Shadows aren’t always black on the Moon
Moon Zoo team member Dr Tony Cook sent me a link to Censorinus Crater which is south-east of Mare Tranquillitatis. At first glance it looks like one of the many craters we see in shadow. But this one is different. As Tony says:
“Despite the floor being almost shadow filled, plenty of interior detail is visible, including some shadows off boulders and craters. Presumably the source of illumination is from the sunlit side of the rim just out of the image field of view.”
The above image is just a taster. Better still have a look at the NAC image: M117277348RE and zoom in to see landslides and boulders clearly visible in the shadows. Stare into the depths – the more you stare the more you’ll see. You will come across these shadowy images from time to time so have a good look around them when you do but be careful to interpret what you see correctly. As Moon Zoo team member astrostu points out even though there are some features that can be clearly seen in shadowed areas, we must be careful about reading too much into image artifacts. Moon Zoo images have been compressed and will display blocky-looking features in regions of lower contrast.
Dr Tony Cook is a research lecturer at the Institute of Physics and Mathematics at Aberystwyth University. He researches into automated planetary cartography, and impact flash and change detection on the lunar surface. He is also Assistant Director of the British Astronomical Association Lunar Section.
Making Sense of Shadows
Forum member Caidoz13 posted this picture last week:
“There seems to be something really tall, casting a long, thin shadow. There are boulders nearby that are casting similar, but much shorter shadows. It looks like the object on the right is a really tall, thin rock, almost a column.”
Another forum member jumpjack took this further and suggested that rather than simply being a tall rock casting a long thin shadow in low sun what we were looking at was the rock shadow cast over a dip in the terrain. He explained with a sketch:
“If you look closer, you can notice that shadows describe a crater to the right of the rock, hence the terrain goes down as much as far it is from the rock, thus causing the long shadow.”
jumpjack remembered seeing a similar effect somewhere else and it reminded me of the shadow we found at Milichius A Crater. Phil Stooke LRO image scanner extraordinaire also agreed that although the Milichius shadow did look unusual it was likely to be nothing more than a linear shadow of an appropriately placed and shaped rock near the terminator. Although the Milichius image isn’t as clear it does look as if the shadow might be cast over a dip in the terrian too though this particular feature would benefit from further scrutiny when additional images are available. Several examples of this type of shadow taken when there is a very low sun (lunar sunrise / sunset) have appeared in the “Interesting Terrain” and “Spacecraft or Space Debris” threads as at first glance they do look unusual and, as jumpjack points out, the effect is exaggerated if the shadow is cast over a drop in the terrain. This combination of a low sun shining on a tallish boulder – especially one at the edge of a dip – can give the illusion of a tall, thin, man made mast-like structure. So I thought it worth highlighting this long thin shadow effect to help people sort the rocks from the space debris.
Here are a few more examples:
And for comparison here’s Apollo 14. OK the tracks give this away too!
Keep looking closely at these low sun angle pictures though – as any monolith out there will cast a very similar shadow.
Jules is a volunteer moderator for the Moon Zoo Forum