Thursday, March 1, 2012

LROC: Sunset Boulder

A tall boulder on the floor of a crater catches a few last rays of light at sunset. Image width is 356 meters, LROC Narrow Angle Camera (NAC) observation M134442210R, orbit 4946, July 22, 2010; angle of incidence 68.31° with a resolution of 0.61 meters per pixel from 59.43 kilometers altitude. View the field of view in the LROC Featured Image, HERE [NASA/GSFC/Arizona State University].

Sarah Braden
LROC News System

The floor of this 3.5 km diameter crater (located at 9.900°N, 113.574°E) is covered with impact melt and boulders. At the time the LROC NAC took this image, the Sun was setting. One of the tallest boulders in the Featured Image was able to catch light from the setting Sun, although the rest of the surrounding terrain was already completely in shadow.

Look closely at the shadowed area on the left side of the image. Can you see the boulders on the shadowed side of the image? The Moon has no atmosphere to scatter light, so a shadowed area on the Moon is as dark as night. However, the LROC NAC is sensitive enough to detect the low light levels being reflected and scattered by other sunlit rocks in the crater floor. That little bit of light makes the shadow not completely dark, and therefore you can still see boulders in the shadowed area!

Why are shadows on the Moon so dark? Think about the case on Earth, where there is an atmosphere. Go outside on a sunny day and look into a shadowed area. Maybe it's the shadow of a large building, or maybe your own shadow! Is your shadow completely dark (like middle of the night with no Moon dark)? No, because our atmosphere scatters light into the shadow. However, if you were an astronaut on the Moon, your shadow would be as dark as night since there is no atmosphere. Aren't atmospheres great?

LROC Wide Angle Camera (WAC) context mosaic draped over LRO LOLA laser altimetry data, viewed through the NASA (LMMP) ILIADS application, simulates an oblique view of the farside highlands crater Lobachevskiy from 50 kilometers altitude over the lunar equator. The yellow arrow points to a bright younger crater within which is the field of view shown in the the LROC Featured Image released March 1, 2012. The context image accompanying that release can be viewed HERE [NASA/GSFC/Arizona State University].
This small, fresh crater is located in the eastern half of the floor of the crater Lobachevskiy. Lobachevskiy is an 84 km diameter crater located on the farside of the Moon at 9.9°N, 112.6°E. Explore the entire NAC frame to see more of the floor of Lobachevskiy.

Related Posts:
Perched Boulders
South Pole Illumination Map
Bhabha sinks into the shadows
Lunar South Pole: Out of the Shadows

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