Lunar maria

The Moon’s maria (literally “seas”) are low-elevation areas of the lunar surface, in contrast to the lunar highlands. Interest in lunar maria reached international prominence in 1969 when the National Aeronautics and Space Administration’s (NASA) Apollo 11 astronaut Neil Armstrong set foot upon the surface of the Moon in the Sea of Tranquility. There, Armstrong and fellow astronaut Edwin E. Aldrin collected the first samples from the surface of another natural body in space.

Overview

Mare (plural, maria) is the Latin word for “sea”; the term reflects seventeenth-century ideas about the nature of the lunar surface. Similarly, there are other places on the lunar surface named for bodies of water: the Oceanus Procellarum (Ocean of Storms), some areas bearing the name lacus (lake), others called sinus (bay), and still others called palus (marsh). Returned lunar samples have established not only that there is no surface water on the Moon but also that the Moon as a whole has so little water that there are no hydrated minerals in the rocks. In reality, the maria are areas of basaltic lava flows. These lavas occupy approximately 16 percent of the lunar surface, with 80 percent of those located in the equatorial area on the side of the Moon that is always turned toward Earth—a total of more than six million square kilometers. Although extensive, mare basalts probably represent less than 1 percent of total lunar crust by volume.

Volcanoes form some of the highest features on Earth, but the majority of the lunar lavas are situated in low regions. These lows were excavated by meteorite impacts that left craters varying in size from microscopic to 2,500 kilometers in diameter. The largest of these craters, called basins, were later filled by lavas to form the maria. Smaller craters, particularly those immediately surrounding the nearside maria, also became the sites of lava eruptions. The depth of basalt fill appears to be related to the age of the impact-basin-forming events, the deeper fill being found in the younger basins. Nearly thirteen hundred separate eruption locations have been identified. This number does not include those buried by their own or younger erupted materials. The true number of eruption sites may be closer to thirty thousand.

Returned lunar samples have radiometric ages that fall within a range of three to four billion years. The presence of basaltic fragments in breccias produced by the large-basin-producing impacts dating from about four billion years, along with dark materials excavated by impacts from beneath a younger impact depositional cover, indicates that the age of volcanism extends further back in time than the ages of the returned samples. Furthermore, ages determined by crater-counting techniques have indicated the presence of lavas, perhaps as young as two billion years. Thus, although now long absent, the majority of volcanic flows on the Moon took place over a time span of about one billion years. Additionally, crater-counting data indicate that individual maria may have witnessed eruptions for a similar time span, the older materials being deeply buried near the basins’ centers. Crater counting and superposition relationships have led to the construction of a relative timescale based on the time of formation of large impact craters and basins. Subdivisions of this timescale in order of decreasing age are pre-Nectarian, Nectarian, Early Imbrian, Late Imbrian, Eratosthenian, and Copernican. Two-thirds of the nearside surface of the maria is of Late Imbrian age; much of the remainder is of Eratosthenian age. Early Imbrian, Nectarian, and pre-Nectarian lavas could have been buried by impact ejecta and younger lavas; Copernican-aged lavas are restricted to small areas of the western near side.

Morphological evidence for volcanism is found in the form of lava-flow fronts, sinuous rilles, mare domes, cones, and pyroclastic deposits. Measured flow fronts have heights that average about thirty meters but range from ten to sixty-three meters. It is highly probable that thinner flows once existed, but they were subject to obliteration by more than two billion years of erosion from meteorite impacts. Flow fronts outline individual eruptions more than several hundred kilometers in length. The great size of these features is attributable to the extremely low viscosities of erupted materials and their high-volume output rate. Sinuous rilles superficially resemble terrestrial river channels and range from hundreds of meters to three kilometers in width and a few kilometers to 300 kilometers long. These features are believed to be lava channels or collapsed lava tubes. The Apollo 15 mission included exploration at the edge of one of these structures, Hadley Rille. More than three hundred mare domes have been identified on the Moon. They have shapes and dimensions comparable to small terrestrial shield volcanoes.

Conical structures are common in the Marius Hills volcanic complex within Oceanus Procellarum but are relatively rare elsewhere. It is believed that these structures are the lunar equivalent of a terrestrial strombolian eruption. The Marius Hills area had been seriously considered as a landing site for Apollo astronauts, but it did not make the cut when three Apollo missions were canceled as a result of budget cuts.

