Sunday, May 26, 2013

Eimmart A: a Crater of Contrasts

John Moore

His latest YouTube video 3D tour,
of crater Eimmart A.

Eimmart A is a relatively small (7.34 km), fresh-looking crater that impacted close by the east rim and ejecta of crater Eimmart (23.97°N, 64.8°E, 44.99 km in diameter).

The impactor that produced Eimmart A may have initially struck on an odd 'contact point': between where ejecta from Eimmart, at its west, met the lava-flooded floor of Mare Anguis (associated to formation of the Crisium Basin), at its east.

As a consequence, a wonderfully, bowl-shaped crater formed where material excavated from it was shot out in every direction - whose signatures, today, mainly shows up particularly as bright rays emanating away from the crater (observable through any amateur-sized telescopes ~ 4-inch or greater).

Huge reduction of a slightly oblique (slew -9.52°) LROC Narrow Angle Camera (NAC) observation of Eimmart A. (Full Resolution NAC mosaic can be viewed HERE.). LROC NAC M1098408548LR, LRO orbit 14225, July 31, 2012; illumination incidence angle 50.96° at an overall average resolution of 1.46 meters per pixel, from 140.93 km over 24.14°N, 66.53°E [NASA/GSFC/Arizona State University].
Close-up of the crater, and its inner walls, displays a 'contrast of sorts'! On the eastern side, we see dry-debris flows of materials that have 'slid' down towards the crater's floor, while on its western side, sheets of melted rock have solidified on the surface, in place

Why the contrast? Why do dry debris flows on one side of the crater contrast against melts on the other side?

New (above) and slightly older (Google/USGS/JAXA) digital elevation models show the anatomy of the Eimmart A impact on the east rim of Eimmart. The view above derived from the LROC QuickMap WAC/NAC GLD100 DEM, below LROC WAC monochrome image overlaid upon the USGS DEM derived from Japan's Kaguya and the United State's DOD Clementine laser range topography.
An oblique impact scenario might be presumed; where the impactor came in at a low angle from the east; whose main energy then produced a predominance in melting on the westwards (down-range), western-side wall. But such impacts usually result in asymmetric-shaped craters -- like in what we see at crater Proclus, an others.

But Eimmart A certainly isn't asymmetric: it is perfectly, circularly-formed from what we would expect by an impactor coming in at a high angle. So what would have produced the 'contrast of sorts'?

Eimmart A from above, in an image used to illustrate the LROC Featured Image "Rootless Impact Flows," July 26, 2011. LROC WAC monochrome (643 nm) observation M119415370M, spacecraft orbit 2732, January 29, 2010; 64.5 meters resolution from 46.4 km [NASA/GSFC/Arizona State University].
The wider region in the vicinity of Eimmert A (above left center), under a higher Sun, shows the wider range of the younger crater's bright  ejecta (and the darker material of its interior). From a LROC WAC mosaic stitched from four sequential orbital passes, May 8, 2011 [NASA/GSFC/Arizona State University].
Would it be that the impactor, on impact, encountered a more 'harder' surface (the lavas of Mares Anguis) at its eastern side, and a more 'softer' surface (Eimmart's ejecta) at its western side? If so, each side may have had a different, rock-melt gradient (that is, a difference in melting because of previous, dynamically-altering of material events), which might explain the resultant contrast of dry debris versus melt in Eimmart's walls that we see.

In an above average eyepiece (actually a stacked mosaic, more specifically "Matsutov-Cassegrain Santel D=230mm F=3000mm, Mount WS-180GTl Unibrain Fire-i 702 CCD black and white camera (1388x1040 pixels); filter: Baader IR-pass 685nm+ stacking in AutoStakkert! 2, Deconvolution in Astra Image; Mosaic of 17 images") Eimmart A in line-of-sight-from-Earth context. Image by Yuri Goryachko, Mikhail Abgarian and Konstantin Morozov of Astronominsk, Minsk, Belarus, January 29, 2012.
Whatever the scenario, while Eimmart itself is impressive enough in the eyepiece, Eimmart A is really the 'eye-candy', the 'stealer', the one that we should appreciate more - given from what we see in this flyover.

Rootless impact melt flows (July 26, 2011)

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