Lunar Geology Notes and References

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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Regional Geologic Maps

Scale: 1 pixel = approximately 4.7 km. Since the Moon differes in size from the Earth, there is little point in trying to maintain "rational" scales. This scale was chosen to fit the near side maria onto a single page. The grid interval is 10 degrees.

The projection is Lambert Azimuthal Equal Area, which can actually portray the entire sphere, although distortion becomes extreme toward the antipode. The maps are cut off 150 degrees from the map center, so objects that look foreshortened are actually well beyond the lunar limb and each map covers considerably more than a hemisphere.

Small Scale Geologic Maps

Scale: 1 pixel = 2 km. The grid interval is 5 degrees. Projections are Mercator, Lambert Conformal Conic and Azimuthal Equal Area depending on latitude.

On the geologic maps, named craters larger than 25 km are outlined in white. In many cases the craters would be obvious even without the outlines, but not all. Many craters are centered within their ejecta blankets, or marked by upraised rims older than their surroundings. However, many other craters are morphologically distinct but obscured by younger material. The great far-side basin Hertzsprung, for example, is completely invisible on the geologic map because it is mantled by Mare Orientale and other ejecta.

Notes on Lunar Geology

Mapping lunar geology is something like mapping a wall at a paintball range. It may be possible to identify specific shooters and the sequence in which they fired, but there is no significance to the actual arrangement of hits.

Lunar geologic mapping can identify some specific formations, like mare lava flows and ejecta blankets from specific large impacts, but much of the mapping is geomorphic mapping. As one of my fellow students put it when the maps first appeared, "The map calls a unit 'crater floor' material, and son of a gun, there it is, right on the crater floor!" Nevertheless, geomorphology reveals geologic history on the Moon to a greater extent than on Earth.

Only a handful of lunar sites have been visited and sampled directly, none at all on the far side. Lunar ages are estimated from:

It's unlikely that future lunar geologists will ever be able to map units the way geologists do on earth. The erosion that so often covers information on earth also uncovers it and sometimes reveals cross-sections. On the Moon, outcrops are rare. Almost everything is covered with a regolith of impact debris. Craters reveal otherwise buried material in their ejecta. Mapping from ejecta would be analogous to mapping in glaciated or deeply weathered areas on earth by looking at the rocks in the overburden.

Because the maps were compiled by different authors at different stages in lunar exploration, units don't always match between maps (this is true for terrestrial mapping as well). There are a few straight-line "contacts" and features near the visible limbs, like Orientale Basin ejecta, were not recognized from Earth-based mapping.

On waterless bodies like the Moon, zero elevation is usually taken as the average radius of the planet.

Lunar Gravity Maps

Lunar gravity was mapped in detail by the GRAIL mission (Gravity Recovery and Interior Laboratory) in 2012. A pair of satellites in identical orbits detected lunar mass concentrations by measuring tiny variations in the separation of the two spacecraft.

Raw Gravity Data are dominated by the mass effects of topography, as one might expect. Crater rims show up as highs. The significant exceptions are the large positive anomalies under many maria. These were observed on the first lunar orbiter missions in the 1960's and were dubbed "mascons" ("mass concentrations"). They are due to kilometers of basalt lava flows filling impact basins.

Bouguer Gravity Anomalies correct for elevation (not so much an issue with satellite data) and excess mass due to topography. On the Bouguer maps, craters are mostly invisible. Large craters frequently have a central high, and the South Pole-Aitken Basin shows up as a very large high, probably due to thinning of the linar crust and shallower depth to dense mantle material.

The most interesting discoveries on the Bouguer maps are parallel strips of high gravity under Oceanus Procellarum. This pattern suggests that the Procellarum Basin is not an impact basin but a series of flooded rifts. The gravity highs are asymmetrical, suggesting tilted fault blocks.

Data Sources

The geologic maps are digitized versions of NASA 1:5,000,000 lunar maps original published as:
[1] Wilhelms, D.E. and J.F. MacCauley (1971) Map I-703 (Near Side).
[2] Wilhelms, D.E., et al. (1979) Map I-1162 (South Pole).
[3] Lucchitta, B.K. (1978) Map I-1062 (North Pole).
[4] Wilhelms, D.E. and F. El-Baz (1977) Map I-948 (Eastern Quadrant).
[5] Stuart-Alexander, D.E. (1978) Map I-1047 (Far Side).
[6] Scott, D.H. et al. (1977) Map I-1034 (Western Quadrant).

Gravity maps are contoured from GRAIL data.

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Created 11 April 2014, Last Update 23 December 2014
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