April 20, 2013

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Secondary Considerations

Lunar Orbiter 4 mosaic (left) from USGS, and Apollo 16-M2468 excerpt from Apollo Image Archive. P,L, S, I , R and D are Piazzi C,
Lacroix, Shickard, Inghirami A, Ritchey and Abulfeda D.

Hooray for Boston!

When we think of impact craters it is often of glorious features like Copernicus or small pinpricks of brightness like Linné. But both of these are in the minority for lunar craters - they are primary impacts, created by a projectile from space smashing into the lunar surface. But secondary craters are far more common than primaries. Just think of Copernicus; one 93 km wide crater surrounded by hundreds of secondaries, each typically smaller than a few kilometers in diameter. And there more be millions of secondaries with diameters of tens of meters. There are many more secondaries of large size than commonly recognized too. We can easily recognize that aligned and overlapping craters in the Rheita and Snellius valleys are secondaries from the Necataris Basin. But in addition, nearly 40 years ago US Geological Survey mapper Don Wilhelms warned us that basin secondaries are widespread. For example, the left image, recreated from Don's paper, shows a variety of ejecta effects derived from the formation of the Orientale Basin; the crater marked S is Schickard. The Ori arrow points to the center of the basin and crater chains and ridged deposits are clearly radial to the basin. Don argues that many of craters are also Orientale secondaries. Craters labeled a are well-formed secondary craters, those with a b are pairs of "secondaries whose rim intersections are straight septa", to quote Don. Craters marked c are degraded or incomplete secondaries embayed by ejecta that travelled across the surface as surges. d craters may be primary craters or possibly secondaries resulting from high angle ejection of debris. The south up image on the right is the area between Abulfeda (top left) and Albategnius (off image to bottom right). Don proposes that many of the depicted craters are Imbrium Basin ejecta, including the chain below Abulfeda D (probably a Nectaris secondary), and Ritchey (R) and its overlapping ears. Based on Wilhelms' work many craters smaller than 30 km in diameter may be basin secondaries. Clearly this would affect inferred ages of features based upon crater counts, and from the point of view of observers, makes another goal - can primary and basin secondary craters be distinguished at the eyepiece?

Chuck Wood

Related Links
Rükl plate 61 and 45
21st Century Atlas chart 24 & 13.

Yesterday's LPOD: Unseen

Tomorrow's LPOD: Moon Drop


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