Monday, July 4, 2011

That Crisium 'ghost crater,' east of Shapley

The Ghost Crater of southern Mare Crisium has been of particular interest to the Lunar Pioneer group for a variety of reasons, mainly because it is situated in the Apollo metric camera digital elevation model and can, therefore, be studied in three dimensions by anyone using the Google Earth application. That's why we were excited when LROC's Drew Enns chose to spotlight a recent LRO Narrow Angle Camera (NAC) cross-section of the formation. The image above shows the 10 kilometer-wide feature swept up in the long shadows prior to local sunset, LROC Wide Angle Camera (WAC) observation M11710778M, from LRO orbit 2392, 42 km overhead, January 2, 2010 [NASA/GSFC/Arizona State University].

A self-defeating miniature of an astoundingly detailed montage of LROC NAC and WAC observations overlaying the Apollo 15 & 17 "J" mission orbital metric camera survey imagery, easily seen when viewing the Moon using Google Earth. During the Apollo science missions, Crisium was under a mid-day sun, to facilitate a landing at Hadley Rille Valley, for Apollo 15, half a hemisphere away. LRO fills in the detail washed out of that bright landscape photographed from orbit in 1971.

A more recent addition to the Crisium Ghost Crater study (LROC NAC observations M150138095R & L, LRO orbit 7260, January 20, 2011), from which the LROC Featured Image release June 23, 2011 was cropped, is a fine late-day close-up of the inundated rim, including calving boulders more than a meter across, and rising 100 meters over the mare-flooded basin's elevation, roughly 3,300 meters below the lunar global mean elevation. The high mountains to the south and completely surrounding the 550 km-wide Crisium impact zone tower over the vicinity, rising quickly up to that global mean in that same lateral distance [NASA/GSFC/Arizona State University].

Crest of a wrinkle ridge in Mare Crisium lined with boulders that have most likely weathered out of the summit. LROC NAC M150138095LE, image field of view is 500 meters. See the outstanding full-sized LROC Featured Image, from Ghost Crater in Southern Mare Crisium!, June 23, 2011, HERE [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

This wrinkle ridge is part of a larger circular network of wrinkle ridges in southern Mare Crisium. Wrinkle ridges are the result of tectonic stresses which have compressed layers of material. On the Moon, these layers are made up of mare lava flows stacked one on top of the other.

When there exists uneven topography, such as a buried (ghost) crater below the lava flows, the lava may deform and produce wrinkle ridges. Deformation occurs preferentially over the buried crater rim, and a circular wrinkle ridge is formed, hinting at the ghost crater beneath.

From Lunar Pioneer Album 5 -
Context image of the ghost crater within Mare Crisium (located at 11.1°N, 59.7° E). The LROC Featured Image, june 23, 2011, is located at the tip of the white arrow. From the LROC WAC 100 m global mosaic, field of view 20 kilometers [NASA/GSFC/Arizona State University].

Ghost craters are valuable scientific tools as they can provide minimum thicknesses of mare units. Knowing the diameter of a crater can quickly yield the depth of the crater, thanks to observations and tests performed by scientists. If we know the diameter of the ghost crater, we can estimate the minimum thickness of a given mare unit! This ghost crater is 16 km and using Pike's equations the crater depth should be 1.7-2.4 km.

Explore the wrinkle ridge further in the full NAC frame!

Related Posts:
Lava Flows Exposed in Bessel Crater
Sublunarean void!
Wrinkle ridge in Oceanus Procellarum

Looking south from the surface of the inundated crater rim, in a virtual environment, the high Crisium basin rim can be seen, more than three kilometers over the basin's interior, and another two thousand meters above the original basin floor, below the deep basaltic lava floods nearly covered it completely, nearly 4 billion years ago.

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