Effects of impacts and volcanism
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The dominant consequences of impacts are observed in every lunar scene. At the largest scale are the ancient basins, which extend hundreds of kilometers across. A beautiful example is Orientale Basin, or Mare Orientale, whose mountain walls can just be seen from Earth near the Moon’s limb (the apparent edge of the lunar disk) when the lunar libration is favorable. Its multi-ring ramparts are characteristic of the largest basins; they are accented by the partial lava flooding of low regions between the rings. Orientale Basin appears to be the youngest large impact basin on the Moon.
Orientale’s name arises from lunar-mapping conventions. During the great age of telescopic observation in the 17th–19th centuries, portrayals of the Moon usually showed south at the top because the telescopes inverted the image. East and west referred to those directions in the sky—i.e., the Moon moves eastward and so its leading limb was east, and the portion of the basin that could be seen from Earth was accordingly called Mare Orientale. For mapping purposes lunar coordinates were taken to originate near the center of the near-side face, at the intersection of the equator and a meridian defined by the mean librations. A small crater, Mösting A, was agreed upon as the reference point. With the Moon considered as a world, rather than just a disk moving across the sky, east and west are interchanged. Thus, Orientale, despite its name, is located at west lunar longitudes.
Smaller impact features, ranging in diameter from tens of kilometers to microscopic size, are described by the term crater. The relative ages of lunar craters are indicated by their form and structural features. Young craters have rugged profiles and are surrounded by hummocky blankets of debris, called ejecta, and long light-colored rays made by expelled material hitting the lunar surface. Older craters have rounded and subdued profiles, the result of continued bombardment.
A crater’s form and structure also yield information about the impact process. When a body strikes a much larger one at speeds of many kilometers per second, the available kinetic energy is enough to completely melt, even partly vaporize, the impacting body along with a small portion of its target material. On impact, a melt sheet is thrown out, along with quantities of rubble, to form the ejecta blanket around the contact site. Meanwhile, a shock travels into the subsurface, shattering mineral structures and leaving a telltale signature in the rocks. The initial cup-shaped cavity is unstable and, depending on its size, evolves in different ways. A typical end result is the great crater Aristarchus, with slumping terraces in its walls and a central peak. Aristarchus is about 40 km (25 miles) in diameter and 4 km (2.5 miles) deep.
The region around Aristarchus shows a number of peculiar lunar features, some of which have origins not yet well explained. The Aristarchus impact occurred on an elevated, old-looking surface surrounded by lavas of the northern part of the mare known as Oceanus Procellarum. These lava flows inundated the older crater Prinz, whose rim is now only partly visible. At one point on the rim, an apparently volcanic event produced a crater; subsequently, a long, winding channel, called a sinuous rille, emerged to flow across the mare. Other sinuous rilles are found nearby, including the largest one on the Moon, discovered by the German astronomer Johann Schröter in 1787. Named in his honor, Schröter’s Valley is a deep, winding channel, hundreds of kilometers long, with a smaller inner channel that meanders just as slow rivers do on Earth. The end of this “river” simply tapers away to nothing and disappears on the mare plains. In some way that remains to be accounted for, hundreds of cubic kilometers of fluid and excavated mare material vanished.
The results of seismic and heat-flow measurements suggest that any volcanic activity that persists on the Moon is slight by comparison with that of Earth. Over the years reliable observers have reported seeing transient events of a possibly volcanic nature, and some spectroscopic evidence for them exists. In the late 1980s a cloud of sodium and potassium atoms was observed around the Moon, but it was not necessarily the result of volcanic emissions. It is possible that interactions of the lunar surface and the solar wind produced the cloud. In any case, the question of whether the Moon is volcanically active remains open.
Telescopic observers beginning in the 19th century applied the term rille to several types of trenchlike lunar features. In addition to sinuous rilles, there are straight and branching rilles that appear to be tension cracks, and some of these—such as Rima Hyginus and the rilles around the floor of the large old crater Alphonsus—are peppered with rimless eruption craters. Though the Moon shows both tension and compression features (low wrinkle ridges, usually near mare margins, may result from compression), it gives no evidence of having experienced the large, lateral motions of plate tectonics marked by faults in Earth’s crust.
Among the most enigmatic features of the lunar surface are several light, swirling patterns with no associated topography. A prime example is Reiner Gamma, located in the southeastern portion of Oceanus Procellarum. Whereas other relatively bright features exist—e.g., crater rays—they are explained as consequences of the impact process. Features such as Reiner Gamma have no clear explanation. Some scientists have suggested that they are the marks of comet impacts, in which the impacting body was large in size but had so little density as to produce no crater. Reiner Gamma is also unusual in that it coincides with a large magnetic anomaly (region of magnetic irregularity) in the crust.
Small-scale features
On a small-to-microscopic scale, the properties of the lunar surface are governed by a combination of phenomena—impact effects due to the arrival, at speeds up to tens of kilometers per second, of meteoritic material ranging in size down to fractions of a micrometer; bombardment by solar-wind, cosmic-ray, and solar-flare particles; ionizing radiation; and temperature extremes. Subject to no meteorological effects and unprotected by a substantial atmosphere, the uppermost surface reaches almost 400 kelvins (K; 260 °F, 127 °C) during the day and plunges to below 100 K (−279 °F, −173 °C) at night. The top layer of regolith, however, serves as an efficient insulator because of its high porosity (large number of voids, or pore spaces, per unit of volume). As a result, the daily temperature swings penetrate into the soil to less than one meter (about three feet).
Long before human beings could observe the regolith firsthand, Earth-based astronomers concluded from several kinds of measurements that the Moon’s surface must be very peculiar. The evidence from photometry (brightness measurements) is particularly striking. From Earth the fully illuminated Moon is 11 times as bright as one only half illuminated, and it appears bright up to the edge of the disk. Measurements of the amount of sunlight reflected back in the direction of illumination indicate the reason: on a small scale the surface is extremely rough, and light reflected from within mineral grains and deep cavities remains shadowed until the illumination source is directly behind the observer—i.e., until the full moon—at which time light abruptly reflects out of the cavities. The polarization properties of the reflected light show that the surface is rough even at a microscopic scale.
Before spacecraft landed on the Moon, astronomers had no straightforward means by which to measure the depth of the regolith layer. Nevertheless, after the development of infrared detectors allowed them to make accurate thermal observations through the telescope, they could finally draw some reasonable conclusions about the outer surface characteristics. As Earth’s shadow falls across the Moon during a lunar eclipse, the lunar surface cools rapidly, but the cooling is uneven, being slower near relatively young craters where exposed rock fields are to be expected. This behavior could be interpreted to show that the highly insulating layer is fairly shallow, a few meters at most. Though not all astronomers accepted this conclusion at first, it was confirmed in the mid-1960s when the first robotic spacecraft soft-landed and sank only a few centimeters instead of disappearing completely into the regolith.
Lunar rocks and soil
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).