals rose as they formed and heavier ones sank, appears to have resulted in the crust and mantle that exists today (see below Origin and evolution).