March 24, 2013

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Dents, Chains And Superposition

image by Michael Wirths, Baja California, Mexico

Everytime LPOD shows an image of a famous area I try to find something new to say. For a recent Iridum image it was the use of topo data to understand why an Imbrium lava flow stopped where it did. On Mike's new image there are three things I haven't discussed before. Mike noticed the small shallow crater near the center of Iridum, and there is a second one close to the northern edge of the Iridum lavas near Bianchini H. The ghost crater near the center has a slight raised rim and is about 6.5 km wide and 80 m deep, and the dent-like one near H is a little smaller in diameter and only 50 m deep (according to transects with the QuickMap Path tool). Such shallow craters are usually interpreted as impact craters formed on an earlier surface and covered by subsequent lava flows. A second feature of interest is the line of secondary craters that diagonally crosses the bay and ends by the dent near H. We know these are secondary craters because there is a line of them, they lie on a faint ray, and they seem to be radial to Copernicus. A shorter line of touching secondary craters is west of the center ghost crater, and other clusters and short lines are visible. In fact, other than the largest half dozen or so craters it is likely that the majority of small craters within Iridum are secondaries. This is because so many appear to be in groupings or associated with bright splotches, and because other studies on both Mars and the Moon demonstrate that at small sizes (typically less than 4-5 km) the majority of craters are secondaries; makes it dangerous to count small craters to determine ages. The third feature in this image is the thickly ejecta-draped Maupertius crater at upper right. Three major ribbons of ejecta are generally radial to the Sinus Iridum impact crater, showing that Maupertius was there before Iridum formed. Bianchini at top center obviously formed after Iridum because it cuts Iridum's Jura Mountains rim. Notice however, that no ejecta from Bianchini are visible on the nearby lavas so we can bracket the time of Bianchini's formation to the interval between the formation of the Iridum crater and when the topmost Iridum lavas erupted.

Chuck Wood

Technical Details
Mar 22, 2013. Starmaster 18" (Zambuto optics), ASI120MM camera, 2.5X's powermate
barlow, R/IR filter, Firecapture, and PS CS, topaz infocus

Related Links
Rükl plate 10
21st Century Atlas chart 19.
Mike's Baja Dark Skies Inn

Yesterday's LPOD: Crater Morphology And Process

Tomorrow's LPOD: IAU Map Update


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