Lunar craters

Lunar craters are the erosion scars of debris colliding at high velocities with the surface of the Moon, many of which are left over from the origin of the solar system. Studies of sizes and time distributions of lunar impact craters allow scientists to make estimates of the same process acting on Earth, where much of the evidence has been removed by erosion. Volcanic craters enable researchers to determine the eruption characteristics and thermal evolution of the Moon.

Overview

All the large lunar craters are named; many of these names are attributable to a 1651 publication by Italian astronomer Giambattista Riccioli in which they appear on a map drawn by P. Grimaldi. Riccioli divided the nearside of the Moon into octants. This map was drawn with the aid of the Galilean rather than the astronomical telescope, and so was not inverted. On this map, Octant 1 extended from the ten o’clock position to just past eleven o’clock and was succeeded clockwise by the seven other octants. Craters were named for astronomers, beginning with the most ancient in Octant 1 and concluding with Riccioli’s contemporaries in Octant 8. This practice has continued to the present, with the restriction that a crater is always named for a scientist no longer living.

Three principal processes have created the lunar craters. There are those directly excavated by the impact of a meteorite; there are those, called secondaries, that result from the impact of material excavated to form the crater of the primary meteorite impact; and there are those of volcanic origin. Until the return of lunar samples from the Apollo missions (1969–1972), the scientific community had been sharply divided into those who believed most lunar craters to be of impact origin and those who believed most to be volcanic in origin. Evidence gained as a result of the Apollo missions has established that the vast majority of lunar craters are of impact origin, resulting from collisions of meteors, asteroids, comets, and minor planets (large objects that failed to achieve independent planetary status) at velocities of from five to fifty kilometers per second (10,000 to 100,000 miles per hour).

No one has counted the total number of lunar craters, as they range in size from the microscopic to the giant (2,500-kilometer-diameter) South Pole-Aitken basin. It has been estimated, however, that there are about 1,850,000 craters with diameters in excess of one kilometer on the lunar surface and 125 with diameters greater than 100 kilometers. A 3,200-kilometer-diameter Procellarum basin has been tentatively identified, which, if placed over a map of the United States, would stretch from Washington, DC, to western Utah and from Brownsville, Texas, up into central Canada. At the other end of the scale, microscopic impact craters are produced on the Moon because of the lack of a lunar atmosphere. Similar-sized particles would rapidly burn up in the Earth’s atmosphere.

Primary impact craters increase in morphological complexity with increasing size. Small craters are bowl-shaped, with a well-defined, generally circular rim, smooth interior walls, and a depth-to-diameter ratio of 1:5 to 1:6. The floor of a fresh crater is invariably at a lower elevation than the preexisting terrain. The rim of the crater is surrounded by a generally circular continuous ejecta blanket, followed outward by the discontinuous ejecta blanket. This discontinuous ejecta often takes the form of rays that radiate outward from a zone close to the center of the primary impact site. An exception is found in craters produced by highly oblique impacts. These craters are generally elongated and have ejecta blankets preferentially distributed downrange or exhibiting a bilateral symmetry, with “wings” on either side of the crater.

An abrupt change in the crater’s shape occurs at a diameter of about sixteen kilometers in the maria (the Moon’s dark lava expanses) and twenty-one kilometers in the highlands. At larger diameters, craters develop terraces on the interior walls; have a generally broad, level floor interrupted by small hills and mounds; develop a central peak; have a less uniform rim elevation; and have a depth-to-diameter ratio reduced to about 1:40. Flows and ponds, which are often seen both within and exterior to these craters, are impact melts resulting from liquefaction of the impactor and target rocks. At diameters in excess of 140 kilometers, the central peak becomes modified into a centralized peak ring. At diameters in excess of 350 kilometers, multiple rings of alternating elevated and depressed terrains, the giant multi-ringed basins, are witnessed.

Secondary impact craters are generally less regular than primaries because they are formed at lower collisional velocities, an upper limit being the lunar escape velocity of 2.4 kilometers per second, at which speed objects ejected from the surface would leave the Moon’s gravitational field. The size of the secondaries largely depends on the size of the primary. Large secondaries have diameters between 2 and 5 percent of that of the primary. Generally, secondaries have smooth interior profiles and are shallower than primaries of the same diameter. They also differ from primaries in that their distribution is non-random. They frequently occur in linear or curving chains, patches, or clusters surrounding the primary. Another common feature of secondaries is the presence of a herringbone pattern produced by small ridges plowed up by impacting objects closely spaced in both time and distance. The apex of the V-shape points back toward the primary.

