Lunar Geology

by Jeff Medkeff

To the observer, the process of the formation of the moon is of relatively little interest, except academically. It cannot be observed, nor can its results be observed except for noting the fact that the moon is, in fact, there; and this is a condition the novelty of which, we expect, has worn off, being familiar with it as we all have been since the time of our childhoods. Additionally, the process of the formation of the moon is one which is rather obscure on theoretical grounds. Certainly some theories have been discounted, and one is generally favored; but the field is crowded with different ideas of the particulars, and, in the end, it seems every particular theory has something substantial to be said against it. While we may think we have the broad issues of the formation of the moon worked out, it seems we are still to argue for quite some time about the particulars.

For this reason, it is probably not especially profitable to dwell on the events of lunar formation in a work for the observer. Hence, we won't.

Signs of processes occurring very shortly after the formation of the moon are observable, however. By far the most significant of these processes to the observer is cratering.

Craters

Impact or Volcano?

The vast majority of craters on the moon were formed by impact. Several reasons can be given for this assertion:

• The first, and rather obvious, hint is that potential impactors such as meteors and planet-crossing asteroids are still so numerous that impacts on all large solar system bodies are inevitable. Because each impact diminishes the supply of these potential impactors, the supply, and hence the rate of impact, must have been much higher in the distant past.
• The fact that craters are circular, and that their ejecta is usually radially symmetrical, points to the origin of craters from a very small centralized source. The material ejected from large craters is substantial and indicates that large amounts of material were displaced from the site of the crater. In some cases, this ejecta and secondary craters are found thousands of kilometers from their point of origin. This requires that they have been ejected at nearly the lunar escape velocity. The energy required to cause this kind of mass movement from a small central zone can only come from impact from space. The lunar crust is not strong enough to contain such pressures at such a small point without releasing the energy in amounts too small to create large lunar craters. It has been suggested that collapse could cause large lunar craters, but collapse cannot explain the ejected material that lies scattered around craters.  
• The appearance of the lunar craters more closely resembles that of terrestrial impact and explosion craters than it does volcanoes. This extends even to quantifiable terms - such as the interior dimensions of the craters, and the fact that in impact craters the floor lies below the surrounding terrain. These morphologies are not reproduced in volcanic craters.
• Craters on the moon are randomly distributed on a surface of a given age, while volcanic features and other structurally-controlled forces form features that are not randomly distributed. In addition, the size/frequency distribution of craters on the moon matches the size/frequency distribution of potential impactors in space.
• Finally, shock-metamorphic rock covers the lunar highlands areas sampled and is very common on the mare. These rocks were formed at pressures far exceeding any possible volcanic pressure. Their frequency demands substantial impacts at every highlands site sampled. 

Small to Large

Impact craters come in a variety of sizes, and with differing characteristics. Some of the characteristics are influenced by their size, and craters have been divided into several morphological types to help classify and distinguish them. Since microcraters are not visible telescopically (!), we'll not discuss them in depth. But the observer may be interested to know that craters on the moon start at about one micron in size and work their way up to a size that is a substantial fraction of the surface of the moon.

Simple Craters:

Moltke. This is an archetypal simple crater. Apollo 10 H-4324.

Small craters predominate on the surface of the moon. Although the larger craters naturally attract more attention, small craters are found ubiquitously across the surface. Craters in this class are most frequently of a simple morphology, and in a fit of simplistic genius, they are usually referred to as simple craters. Classical examples of simple craters are Linne and Moltke.

Both of these craters have:

• bright ejecta deposits;
  
• circular raised rims;
  
• an interior slope that is steepest near the rim, smoothly decreasing toward the crater center;
  
• no flat central floor;
  
• the floor shape is nearly paraboloidal.

For the numerically inclined, the rim-to-floor depth of simple craters tends to be about 1/5 the diameter, while the rim height above the surrounding terrain is around 4% of the diameter. Craters like this are formed by projectiles faster than a few kilometers per second; slower projectiles form a different morphology, treated below in "secondary craters."

Simple craters, because they tend to be small, can be difficult to observe on the moon. They will give every appearance of smooth walls, a bowl-shaped interior, and a relatively smooth ejecta blanket which is often more easily visible under high lighting.

Complex Craters:

 

Bessel. This crater shows material on the floor that slumped from the walls, making a clumpy texture near the walls and a flatter floor than Moltke. Apollo 15 P-9328.

