The Moon Up Close

Gassendi Double Crater
Below we see a picture of the double-crater system of Gassendi, taken by the Clementine Army Orbiter in the early 1990s:

Now, supposedly, this photograph was copied off of the net just before the Army purged it from public access. Frankly I doubt it, since a much higher resolution image of Gassendi was taken by Apollo 16, and is shown below. This photo pretty much puts the kabosh on any theories of Gassendi being a moonbase. The crater floor is highly fractured and eroded. The reason the crater looks man-made is because of the geometric quality of the fractures, but this is explained by the crustal tensions and stresses within the crater and differential erosion. The central domes are blocks of crustal material perched on hard-rock pedestals, around which the surrounding maria material has been eroded over long aeons. The erosion acts on a scale of centimeters, explaining not only the smooth look of the central peaks, but the appearance of an electrostatic dust halo on the main central peak.

Stranger than fiction! 

The scale of this terrain can be compared to the San Francisco Bay, as shown below. One can see some details of man-made construction in the Bay photograph, such as the thin lines of bridges and the white grains of building construction. There are no such identifying marks on the Gassendi photo. The middle distance of the Bay photo is about 30 miles from left to right. The far rim of the Gassendi crater as viewed above is also about 30 miles from left to right.

The Moon is a truly alien place. How beautiful by comparison our glorious Earth!


Explaining Gassendi’s Smooth Look
In 1963 scientists were trying to figure out what the surface of the Moon would be like. This was critical if a manned mission was to land there successfully. Several labs including Cornell and Northrop Aircraft tried to simulate the lunar surface, with the basic parameters of being in a vacuum under solar and cosmic particle bombardment.

Cornell conducted some experiments with silicates in a powder form, and concluded based on their results that rock or mineral surfaces, in the presence of ionizing radiation and in a high vacuum, would be broken down into dust to a depth of several millimeters, and that the dust in turn might coagulate into intricate clumps.

Cornell was wrong about the clumps but right about the dust. It turns out that General Mills came up with the key insight. Using a more intense plasma flux in their experiments, they could find no evidence for a heavy surface breakdown as Cornell did. Instead, the work by General Mills indicated that lunar soil exposed to intense proton bombardment from the Sun would develop a light crust. This is exactly what the astronauts found when taking their first steps on the Moon.

General Mills came to their conclusion by simulating the effects of sputtering on the Moon’s surface by particles of solar origin. Based on their lab results, they hypothesized that if the bombardment process has gone on for 1 billion years, the heavy atoms on the lunar surface would be significantly enriched – that is, they would become more radioactive and hotter – and loose dust particles would be fused by sputtered atoms. The surface should have been smoothed on the 10-centimeter scale. The General Mills scientists concluded that micrometeorite bombardment may have converted the surface into a porous but rather firm crust.

Cornell wasn’t all wrong, since a layer of dust a few millimeters thick coats the lunar surface. There are plenty of pictures of unhappy Apollo astronauts covered in this dust. In the main, two complementary processes are going on. First, pressures deep within the crust have caused silicates to fractionate, and over long ages have heated basalt and forced it to the surface in maria upwellings. The maria surfaces are therefore smooth. Second, bombardment of the lunar surface by the solar wind has caused a slow erosion of the top-layer. At the same time uranium and thorium silicates in lunar dust have been heated over millions of years, thereby creating a firm crust in some parts of the lunar terra.


The Copernicus crater, which is very similar to Gassendi, was imaged by the Lunar Orbiter 2 in 1966:

The Copernicus crater is so large that this oblique view had to be photographed over about twenty laps, because the orbiter could only process so much data. You can see that the strips don’t always line up, and that’s because the spacecraft’s angle changed slightly from orbit to orbit, as did the position of the Sun. Both Gassendi and Copernicus are about 60 miles in diameter and Copernicus is more than 2 miles deep.

To appreciate the scale of the Copernicus photograph, pretend that you’re about to land at San Francisco International Airport, and as the plane is making its final descent, you’re looking out over the Bay from about 20,000 feet. That central bright area might be the Bay itself, and the East Bay hills from Oakland to Fremont would lie beyond, although the far rim of the crater is three times the height of Mount Diablo. You could live in this crater and only be dimly aware that you were even inside of a crater. Those half-dozen central peaks are about a mile high – a day’s hike to the top of any one of them.


The Word "Crater"
In 1949 Willy Ley, a German scientist who came to America in 1936, wrote a popular book called The Conquest of Space. In his exhaustive chapter about the Moon, Ley wrote:

While the [lunar] maria and the mountain chains compared (or seemed to compare) directly with their terrestrial counterparts, a third type of lunar formation, most common of all, presented a puzzle. Wherever astronomers looked with their new enlarging instruments they saw "circular mountains," round structures now commonly called "craters." It is not a good name; the term "ringwall" which German astronomers have tried from time to time to introduce is much better because of its lack of connotations. Unfortunately it failed to take hold.