Pyroclastic deposits on the Moon can be subdivided into two major groups: dark halo deposits and regional dark mantle deposits. Dark halo deposits extend to ranges of about five kilometers from an endogenic crater. Dark mantle deposits cover areas of up to 40,000 square kilometers. The Apollo 17 spacecraft carried the only geologist astronaut, Harrison “Jack” Schmitt, to fly to the Moon to do fieldwork on its surface. On that final lunar landing mission, the astronauts returned samples of this pyroclastic material in the form of orange “soil” found in the Taurus-Littrow valley. Stratigraphy and compositional variations of lunar pyroclastic materials suggest an age range comparable to the mare basalts.

Compositionally and texturally, lunar mare samples returned to Earth are basaltic lavas and glasses. Samples from each landing site are unique in terms of their major and minor element chemistry. Additionally, different chemistries can be recognized in the basalts at each site. General distinctions have been drawn between high, intermediate, low, and very low titanium basalts and feldspathic basalts. It must be remembered that samples have been returned from only six mare sites. Fortunately, basalts are rich in the transition group metals, particularly iron and titanium. It has been possible to map the distribution of compositionally different basaltic materials across the nearside of the Moon on the basis of their spectral characteristics. More than one dozen spectrally different units have been identified. There appear to be no simple age-composition relationships or composition-location relationships. For example, there are both old and young titanium-rich basalts and titanium-rich basalts in both the eastern and the western hemispheres of the Moon.

There are also dome-shaped features on the lunar surface whose shapes and spectral characteristics serve to distinguish them from mare domes. These features have heights of up to slightly more than 1 kilometer and areas of up to 500 square kilometers. They are all close to the highland-mare boundary in Oceanus Procellarum. Their spectra have suggested a comparison with KREEP basalts, which are rich in potassium, rare Earth elements, and phosphorus. If they were of volcanic origin, their shapes would indicate formation by higher-viscosity materials or at lower volumetric eruption rates than mare basalts. Stratigraphic analysis indicates that they were placed at the lunar surface at the same time basaltic eruptions were taking place elsewhere.

In summary, volcanic activity, perhaps dominated by KREEP-rich materials, preceded the development of younger large impact basins. Cavities created by the large impacts became the sites of basalt deposition, and deeper, central portions of the basins were the first to be filled. Widespread, flood-type volcanism was locally accompanied by the eruption of more volatile-rich pyroclastic material. With progressive infilling, the magma had an increasingly difficult task in penetrating the thick, high-density basin fill, so volcanism shifted to the periphery of the basins, found outlets in the floors of circum-basinal craters, and overflowed into the surrounding terrains.

Methods of Study

The locations and stratigraphic relations of the lunar maria have been determined by geologic mapping using Earth-based and orbital photographs as the database. Relative ages of different surface units have been determined by measuring the number of impact craters of a given size within a specified area or by determining the extent to which craters of a given size have been modified with respect to fresh, similarly sized craters. Both of these techniques rely on the fact that the Moon is under constant bombardment by meteorite particles, so the longer an area has been exposed to the lunar surface, the more craters it will have and the more the pre-impact surface will be affected by erosion and deposition. In many cases, it is possible to identify secondary craters created by material excavated by a primary impact. These secondary craters (and the primary craters) must be younger than the material into which they impacted. By mapping these relationships, scientists have established the lunar stratigraphic column.

The advantage of the return of lunar samples was that the relative ages calculated by the cratering techniques could be related to the absolute ages determined by radiometric dating. Furthermore, exposure times (rather than crystallization or metamorphic ages) of the returned samples at the lunar surface have been established by determining their amounts of solar-derived particles. Sizes of such structures as lava-flow fronts have been determined by measuring the lengths of shadows and through the use of laser altimeter carried aboard orbiting spacecraft. These data have been compiled to make topographic maps.

