Rays of Light
Explanation:
As the sun rises over a crater, its rim casts a long shadow across
the floor. If the rim were of even height and the floor were perfectly flat, the
shadow would be a smooth curve. This is what most small simple craters look like
(the BIO types). But larger
craters have more topographic variations in their rims and floors. Long sunrise (or
sunset) shadows map out local high spots in the rim, and bright bands or rays between
shadows mark where the peaks are lower. And the neat thing is that these bright and
dark rays are fun to observe, partially because they are only visible for a few hours
each month. Here Robert Spellman has captured shadows and bright cones streaking
the floor of the crater la Condamine in the rubble behind Sinus Iridum. By use of
a video camera, he documented variations in the bright and darks bands through time.
The upper left image was taken at 2:14 UT and the bottom right one was 26 minutes
later. You can see that the shadows shortened slightly. On the mouseover image I
have drawn lines that bound the bright and dark bands and show how they connect back
to locally high (H) and low (L) spots on the crater rim. On Robert's web site he
also shows the Lunar Orbiter IV image that I have annotated to match his photos.
The Orbiter image shows that the rim of la Condamine is mostly even except for where
it has been cut by secondary craters. The image also shows that la Condamine is a
floor-fractured crater with an uneven floor. So here, I would guess, shadows and
bright bands are mostly due to rim height variations, with some complexity added
by floor roughness.