Apparently for a while the largest craters were also called "walled plains." Ley objected to the word "crater" because of its association in 1949 with volcanoes on Earth. He pointed out that there was no resemblance whatsoever between lunar craters and volcanic craters. Instead, Ley made a case for impact craters. Today, most people understand a lunar crater as an impact crater.

Of the characteristics of lunar craters, one in particular stands out: in most cases the volume of the ringwall or rim is just about what would be needed to fill the depression. Thus Copernicus, which is a deep crater, has a higher ringwall than Gassendi, which is shallower. It’s true that the displacement from a meteor impact would produce this effect, but this doesn’t automatically mean that the larger craters on the Moon are impact craters.

Ley brings up the case of the "walled plain" of Clavius, over 150 miles in diameter on the southern nearside. Apparently, since it was formed, four smaller craters have formed on its ringwall, and there are four major and several minor craters in its interior. Ley writes: "Obviously these smaller craters are younger than Clavius since they are superimposed upon it. But they also look younger. The old walls look as if they had been subjected to erosion. This is true for a few other walled plains too. What kind of erosion?"

Ley points out is that the distribution of craters on the Moon in general is completely random. There are no "chains" of lunar craters as there are volcano chains on Earth. So whatever the craters are doing they are doing it in a random fashion, probably related to the random formation over billions of years of the Moon itself.

The photograph below shows the base of one of the central peaks of the Copernicus crater. Can you tell which one from the main photograph? The thing that looks like a landslide on the middle right is caused by thermal erosion. As Ley explains it, during the Moon’s 27 day axial period full sun and then full shadow creates a temperature difference of about 400 degrees Fahrenheit. Rocks are heated and then cooled, and the repeated expansion and contraction causes the crystalline structure to crack and the rocks to flake. Gravity takes care of the rest. Like glaciers on Earth, the downward flow is fastest in the middle of the landslide, and slowest at the edges, where friction is greatest.

On Earth, glaciers tend to form in arctic regions, or on the leeward side of mountains where they’re sheltered from wind and sun. Only then is the ice cold enough to become dense enough to be affected by pressure and shearing. On the Moon orientation also plays a role, but since ice is entirely absent, the critical factor is direct solar exposure. Copernicus, slightly above the lunar equator, receives direct exposure from the South with the Sun high in the sky at its zenith. The orientation of the Copernicus peak is such that the most stress is on the side facing the camera. A second landslide can be seen to the left of center (in shadow), and a third just to the right. Several landslides can be seen on the far rim of the crater in the main photograph.

The mountain is embedded with huge boulders similar to manganese nodules at the bottom of the sea, only much larger. As the material that holds them in place starts to erode, the Moon’s gravity unseats them and they topple to the crater floor. The boulders at the bottom right, which throw distinct shadows onto the surface, must be about 200 feet wide. At upper left a pentagonal arrangement of boulders still embedded in the mountainside can be seen. The suspension of such large boulders in the mountainside might seem like a physical impossibility – and on Earth it would be – but remember that the gravity of the Moon is one-sixth that of Earth.

It would be nice at this point to say what these craters really are. Other pages in the Outer Space section will look at the Moon from other viewpoints, but in terms of its craters, one might say that craters can be fully described as "gamma flux-tube radiation resonance rings." Craters can be conceived as the resonant artifacts of incredibly powerful radiation processes going on in the Sun. The movement of crustal material from the interior of the crater to the ringwalls (and beyond) hasn’t happened instantaneously, but over billions of years of evolution.

One might hypothesize that craters are the results on the surface of concentrations of mass and heavy elements in the lunar interior. These concentrations have naturally acted as sinks for solar flux-tube emanations. Over billions of years craters have been excavated to greater or lesser extents depending on the strength of the flux. As the potential of a major crater fades away after billions of years of activity, the flux disperses into smaller emanations and thus produces smaller, fresher craters.

This description probably sounds alien, but so is the Moon. We naturally think of crater formation as being instantaneous via a meteor impact. But it may be that crater formation is completely alien to the human timescale, having taken place over billions, literally billions of years.

The macro-relationship between the Sun and the Moon is far from well understood. Nobody really knows, for example, how hot spots on the Moon relate to solar activity, or what the specific functions of craters in the solar regime really are. If this can be understood it would explain a great deal more of the various internal and external processes going on. It would explain too related processes that are happening on the Earth. For more information on the mysterious processes of the Moon, see the web page entitled A Very Strange Operation.


United States Space Science Program: Report to COSPAR
(Committee on Space Research), 1963, Section 6.4: Laboratory Studies of Simulated Lunar Conditions. Google Books.
Bonestell, C., Ley, W. The Conquest of Space, Viking Press, 1949, 1952

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