Many large lunar craters were once considered to be analogous to terrestrial calderas. Calderas form as a result of collapse following evacuation of a large, near-surface magma chamber. Analyses of returned lunar volcanic materials established, however, that they were derived from great depths (150–400 kilometers), with little evidence of residence at shallow levels for any extended periods. True lunar volcanic craters are recognized primarily on the basis of their distribution, which, like that of secondaries, is nonrandom. Volcanic craters or endogenic craters (those of internal origin) are found at the summits of volcanic domes and cones, at the heads of sinuous rilles, or in association with linear fractures. The volcanic craters are generally small (less than twenty kilometers in diameter) and have outlines that range from circular to elliptical to highly irregular. Some volcanic craters are surrounded by a halo of dark surface deposits believed to consist of pyroclastic materials ejected during strombolian- or vulcanian-style eruptions.

Impact craters are the product of an instantaneous geologic event, yet the lunar surface has been subjected to the formation of these features for at least the last 4.2 billion years. Many lunar craters have, thus, become highly modified from their original pristine form. Since the Moon lacks an atmosphere, this modification results primarily from two agents: later impacts and volcanism. An impact has two principal effects. First, it will result in the total or partial obliteration of any crater smaller than itself that was located within the area of the younger crater. Second, ejecta from the younger crater will erode the walls and infill the floors of the older craters surrounding it. At the extreme, the ejecta deposits could infill the preexisting surrounding craters. The net result of this process is that older craters are shallower than newer ones of similar size. The effects of volcanism on impact craters are largely restricted to areas around and within the major maria.

One feature attributed to volcanic modification of impact craters is the presence of floor fractures. These features are primarily found on the level floors of craters with diameters of 30 to 100 kilometers and consist of radial and concentric arrangements of fractures resembling spiderwebs. They are attributed to uplift of the crater floor by subfloor intrusions of magma. Some of these magma bodies found outlets to the surface and, with limited volcanic output, resulted in the formation of volcanic dark-halo craters aligned along the floor fractures. The crater Alphonsus is a typical example. With more extensive volcanism, the floor of the crater becomes flooded with lava until even the central peak becomes buried. At this point, there is too thick an overlying lava pile, and the magma seeks alternative routes to the surface around the periphery of the crater. Some craters have experienced postflood floor fracturing, which results from the sinking of the dense, thick lava pile or posteruptive intrusion, leading to renewed uplift. Flooding of the larger basins appears to have begun within the central low and later extended to the topographic lows between the mountain rings. Small impact craters were constantly being created during the period of basin filling. Many of these craters were either partially or totally covered by younger lava flows.

Methods of Study

Lunar crater studies began in earnest with the availability of the first crude telescopes. At this point, it was realized that the Moon’s surface was not perfectly smooth, as the Greeks had hypothesized, but instead was pockmarked with features that ranged in size from depressions barely resolved in the telescope to enormous basins. During this telescopic era of investigation, most scientists believed that lunar craters were formed volcanically. However, Eugene Shoemaker of the US Geological Survey became a champion of an impact origin for most of the Moon’s craters about the time that the space age dawned and early probes began to be sent to investigate the Moon at close range.

Early Pioneer probes (from the late 1950s to the early 1960s) largely failed to achieve lunar goals, but Soviet Luna and American Ranger probes purposely crash-landed on the Moon in several sites and transmitted images up until nearly the instant of impact. Those pictures revealed that the lunar surface displayed cratering down to the smallest scale, providing further evidence for their impact origin, with an enormous number of secondary craters formed out of each primary impact. Several US Surveyor spacecraft (in the mid-1960s) and Soviet Luna spacecraft (from the mid-1960s to 1976) soft-landed on the lunar surface to provide far more images of the nature of the heavily cratered surface, even in the mare regions that appear relatively smooth through backyard telescopes. The Surveyors were followed by the Lunar Orbiter and more Soviet Luna spacecraft, which provided detailed maps of cratering across the lunar surface, thereby assisting scientists in both the United States and the Soviet Union to identify safe landing sites to which they could dispatch astronauts and cosmonauts.

Between July 1969 and December 1972, six Apollo missions landed at six different sites on the Moon, returning samples that bore evidence of impact origin (the breccias) and volcanism (the igneous rocks). In 1976, Luna 24 returned samples to Russia robotically. Then, interest in directly investigating the Moon waned for nearly two decades.