At a diameter of around 12km, a change in the appearance of craters on the moon takes place which reflects various processes of modification that a crater undergoes after its formation. The interior walls slope steeply to the floor, and are often near the angle of repose. Some craters in this size range, such as Bessel, have a fairly flat floor with landslide deposits in the bottom of the crater. For larger craters, such as Euler, there may be slump terraces on the walls and a central peak in addition to a flat floor. Craters of this size have collapsed more than the smaller ones, and these craters represent a transition form between the simple and complex craters.

Theophilus and Copernicus are textbook complex craters. The crater floor lies 2.8km below the surrounding plain and the rim 1.3 km above it. The rim is scalloped, and a series of terraces starts inside the rim and descends stepwise to the crater floor. The terraces tilt outward, and most of them have captured a pool of impact melt. In the case of Theophilus, the central peak rises 3km above the floor. The central peak diameter is typically around 22% of the rim diameter.

Complex craters have ejecta blankets that extend about a crater diameter from the rim. The outer parts of these blankets often show a radial structure, signs of being deposited and scoured by rapidly-moving ejecta. Secondary crater fields extend for many times the diameter of the crater, sometimes extending across a significant fraction of the lunar surface.

In craters larger than about 140 km diameter, the central peak is replaced by an inner concentric ring of peaks. In these cases, the diameter of the ring of peaks is about half that of the crater. 

Basins:

The largest impact scars on the moon are of such size that they are referred to as impact basins. These structures are huge, most of them being of a size that would render them easily visible to the naked-eye, if something were to set them off from the lunar highlands - and mare flooding frequently does.

Orientale Basin. This basin lies on the western limb of the moon and is not easily observed, but shows the essential features of basins on the nearside. Nearside basins tend to be more flooded with mare than Orientale. IV-187-M

The floors of basins lie several kilometers below the surrounding terrain; 2km in the case of Orientale and more in the case of other, larger basins. Their rims are thrown-up scarps that can be ten or more kilometers high. This scarp's steep side faces inward, and gives the appearance of a curved range of high mountains. In addition to this primary scarp, one or more other curved ranges may form. In well-preserved basins, all of these curved ranges are draped with ejecta which arrived a matter of minutes after the scarps themselves formed.

The basins have a significant impact on two characteristics of the moon that are worth reviewing here. The first is the thickness of the lunar crust. It is known that the nearside crust is thinner than the farside crust. This is very probably the result of the large number of overlapping basins on the nearside, which serve to depress the surface and throw large amounts of material completely off the moon. The other process that basins mediate is mare flooding. The large mare of the nearside have welled up through large basins such as Imbrium, Serenitatis, Nectaris, Tranquillitatis, Humorum, and others, which themselves lie within the gigantic Procellarum Basin. Other mare units occur inside basins or inside craters that have further weakened the crust inside basins.  

Secondary Craters:

Secondary craters from the Copernicus impact. Consolidated Lunar Atlas D19.

Secondary craters are formed by boulders ejected during a large impact. These boulders are of considerably slower speed than interplanetary impactors, perhaps 1km per second or less.

Secondary craters tend to form in loops, clusters, and lines, and are typically asymmetrical in shape. As the distance from the primary increases, the secondaries become more circular, and more widely dispersed in their groups. They tend to have a steeper interior slope on the side closest to the primary from which they were formed. The size of the largest secondary is typically about 4% that of the primary, which means that basin-forming impacts have the potential to produce some very large secondary craters.

One of the more interesting appearances of secondary craters that form close to one another is the chevron-shaped dune that forms between them. It is the result of interference between the ejecta of the two craters which formed nearly simultaneously, and the chevron actually encloses the crater which was formed second. 

Weird Craters:

Referred to as "aberrant types" by Melosh, craters which simply do not fit the profile of the archetypal impact craters are generally the result of special circumstances either in the impactor or the target:

• most elliptical craters are the result of an extremely oblique impact (Schiller is an example)
• most craters with duplicate, concentric rims are the result of layers of rock of differing strength in the target material
• strong regional joints or faulting may result in craters with squared-off or polygonal shapes
• extra-wide terraces can be produced in craters adjacent to high terrain.

Halo Craters:

Some craters on the moon are surrounded by material darker or lighter than that of the surrounding terrain. It is presumed the difference in brightness is caused by the crater excavating material different than that found on the surface. These excavations generally do not exceed about 10% of the crater diameter, but it does explain why sampling rocks at or near crater rims was an important goal for the Apollo science program.

Crater Rays

Crater rays west of Censorinus. The ray areas are lighter and more heavily cratered; the extend from upper right to lower left. Lunar Orbiter V-63-H3.

Crater rays are seen as bright streaks radiating away from the crater, or crater rims, of some craters. The size of the ray system extends for just a few miles, for small lunar craters, to nearly global coverage, for the rays of Tycho. These rays are best seen at high sun when albedo differences on the moon are most visible.

