Astronomical Society of Coonabarabran

Lunar craters: inner wall basics

by Harry Roberts

Point a small ‘scope at a lunar crater, and every viewer from eight to eighty will shout “wow! Look at that!”

They sure are amazing landforms, but amateurs who have seen them often can become jaded with repeat viewings. This article will highlight crater wall formations that may have been overlooked by amateurs, and suggests an area of study for current workers.

When I recently sketched the inner walls of Tycho I found that much more detail could be seen in the 8” than I had expected. It seemed to me that perhaps (like others) I had not really “seen” the crater before, although I’d “looked” at it often!

After finishing the sketch I dipped into Hill’s “Portfolio of Lunar Drawings” to find, surprisingly, that he records little wall detail, although his views of Moretus (p123) and Eratosthenes (p49) do show some. Why would this be? Hill is the finest lunar draftsman I know of; and Alika Herring, too, made excellent Moon drawings. But both tried to map ever smaller craters, rilles, ridges, etc. leaving larger scale detail of crater walls somewhat ignored.

Before discussing crater wall features we need a terminology for them. I’ve borrowed this from Wood (“Modern Moon etc”) and Murray et al (“Earthlike Planets- etc” W. H. Freeman and Co. 1981). And Schultz’s “Moon Morphology” is indispensable for its high resolution Orbiter images, that seem not to be available on-line.

It’s now accepted that G. K. Gilbert’s analysis of 1893 was correct, and that a crater whose floor is lower than its surroundings is an impact feature, and a crater whose floor is higher than its surroundings is a volcano. It sounds obvious now, but what a lot of effort was wasted trying to prove most lunar craters were volcanic, when clearly they were not

I do not intend to deal with the impact event in detail, except to note that a typical impactor hits the Moon at hundreds of kilometres per second, and explodes like a nuclear device, excavating a roughly hemispherical cavity in the Moon’s surface (I’d love to watch an impact from a safe distance!)

Upon impact, melted regolith (“melt”) is generated, that may flood the cavity’s floor; and shortly after, the steepest walls of the cavity collapse because supporting material has been removed. Of course we view craters millions of years (Tycho, Aristarchus), or billions of years after the impact, and need not spend much time on the initial event.

After the impact a major “weathering” force acting on a crater is the seismic shaking of later impacts nearby. This shaking and compression can trigger further collapse of the subject crater’s walls that had previously been steeper.

From Murray (above) we learn that crater walls collapse in two principal ways: one is “rotational slumping”; the other is termed “rockslide”. Both are familiar to Earthlings who live in mountainous areas. In Australia with less vertical relief rockslides are uncommon; but elsewhere, e.g. NZ, slides often occur.

Fig 1 shows a typical fresh (Copernican Era) crater, maybe 80 km wide, 5 km deep and about a billion years old. In the figure the vertical scale has been doubled for clarity, as lunar craters are much shallower than they look in the eye-piece! Note that almost none of the original crater rim (dotted) survives after the impact. Dark areas of impact “melt” collect on the crater floor as well as the tops of scarp blocks. Of course, many crater floors have been covered by basalt lavas, the result of radioactive heating 3 billion years ago. Such flooding is found mostly nearer the centre of the Moon’s Earth-facing hemisphere.

Fig 2 shows the crater wall in detail, and names some of the features. Note the large blocks of material that collapse by rotational movement: the “slump blocks”. These expose steep “scarps” that are usually very bright. The highest scarp is often called the “main scarp”, and it approximates the position of the original crater rim. The tops of the slump blocks are called “terraces”, and they can be partly covered in dark “melt”. The terraces are often steeply inclined, and can throw obvious shadows under low altitude lighting.

Near the floor of the crater the rotated material fractures into smaller units and can spill across the crater floor, often as far as the central peak. This feature is termed a “rock slide” or “slippage” zone. At the front of the “slide” individual boulders the size of small mountains may separate from the mass, and be detectable as single units about 2 to 4 km in diameter. (Very big rocks!) This feature is called a “toe” (Fig 3).

The mix of scarp, terrace, slippage zone, and rock slide can vary a lot inside any crater. Subsequent impactors can further modify the inner wall. The result is that large craters display a wide variety of collapse landforms, of which Tycho provides a good example (Fig 3).

Try seeing some of these landforms when next viewing the Moon, using about 30 to 40 per inch of aperture if “seeing” permits. You may be surprised by your ‘scopes performance.

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