A nearly two-decade drought in lunar exploration ended with the Clementine spacecraft, launched on January 25, 1994, by the National Aeronautics and Space Administration (NASA) and the Ballistic Missile Defense Organization. Although principally a test of new sensor technologies and spacecraft systems, Clementine used the Moon as its target and provided new insights into the Moon’s surface distribution of chemicals and minerals. Clementine examined the lunar surface in several different bands of the electromagnetic spectrum, created a laser altimeter-generated map of lunar stratigraphy, and provided a more detailed gravity map of the Moon before its mission ended in June 1994. Clementine’s most exciting finding was that there were likely to be deposits of water ice on the Moon inside permanently shadowed craters, enough water to support human outposts with both potable water and fuel (by breaking down water into hydrogen [fuel] and oxygen [oxidizer]). Clementine’s water detection was indirect in that its sensors picked up the presence of hydrogen (protons), which was then interpreted to be bound in water ice.

Clementine was followed by Lunar Prospector, which launched on January 7, 1998, and conducted a 570-day examination of the lunar surface with alpha particle, neutron, and gamma-ray spectrometers. Lunar Prospector verified the signature seen by Clementine and provided a better estimate of the amount of water ice that might be available in shadowed craters on the Moon. Lunar Prospector was directed to impact the lunar surface on July 31, 1999, in an area (the shadowed crater Shoemaker) where water ice was expected to be found. It was hoped that the impact process might liberate the water ice so that it could be detected from Earth. The final experiment was disappointing: no water plume was seen.

International interest in the Moon increased in the early twenty-first century. The European Space Agency sent the Small Missions for Advanced Research in Technology 1 (SMART 1) spacecraft to the Moon using an advanced ion engine that took thirteen months to reach lunar orbit. From orbit, SMART 1 examined the surface with X-ray and infrared sensors to search for frozen water near the Moon’s south pole. Until the mission ended on September 3, 2006, SMART 1 also provided high-resolution optical images of the entire lunar surface, not just craters where scientific interest in ice deposits was centered. SMART 1’s mission ended with a purposeful impact at two kilometers per second in another attempt to kick up a water plume; results were not definite, although an impact flash was observed from Earth.

Japan launched Selene on September 14, 2007. Two weeks later, it entered a highly elliptical initial polar orbit, which was later adjusted to a circular orbit just 100 kilometers above the surface. Its mission was to help set the stage for returning humans to the Moon.

Applications

Because impact craters are instantaneous events, they are superb geologic time markers. Any material on which an impact crater and its ejecta are superposed is older than the crater; any material overlapping the crater or its ejecta is younger. Analysis of these relationships led to the development of the lunar stratigraphic column consisting of five systems. The pre-Nectarian system, comprising all lunar surface features formed before the excavation of the Nectaris Basin, and succeeding systems defined by the formation of the Imbrium Basin and the Eratosthenes and Copernicus craters.

Primary impact crater densities indicate the relative ages of different units on the lunar surface: the more craters, the older the surface. Craters employed in such studies are usually larger than four kilometers in diameter, and the densities are obtained by the extremely tedious task of simply counting them on a photograph. Crater density divided by the average crater-production rate gives the approximate absolute age of the surface units. In the case of the Moon, a calibration curve for production rate can be obtained by comparing the radiometric age of samples returned from the landing sites with crater-count statistics of those sites. These data indicate an exponential decline in crater production from about four billion years ago to the present. The details of crater production prior to about four billion years are the subject of debate, but because most of the lunar surface postdates this period, the debate is of little relevance to age determinations.

Small craters (less than three kilometers in diameter) have also been used for dating purposes. These techniques are based on the fact that morphologies of small craters are modified in a consistent manner with time. One of these, called the D_L method, is based on the interior slope of the crater. As craters become progressively infilled, the length of the shadow cast by the rim decreases. For a given illumination angle, it is, therefore, possible to define the largest crater within an area that has reached a specified shadow limit. If a crater in another area is wider than the limit, the second area is older.

Predictable depth-diameter relationships of fresh impact craters allow the determination of some of the three-dimensional characteristics of lunar surface features. If a crater has been flooded by a younger lava flow, the extent of departure of that crater from the dimensions of a similarly sized fresh crater can be employed to determine the thickness of the lava (or other material). The effectiveness of this method is limited by the accuracy of the topographic data.

Material forming the lunar highlands has a different composition from that of the maria and results in pronounced spectroscopic differences. By analyzing spectroscopic signatures of ejecta blankets of craters superposed on the lunar maria, it is possible to determine if a crater excavated solely basaltic material or if it penetrated into the crust beneath. The depth-diameter relationship of the crater can then be employed to ascertain the mare thickness.

Mineralogical and geochemical analyses of returned lunar samples have played a large role in scientists’ understanding of the physical processes involved during an impact event. Indirectly, lunar craters have also provided information on the deep lunar interior because the impacts of both natural and human-made objects have generated seismic waves recorded by the Apollo seismic network.