In the past there has been quite a bit of discussion about what formed crater rays. From the late 19th century and until the moon was explored by spacecraft, a few theories predominated. None of them were very satisfactory.

One notion was that rays were the result of emissions of gas from cracks in the lunar surface. This theory had the problem of the invisibility of all such cracks, and that of explaining why rays would form radially away from large craters. It also didn't explain why there were no rays around visible lunar cracks, nor could it explain why many such visible cracks were not straight and radial to a crater as the rays are. Nor was there any explanation of where the gas went after venting.

Another popular explanation was that crater rays were depositions of volcanic ash carried along by the wind during eruptions of the (presumably volcanic) crater that formed them. This theory had the problem of a lack of a lunar atmosphere, and had to postulate an atmosphere in the distant past. It was also rather difficult to explain why wind circulation patterns would carry ash in such relatively straight lines and in all directions of the compass at various times but would only rarely produce a substantially curved ray. There was also the trouble that volcanic ash was quite dark on earth, while quite light on the moon.

These, and all other such theories, were swept aside when the moon was explored by spacecraft. The cause of rays is no longer a great mystery, but for some reason they are still frequently treated as such in amateur guidebooks to the moon.

Rays are in fact nothing more than distant ejecta from the primary crater. This ejecta forms very small secondary craters, some of which might be the result of nothing more than simple digging when the ejecta hits the ground. This gardening of the lunar surface turns up lighter-colored material buried underneath and deposits fractured, and hence light-colored, ejecta onto the surface. By moving around the surface, the ejecta also tends to bury the dark glass material that forms in the very top layer of the regolith. All of these mechanisms lead to the lightening of the rays. Just as secondary craters tend to form in clusters and streaks, and for the same reason, rays do as well.

How Craters Form

Craters form in three basic stages. The formation process begins when the impactor hits the target material, and ends when the last ejecta settles, but modification can result from a variety of events unrelated to the original impact and can continue into the distant future. Each stage is dominated by a different phenomenon, though each stage also blends into the next.

The first stage consists of initial contact and the compression of both the impactor and target material. The fast-moving projectile pushes target material out of its path, which compresses it and accelerates it suddenly to speeds nearly equal to the speed of the impactor. At the same time, the target resistance decelerates the projectile. This deceleration is caused by a shock wave which forms at the point of initial contact and spreads through the impactor and target. The pressures set up by this shock wave will reach millions of bars, far exceeding the strengths of both the impactor and target, which will likely vaporize or melt when the shock wave dissipates. Some of this melt may squirt from the crater immediately. All of this takes much less than a second.

The second stage is the excavation of the crater. The shock wave set up in the first stage propagates through the target material and assumes a roughly hemispherical shape. This shock wave weakens as it encompasses more material. As the shock wave passes, the decompression of the material sets up the excavation - pulverized, fractured, and melted rock rebounds and is thrown out of the crater. The ejecta travels in a sheet-like fashion, with the material flung farthest from the crater falling to the surface last. Ejecta on the rim is from the deepest parts of the excavation. Because much of the ejecta will pass through and interfere with other ejecta, herringbone or braided patterns can appear in the ejecta blanket. Because the shock wave is so strong, the crater diameter will exceed by many times the size of the projectile. This stage takes a few seconds to a few minutes to complete.

The third stage, modification, begins after the crater has been excavated and continues indefinitely. In large craters, the bowl-shaped excavation that results from the hemispherical shock wave is usually unstable and will immediately begin to slump or collapse under gravity. Loose debris may slide down the interior walls, as might impact melted rock, which will pond when it comes to its resting place. In large craters, the walls will slump into terraces and a central peak will form on the floor, while in smaller craters debris will slide down onto the floor, partially filling or covering it.

On much longer timescales, isostatic rebound and erosion by subsequent impacts may flatten the crater into an albedo feature. Or it may be flooded by mare or undergo other modifications.

Illustration credits:

Crater formation: Drawn by Donald E. Davis, from The Geology of the Terrestrial Planets, NASA SP-469.
Moltke: Apollo 10 H-4324
Bessel: Apollo 15 P-9328
Copernicus: Kuiper et al.; Consolidated Lunar Atlas, Lunar and Planetary Laboratory 1967 (plate D19)
Orientale Basin: Lunar Orbiter IV-187-M
Secondary Craters (of Copernicus primary): Consolidated Lunar Atlas, Lunar and Planetary Laboratory 1967 (plate D19)
Crater rays: Lunar Orbiter V-63-H3 (west of Censorinus). Taken from Schultz.

part of Jeff Medkeff's Notes on Lunar Features
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