Wednesday, November 23, 2011

Shades of Grey, east of Lenz C

Virtual ILIADS/LMMP perspective from an imaginary point high over the farside lunar highlands looking southeast past the equator to Mare Orientale on the horizon, clearly the most broadly influential "impact event" affecting the topography of this area of the Moon. The LROC Featured Image, November 22, 2011, focuses on one of the more prominent and more recent superpositioned impact craters (yellow arrow, below center), and what its creation returned to the surface [NASA/GSFC/ARC/Arizona State University].
Focusing in on the northwest quadrant of the relatively fresh, optically immature unnamed crater, seen HERE at the 32 meter per pixel level of resolution using the LROC QuickMap. The yellow arrow indicates the location of the huge boulder in the Featured Image. [NASA/GSFC/Arizona State University].
Two streaks of high and low reflectance blocky ejecta from the same crater. A large boulder rests in the low reflectance deposit. LROC Narrow Angle Camera (NAC) observation M168862555R,, LRO Orbit 10019, August 24, 2011; image field of view 500 meters. See the full size LROC Featured Image HERE [NASA/GSFC/Arizona State University].
Drew Enns
LROC News System

This unusual ejecta is blocky, and exhibits high and low reflectance, and is from a very fresh crater located in the lunar highlands at 3.348°N, 259.724°E

Blocky craters can happen when an impact occurs in a more coherent target material, but what about the difference between the reflectance?

The reflectance difference has two possible causes. The two ejecta streaks could be from separate craters, or the ejecta could be the result of compositional differences in the target rock.

Local mid-morning over the Lends crater group region and the unnamed crater at center, subject for the LROC Featured Image released November 22, 2011. LROC Wide Angle Camera (WAC) mosaic, field of view 140 kilometers. View the full size LROC WAC context image HERE [NASA/GSFC/Arizona State University].
An additional 76 meter per pixel LROC WAC mosaic demonstrates how the appearance, revealing composition and morphology that might otherwise remain hidden can change dramatically under different angles of incidence; here the region of interest, with the subject crater at center, is seen under early evening illumination. One aspect of young, less weathered (and thus brighter than average) crater wall interiors can be that their reflective surfaces are sufficiently bright to illuminate that crater's otherwise darkly shadowed interior. LROC WAC mosaic (604 nm) from four orbital passes averaging 54.4 kilometers altitude during LRO orbits 4804 through 4807, July 11, 2010. Field of view 40 kilometers [NASA/GSFC/Arizona State University].
Since both ejecta streams point back to the unnamed crater, the ejecta contrast is most likely the result of a compositional difference in the subsurface. Since the context views show that the higher reflectance ejecta is much more prevalent, a small lower reflectance rock layer is probably the culprit.

For perspective, scale is vital on the Moon, as NASA discovered early on, during the Ranger impact probe missions. The blocky ejecta may look like simple "reject" pebbles, like those used for gravel roads here on Earth, but that large block described by Drew Enns further above is slightly larger than a city block in length. Once again the multi-storied, apartment-building-sized block, ejected more or less intact from beneath the surface by the relatively recent nearby impact event, is shown under not just two but four kinds of lighting conditions by the LROC Narrow Angle Cameras. Over the course of more than two years the subject probably doesn't change measurably but it clearly takes more than a single good photograph to obtain our best notion of the true shape of this huge resting boulder. LROC NAC observations M114613684L, orbit 2024, December 5, 2009; M135847041L, Orbit 5153, August 7, 2010; M160607763L, Orbit 8803, May 21, 2011 and M168862555R, Orbit 10019, August 24, 2011 [NASA/GSFC/Arizona State University].
Can you find any other low reflectance boulders or ejecta in the full NAC frame?

Related Posts:
Rubble Pile on Fresh Crater Floor
Dark streaks in Diophantus crater
Two-toned Impact Crater in Balmer Basin: A Reflection of the Target?

No comments: