Sun angles, over fresh ejecta, east of Buisson V

Overlapping views of the same 1 km-wide area of the lunar surface, immediately southwest of a fresh impact crater near Buisson V; 68° morning shadow and 8° mid-day albedo, respectfully. LROC NAC observations M146255155R (LRO orbit 6687, December 6, 2010; incidence 67.86° at 62 cm resolution from 60.47 km) and M187520776L (orbit 12701, March 27, 2012; 8.35° incidence, resolution 97 cm from 116.95 km) [NASA/GSFC/Arizona State University].

Hiroyuki Sato

LROC News System

Today's Featured Image highlights differences in how the surface appears as the Sun angle changes. As seen in the opening image, the two images show the exact same area, but their appearances are remarkably different. Both images show the southern portion of the ejecta from an unnamed fresh 1 km crater (0.4933°S, 110.7156°E, 9.8 km west by northwest of Buisson V, an ancient equatorial crater just beyond the east limb.

The darker image, highlighting the granularity and elephant skin texture of the surface, was acquired when the Sun was low on the horizon (incidence angle = 68°), and the image characterized by bright swaths of ejecta was acquired when the Sun was nearly overhead (incidence angle = 8°).  Both were acquired with the cameras looking straight down. The small crater (about 27 meters in diameter) in the upper-middle each segment is a good landmark to confirm both images show the same location. 

Context view of ejecta blanket, source unnamed fresh crater (0.4933°S, 110.7156°E) 9.8 km west by northwest of Buisson V. LROC NAC footprint fields of view designated, area shown at high resolution indicated with an arrow. 46.1 km-wide field of view from LROC WAC monochrome (604 nm) observation M176905846C, LRO orbit 11206, November 26, 2011; 62.67° incidence, resolution 58.44 meters from 43.14 km [NASA/GSFC/Arizona State University].

When the Sun is high no resolvable shadows are cast, and surface brightness (albedo) variations stand out. The low Sun image has sharp shadows, highlighting the surface texture. Due to these drastic changes caused by the Sun angle scientists use NAC images at specific lighting conditions for different studies, of morphology, composition, optical and molecular maturity, topographic mapping, etc).

Granularity, at the expense of albedo, becomes visible at relatively low sun (high incidence angles). See the image at higher resolution HERE.

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"Star light, star bright"

A sharp reflectance contrast is found in an unnamed crater on the east floor of Humboldt (26.593°S; 83.764°E). NAC frame M182974061L, illumination is from the west, north is up, image is ~850m wide [NASA/GSFC/Arizona State University].

James Ashley
LROC News System

Why do we see such a sharp contrast in reflectance in the above Featured Image between the crater wall on the left and the crater floor on the right? This is the floor and wall of an ~8.5-km diameter, unnamed impact crater within the much larger Humboldt crater on the lunar farside.

The reason for the striking dichotomy in terrain appearance is not a mystery -- it has to do with solar illumination angle.

Our star, the Sun is shining from the west at an elevation above the lunar horizon (~25°) that is close to the slope of the western crater wall. Thus much of the wall on this side is receiving low-angle sunlight and appears relatively dark compared to the crater floor surface, which is more directly illuminated. The floor makes a clean contact with the crater walls, and becomes a zone of deposition for boulders rolling down the slope, including one larger and prominent boulder at the southern end of the frame.

Contextual view of the unnamed crater of interest (center) on the far east floor of Humboldt, its interior and floor highly illuminated by a comparatively low angle of solar illumination (a high Sun). From the global albedo lunar photograph mosaic swept up over the mission of China's lunar orbiter Chang'E-2 [CNSA/CLEP]..

For reasons like this, planetary scientists need to be careful when making interpretations of surface features, and must often use several images collected under a variety of lighting geometries to understand the geomorphology.

The entirety of Humboldt crater, from the early LROC GLD100 Wide Angle Camera (WAC) global mosaic. The unnamed crater of interest is designated [NASA/GSFC/Arizona State University].

The WAC mosaic presents the exotic landscape of Humboldt crater for context. Click HERE to see the full NAC frame.

Other examples of lighting effects can be found at the earlier LROC Featured Image posts, Shadows in Egede A, Boulder or Crater?, and Sunset Boulder.