Featured Image - 11/04/2008
Rimless Depression: Brayley G

Brayley G is a remarkable lunar feature. It can generally be described as a rimless depression with a width of approximately 3 km and a length of almost 5 km. It does not have the shape typical of most lunar craters -- bowl-shaped circular depressions with raised rims and surrounded by ejecta. Brayley G is oblong, does not have a raised rim, nor any visible ejecta surrounding it. This unusual feature is located in Mare Imbrium about 300 km east of the Aristarchus impact crater. The edges of Brayley G roll off from the mare and are marked by grooves that parallel the edge of the depression (Figure 1). These grooves may be faults and graben caused by subsidence of the original surface (Masursky et al. 1978). The LROC narrow-angle camera (NAC) on the Lunar Reconnaissance Orbiter (LRO) will search for more features like Brayley G next year in order to discover how frequently rimless depressions occur on the Moon and to allow lunar scientists to study how rimless depressions such as Brayley G form.

Several mechanisms have been proposed to explain how rimless depressions form on the Moon. Some probably form as volcanic vents; these depressions are usually surrounded by pyroclastic materials. Others may form by collapse into a subterranean cavity left as magma emptied out of lava tubes onto the lunar surface. Brayley G is most likely an example of this type of rimless depression. Degraded craters can also appear as rimless depressions, as the erosion and slumping of crater walls and rims can eventually remove the elevated crater rim. However, degraded craters do not show the concentric faults seen at Brayley G.

 
Figure 1: Brayley G is a rimless depression approximately 3-km wide and less than 5 km long found on the boundary between Mare Imbrium and Oceanus Procellarum. The distinctive concentric faulting paralleling the rim suggests local subsidence of regolith into a pre-existing cavity.
(Selected portion of Apollo Image AS15-M-1849 [NASA/JSC/Arizona State University])

References:
Apollo Over the Moon: A View From Orbit (1978) H. Masursky, G. W. Colton, F. El-Baz, eds. NASA SP-362. http://www.hq.nasa.gov/office/pao/History/SP-362/contents.htm

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