Friday, March 29, 2013

Bright small crater ejecta - with a black eye

Fifty meter crater with bright ejecta extending several crater radii. The dark deep interior of the crater could be the disk of of an impact melt pond Field of view 1000 meters across from LROC Narrow Angle Camera (NAC) observation M1117189620R, LRO orbit 16860, March 6, 2013; 0.9 meters resolution [NASA/GSFC/Arizona State University].

Drew EnnsLROC News System

Our impressions (and interpretations) of surface features on planetary bodies are affected by the way they interact with sunlight when we image them.

For instance, the shape of a crater is brought out by shadows in large incidence angles (Sun near the horizon) images.

In today’s Featured Image, we are observing a crater with the Sun nearly directly above the surface. This type of image (small incidence angle) helps scientists understand the physical properties of the surface. Why might the ejecta blanket of the crater be highly reflective? Why is the interior have a much lower reflectance? Two different surface properties could be affecting what we see. First, 'fresh' material should be brighter than surrounding material. And second, the composition of materials affects how they reflect light (see albedo).

A similar, somewhat larger crater for comparison - one also considered to be relatively fresh - in Oceanus Procellarum, northeast of the central eye of the Reiner Gamma albedo swirl. The explicit central melt floor, or disk, may resemble the less clearly resolved fresh crater spot-lighted in this post. You can read the feature story about this comparable crater HERE. LROC NAC observation M111972680L [NASA/GSFC/Arizona State University].

LROC Wide Angle Camera (WAC) context of the region around the small crater highlighted in the LROC Featured Image, located near the red cross (3.022°N, 258.698°E). Image field of view roughly 85 km [NASA/GSFC/Arizona State University].

In the case of today's Featured Image, the crater looks very young. We have some stratigraphic evidence for this as the crater is sitting on top of a larger flesh unnamed crater's ejecta deposit (see context image below).

A quickly put-together crop from the Chang'E-2 (CNSA/CLEP) global medium resolution mosaic, highly emphasizing albedo over the relief made visible by long shadows. Even old and deep craters in this 170 km-wide field of view north of Mare Orientalis seem to disappear under the low solar incidence. If the ejecta blanket from the unnamed crater near center were just a little further east and clearly on the Moon's nearside it would rival the similarly bright ejecta from Tycho, Copernicus or Brygius A. The small crater, clearly overwhelmed in this crop, is marked by a small "X" on he theouter slope of Lents (Lenz) C.

Therefore the brightness of the ejecta blanket is likely due to the young nature of the crater! But that doesn't solve the problem of the crater's interior. The interior could have been mantled by a thin veneer of impact melt which then pooled in the center. We know from many examples that impact melt rock reflects less light than its source material.

The small crater (arrow), situated on the ejecta blanket of a fresh crater further east which, in turn, sits on the wide outer reaches of the Mare Orientalis impact basin. View toward the south, [NASA/ILIADS/LMMP].

The impact melt hypothesis is not certain, though a follow up image at a larger incidence angle to help us understand morphology and could certainly help test this hypothesis!