Concentric Lunar Craters

The Strolling Astronomer Feature Story: Concentric Lunar Craters By Howard Eskildsen, [email protected] Online Readers Left-click your mouse on the e-mail address in blue text to contact the author of this article, and selected references also in blue text at the end of this paper for more information there. This paper by ALPO member and astrophotographer Howard Eskidsen is only one of many that ratios were calculated using Lunar averaged 0.065, much shallower than were presented at ALCon 2013, Reconnaissance Orbiter data. Crater the T/D ratio of 0.20 that is typical for held in Atlanta, Georgia. coordinates were reproducible to within non-concentric craters. Mean crater rim 0.02°. Outer rim diameters (D) ranged elevations were below the mean lunar Abstract from 2.3km to 24.2 km with the mean radius of 1737.4 km with the mean 1.5 8.2 km with error of ± 0.2 km. Inner km lower than the mean lunar radius. Fifty-five concentric caters were toroid rim diameters (T) averaged 4.3 identified and measured for diameter, km, and calculated toroid to diameter depth, toroid crest diameter, ratios (T/D ratio) averaged 0.51. The d/ coordinates, and depth/diameter (d/D) D ratios for the concentric craters Table 1: Eastern Hemisphere Diameter Data Page 36 Volume 56, No. 1, Winter 2014 The Strolling Astronomer Table 1A: Eastern Hemisphere Elevation/Depth Data Concentric Craters pure highland areas. None are found in While the most notable concentrics have A small percentage of craters that would the central maria (Wood 1978). Their rounded, doughnut-like toroids, others normally fall in the classification of ages range from Pre-Imbrian (>3.85 Ga) have flattened inner mounds, and a few simple craters show a curious inner ring to Eratosthenian (1.1-3.2 Ga), with most have sharp inner rims. The space or (torus or toroid) that averages about 50% of the craters being of Imbrian age (3.2- moat between the inner and outer rims of the diameter of the outer ring (Wood, 3.85 Ga). Notably, however, no varies in appearance, and a few have 1978). The crater sizes range from 2-20 Copernican age (<1.1 Ga) craters are confluent lobes of rubble that may give km outer diameter with an average outer known to exist on the moon (Trang et al., the appearance of another ring between diameter around 8 km, though most fall 2011). inner and out rings when viewed at low within the 2-12 km diameter size. (Note resolution. sub-kilometer “bench” craters and multi- Most, but not all, of the toroid rims are ringed basins are not being considered in concentric with the outer crater rims and Chuck Wood in his 1978 paper describes this paper). They also tend to be the ratio of the toroid diameter (T) to the several variations of morphology. Most shallower in depth than normal craters of outer crater rim diameter (D) range from common are typified by Hesiodius A with similar size which would be expected to 0.2 to 0.9 (T/D ratio). Most of the T/D the usual characteristics described above. have a depth (d) to diameter (D) ratio ratios range from 0.3 to 0.6 and average Marth was described as appearing like a around 0.2 (Trang et al., 2011). 0.5. While the outer rim margins cratered dome. Some craters have craters this size are normally slightly elliptical inner and outer rings as typified Concentrics have a non-uniform concave, the outer margins of the toroid by the crater in Struve, while others have distribution over the moon that is similar tends to be slightly convex and the area a round outer ring and elliptical toroid to the distribution floor-fractured craters. inside the torus is usually concave (Wood, such as Crozier H, which is also off Seventy percent of them are found on 1979). center from outer ring. Craters such as the margins of maria, 20% in the smooth Gruithuisen K have raised, rubbly moat floors of lava-flooded crater, and 10% in area that resembles a third ring. A few Volume 56, No. 1, Winter 2014 Page 37 The Strolling Astronomer craters are fractured and cracked, highly concentric craters. Exogenic hypotheses Endogenic formation hypotheses include worn such as the concentric craters near include a fortuitous double impact either successive eruptions from same vent, Mons Jura, and have been described as nearly simultaneously by tidally spilt symmetrical collapse of the outer rim, or having “bread crust” appearing interior. object, a second smaller impact cratering into extrusive domes. Another Garbart J is shown to have an inner rim sometime later than the initial impact, or endogenic hypothesis involves volcanic modification of existing craters by that is low and inconspicuous while an impact into a layered target. distortion from intrusion of laccoliths Chamberlain was described as having a Exogenic origins seem unlikely due to the below the surface or by extrusion of lava high concentric “collar”, but on LROC non-uniform distribution of the craters, above the surface (Wöhler and Lena, images looks as if it could possibly be a due to the presence of non-concentric 2009) due to separate impacts. craters in proximity to concentrics, and also due the lack of Copernican-age Any hypothesis for concentric crater Various mechanisms have been proposed concentric craters (Trang, 2010). formation must account for the to explain the formation of the following: Table 2: Western Hemisphere Diameter Data Page 38 Volume 56, No. 1, Winter 2014 The Strolling Astronomer • The non-uniform distribution of the • Spectroscopic studies showing the described by Wohler and Lena (2011): concentric craters interior composition to be very simi- “At the bottom of the transient cavity, lar to the surrounding terrain. however, the flexural rigidity of the • The lack of Copernican age craters. overburden and the overburden weight Of the various hypotheses, volcanic per unit area were reduced to 30% and • Presence of normal non-concentric modification of the crater appears to be 61% of their original values, respectively, craters of younger age in close prox- most likely candidate, and spectral according to the relations given in [4,10]. imity to some of the concentric cra- studies support the intrusive model. If the impact occurred during the magma ters. Trang (2011) describes the process: “The intrusion phase, the thinned part of the idea suggests that dikes have propagated overburden was probably unable to resist • Presence of ejecta blanket remnants into impact-induced fractures producing the pressurized magma, which in turn around several of the concentric cra- a laccolith. The laccolith grows, uplifting may have lifted up the crater floor, thus ters. the crater floor. The inner torus is leading to the shallow crater depth. In created either because impact melt in the this line of thought, the inner depression • 5. Shallow depth compared to simi- center of the crater is inhibiting uplift or of the concentric crater is a remnant of lar non-concentric craters the floor relative to the sides, or inflation the original bowl-shaped crater floor.” is followed by a large deflation or collapse.” The process is further Table 2A: Western Hemisphere Elevation/Depth Data Volume 56, No. 1, Winter 2014 Page 39 The Strolling Astronomer Purpose of current study described earlier by Chuck Wood, but was subject to cylindrical distortion.) was not in his original paper. Elevation plots were obtained from query The current study was undertaken to use paths drawn across the center of the LROC data to obtain high resolution Methods crater (one from west to east and the images, to determine coordinates of the other from north to south) that extended crater centers, measure the diameters of The craters were located on the LROC well beyond the outer crater rim, and the outer rim crests (D) and toroid rim ACT-REACT QuickMap, the coordinates elevation profiles were obtained from the crests (T), and the crater depths (d). The of the apparent center of the craters Query Results box. Four rim elevation toroid/crater diameter (T/D) ratio and recorded and the diameters and depths data points were obtained and two the dept (d)/diameter (D) ratios were measured and recorded. The diameter central crater elevation points were calculated from the measured data. measurements were made by placing the obtained. The deepest central crater QuickMap cursor over one rim crest and elevation was subtracted from the mean Craters studied included the concentrics tracing a line (query path) through the of the rim elevation to determine the listed by Chuck Wood in 1978, most of center of the crater to the crest of the crater depth. the other craters listed in the http://the- opposite side of the rim and reading the moon.wikispaces.com/ geodetic distance from the Path Query Eastern hemisphere diameter Concentric+Crater website (Wood et al., box. In some cases, where parts of the measurements and calculations are listed 2007), and a few other concentric rim were obliterated in the desired area in Table 1 and elevation measurements craters discovered by internet search. of measurement, the diameters were and crater depth calculations are One concentric, Fracastorius E, was measured from northwest to southeast presented in Table 1A. Corresponding accidently “discovered” while measuring and from northeast to southwest and the western hemisphere measurements are another concentric; the “discovered” averages taken. (Note: Geodetic distance listed in Table 2 and Table 2A. Depth/ concentric, as usual, had already been was used since the cartographic distance diameter ratio and toroid diameter/crater Table 3: Eastern Hemisphere Depth/Diameter and Toroid Ratio Calculations Page 40 Volume 56, No.

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