For a long time, claims were made by observers of the Moon that flashes of light came from some regions of its surface, notably some large craters. Before the Ranger 8 spacecraft's impact, the crater Alphonsus, for example, was an area where some reported seeing flashes of light; they supposed that Alphonsus had active volcanism. Most astronomers scoffed at the reports, and no serious observations of Alphonsus recorded undeniable light flashes. However, in late 2005, a coordinated search by astronomers at NASA began to record hundreds of small flashes of light representing explosions on the order of a few hundred kilograms of TNT. The early supposition was that the flashes were the result of volcanic activity. Those recognized flashes were actually from impacts of fragments of the extinct comet 2003 EH1, the progenitor of the Quadrantid meteor shower. This observation indicated ongoing alteration of the lunar surface, a process that had long been attributed to the production of the lunar regolith but was never before seen occurring routinely. The influx of material-creating microcraters hinted at a threat to astronauts returning to the Moon to stay and establish permanently occupied bases for research.

Context

The heavily cratered surface of the Moon provides a scenario for what was also taking place on Earth at a time for which scientists have no geologic record. They have learned that, as one goes farther back in time, the number of objects that hit the Moon increases exponentially until around four billion years ago. The study of lunar craters has influenced the conceptualization that meteorite impacts may have played a role in terrestrial mass extinctions and, thereby, the evolution of life. It has been suggested that because the Moon contains so many craters, a large number of age determinations of the impact events could provide information concerning a hypothesized correlation between impact bombardment and cyclic extinctions.

Analyses of the sizes and distribution of volcanic craters have provided data concerning the internal thermal evolution of the Moon and the stress distributions within the upper lunar crust. In addition, study of the morphologies of impact craters allows scientists to determine the effects of impacts within a gravitational field one-sixth that of the Earth and on a body with no atmosphere. Much of the work by Ralph Baldwin in formulating the characteristics of impact craters was based on data from small terrestrial human-made explosions. Modern scientists can fairly accurately predict the consequences of very large explosions on land, in water, or in space as a result of lunar impact crater studies. Much of Baldwin’s original work remains viable even decades after the first lunar landings.

It has been suggested that the permanently shadowed floors of some near-polar craters may be reservoirs for trapped volatiles, such as water. Such resources, if present, would play an important role in the location of a crewed lunar base. Furthermore, impact craters have played a major role in forming the lunar regolith. This loosely aggregated material could be mined with limited mechanical processing. Conversely, the myriad craters on the lunar surface pose a major hazard for safe surface travel and will probably result in unavoidable detours for initial crewed expeditions. Both structures and persons would have to be protected against the small impacting bodies that rain onto the lunar surface.

Bibliography

Baldwin, R. A. The Measure of the Moon. Chicago: U of Chicago P, 1963.

Beattie, Donald A. Taking Science to the Moon: Lunar Experiments and the Apollo Program. Johns Hopkins UP, 2003.

Chaikin, Andrew. A Man on the Moon: The Voyages of the Apollo Astronauts. Penguin, 2007.

Consolmagno, Guy, and Martha Schaefer. Worlds Apart: A Textbook in Planetary Sciences. Englewood Cliffs: Prentice Hall, 1994.

Cudnik, Brian. Lunar Meteoroid Impacts and How to Observe Them. Springer, 2009.

Hartmann, William K. Moons and Planets. 5th ed. Thomson Brooks/Cole, 2005.

Harrington, Kirstin. "Discover the Top 10 Largest Lunar Craters on the Moon." A-Z Animals, 31 May 2023, a-z-animals.com/blog/discover-the-top-largest-lunar-craters-on-the-moon. Accessed 20 Sept. 2023.

Melosh, H. J. Impact Cratering: A Geologic Process. Oxford UP, 1996.

Schmitt, Harrison J. Return to the Moon: Exploration, Enterprise, and Energy in the Human Settlement of Space. Copernicus, 2006.

Schrunk, David, Burton Sharpe, Bonnie Li Cooper, and Madhu Thangavelu. The Moon: Resources, Future Development and Settlement. Springer Praxis, 2007.

Spudis, Paul D. The Geology of Multi-ring Impact Basins: The Moon and Other Planets. New York: Cambridge UP, 1993.

Wilhelms, D. E. The Geologic History of the Moon. US Geological Survey Professional Paper 1348. Washington, DC: Government Printing Office, 1987.

Yue, S., et al. "Shape Characteristics-Based Extraction of Lunar Impact Craters: Using DEM from the Chang'E 1 Satellite as a Data Source." Annals of GIS 19.1 (2013): 53–62.