Telescope Reviews: Illuminated crater innards

Tom,

Good luck looking for the shadowed interior of Atlas illuminated by reflected light. I think one has a better chance of seeing such an effect in smaller, fresher craters with more highly reflecting walls; and I'm not sure I understand how processing artifacts (if that's what they are) provide insight into what one would expect to see.

The signature of illumination by reflected light from crater inner walls is a pattern of light and shadow reversed (because the direction of incidence is reversed) from what one would expect from direct sunlight. The pattern might also be expected to become more obvious as the direct illumination of the area fades.

The attachment shows, in the lower left, an August 19, 2008 AstronoMinsk image of the central peaks of Atlas catching (as you describe) the last rays of direct sunlight, at sunset. In the lower right I have attempted to portray what one might expect to see if, after the directly-lit peaks fade, the floor became visible, illuminated from the east by the sunlit east inner wall (as at sunrise, but much more dimly). The simulation was created by using Photoshop to paste in a dimmed version of the floor (and peaks) as they appear with direct sunrise illumination in the Consolidated Lunar Atlas.

Since from the point of view of someone on the lunar surface the inner wall of a crater is a much broader, somewhat lower, and much closer light source than the rising Sun, the shadows would actually be longer and more diffuse, and the floor illumination would show more fall-off as one moves away from the wall; but the point is: one would expect to see the central peaks dimly lit on their eastern sides (to the right in this view), with shadows cast to the west (left). The illumination from Earthshine (which would be exceedingly dim in this case) adds another component that (as seen from Earth) is expected to be shadowless, and essentially the same as the albedo pattern we see at Full Moon.

My intuition is that this would be an extremely difficult observation. Visually, most of us would probably be lucky to detect even a hint of indirectly-lit texture on the shadowed floor, let alone to be sure of its details. Photographically, illumination of shadowed crater interiors by reflected light has certainly been captured by spacecraft orbiting the Moon (the most famous recent case being photography of the interior of Shackleton by Kaguya), but I don't recall having seen a convincing example captured from Earth.

A possible example are the 2003 photos of Tycho by Brendan Shaw shown under "Observing programme 4" on the BAA TLP page, which may show the central peak faintly illuminated by the sunlit west rim, long before the rising Sun directly illuminated it from the opposite side. But even Brendan doesn't claim to have had the sensitivity necessary to see any detail on the floor proper, so it can't be confirmed that the purported peak is casting a shadow to the east.

-- Jim