Many techniques have been employed to determine the thickness of the mare basalts. One geophysical technique relies on the fact that the mare basalts have a greater density than the surrounding highlands materials. This greater density results in the maria exerting a greater gravitational pull on a spacecraft, causing minute changes in its orbital motions. This phenomenon led to the discovery of strong pulls over the younger circular impact basins and the formulation of mascons (short for mass concentrations). Morphometric techniques for determining mare thicknesses rely on the fact that impact craters have regular and predictable dimensional characteristics. For example, a fresh, bowl-shaped crater has a depth equal to one-sixth its diameter. If an impact crater has penetrated a mare surface and excavated pre-mare materials from beneath, then the mare must be less thick than one-sixth of the crater diameter. Similarly, if all pre-mare craters with less than a specific diameter have been buried by lava, then, again, a minimum depth can be established. At the larger end of the scale, it is possible to take a topographic map of a relatively unflooded young basin, such as the Orientale Basin, and artificially raise the lava level parallel to the contours until the basin looks like the more deeply flooded basins. The height difference is then a measure of the mare infill thickness in the more deeply flooded basin.

The composition of mare basalts has been established by standard geochemical techniques applied to returned samples. More Moon-wide data have been obtained from orbital geochemical experiments. Information on radon and polonium variations was gathered by alpha-particle spectrometry; uranium, potassium, and thorium concentrations and the elemental abundances of oxygen, silicon, iron, magnesium, and titanium were determined by gamma-ray spectrometry; and aluminum, silicon, and magnesium variations were determined using x-ray fluorescence data. Various photographic and reflectance spectroscopy techniques have been employed by Earth-based observers to determine transition element variations, mineral compositions, and glass contents of the mare surfaces facing Earth.

Lunar studies ceased after the Soviet Luna 24 spacecraft robotically returned to Earth samples from the Sea of Crises (Mare Crisium) in 1976. The Moon was ignored for nearly two decades in terms of planetary science programs involving robotic spacecraft. Then in the 1990s the Ballistic Defense Organization sent the Clementinespacecraft to test new sensors and, in the process, map lunar resources. NASA followed with the more capable Lunar Prospector. Clementine and Lunar Prospector returned tantalizing suggestions of water-ice on the surface of the Moon in permanently shadowed areas. Data from the Clementine spacecraft in 1994 seemed to indicate an ice field near the Moon’s south pole. Five to ten meters deep and sixteen thousand square kilometers in area, the ice field is mixed with soil; it could be a valuable resource for crewed lunar bases. That finding was substantiated by NASA’s Lunar Prospector spacecraft, which orbited the moon from January 1998 to June 1999 and determined the chemical composition of the lunar surface using alpha particle, neutron, and gamma-ray spectrometers. Renewed interest in the Moon followed. The Chinese, Indian, European, and Japanese space programs sent spacecraft into orbit around the Moon in the first decade of the twenty-first century, and NASA launched the Lunar Reconnaissance Orbiter in June 2009. The Chinese space program launched a probe to return lunar samples to Earth as well as with the Chang'e 5 launched into lunar orbit in 2020.

Context

Maria form the least rugged terrain on the Moon, so they are conducive to the safe landing of spacecraft. These flat areas can also facilitate lunar exploration through the use of surface craft. They are most abundant on the Earth-facing hemisphere, where continuous telecommunications are possible with Earth. Excavation of the maria to produce dwellings may be feasible. Alternatively, natural shields to solar radiation, such as lava tubes, could be turned into habitation sites. Several techniques have been proposed for the mining of mare basalts to be used as raw materials for the construction of spacecraft in Earth orbit and to provide sustenance for lunar inhabitants. These factors make the maria primary candidates for the establishment of a permanent, crewed lunar base.

A farside mare site would be an ideal location for the construction of an astronomical observatory. Communications with Earth-based stations would necessitate a more complex satellite system than that required by a nearside base, but at the same time, the shielding from terrestrial electromagnetic radiation provided by the bulk of the Moon would enable clear views of distant galaxies.

From a scientific viewpoint, the surface materials of the maria provide a diary of solar activity and small meteorite impacts extending over the past few billion years. On Earth, by contrast, this information is less available because it has been removed by erosion, deposition, and the recycling of tectonic plates. Detailed study of the maria can therefore furnish information about what was taking place in the near-Earth solar system from the time of emplacement of the oldest surviving terrestrial rocks through the development of unicellular organisms to the arrival of humans. These data cannot be obtained from the surface of Venus, because of the thick atmosphere of that planet; nor will Mars serve, because of the effects of wind erosion, ice formation, and, perhaps, past erosion by river systems. Therefore, the Moon is a unique natural laboratory, and the maria provide favorable sites for a laboratory inhabited by humans.

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