June 14, 2006

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Primaries or Secondaries?

image by Apollo 10 from Apollo Image Gallery

Moltke crater is probably best known as a guidepost to the Apollo 11 landing site. It is a fresh, 7 km wide simple crater with steep bowl-shaped walls. This Apollo 10 image beautifully shows the ejecta deposits that surround it. On a higher Sun view the ejecta shows up as a bright patch; in this closeup it is seen as a thinning pile of bumpy hills (called hummocks by geologists) and beyond them tiny secondary impact craters. In fact, this area - like every area seen at extreme high resolution - is peppered with tiny craters. Some are clearly secondaries (the wormy chains in the foreground) and any none clump member crater is assumed to be a primary impact. But a study of a very young impact crater on Mars has led some researchers to believe that millions of secondary craters may be created by the formation of a moderate size primary crater. That would mean that most small (with diameters less than 2 km) craters on Mars and the Moon are secondaries. And thus estimates of surface ages based on counts of small craters would be unreliable. Most craters observed telescopically are larger than 2 km and are probably primaries, but when looking at a great space image like this it is possible that nearly all of the small craters are secondaries.

Chuck Wood

Technical Details:
NOTE: I will be at a games + society meeting in Madison, Wisconsin the next few days - look for a new LPOD on Sunday.

Related Links:
Rükl plate 46

Yesterday's LPOD: A Hot Day at Plato

Tomorrow's LPOD: Oozing Ejecta


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