Replies above are correct, a shadow cast downslope will be longer than on flat ground.
Anyhoo, I like to have an idea of what I'm looking at from several LRO images of the same location, taken at different sun angles. Notice that in the
WAC image (the wide angle one) there's no hint of the dark vent; in fact that spot is actually bright. The articles on Mons Hansteen say it's thought
to be "an extrusive volcanic mound". I'm not an expert on volcanism, so maybe this thing is somehow different from a typical volcano. This may be just
a big pile of material pushed out of the interior by volcanic forces.
Here are the NAC images covering that area:
I'll try to zero-in on the "vent" if I can, and post screenies here.
P.S. ok, here they are:
OP's image, M166182355RC
At higher sun angle, M183853525LC
The shadow is radically shorter, similar in length to that of the nearby monolith.
Sun almost overhead, shadow pointing the other way, M127259935RC
While the monolith is obviously the tallest of the nearby rocks, the shadow isn't that much longer.
Here's an interesting image, M1112139088LC
The whole area is in shadow, but the tip of the monolith catches the sun.
Aha! The best image yet, M140237118RC (0.489 m/p resolution)
Sun is overhead, so almost no shadow, but the great resolution shows it to be nothing but a big rock.
P.P.S. That shadowed area that caught your eye seems to be simply a slope of a hill or ridge. It's on the edge of the large relatively flat area to
the right of the monolith.
Oh, just found out that the QuickMap allows you to make a 3D visualisation of the selected terrain
A closeup, very rough geometry, but shows that it's a big rounded slope (click and drage image to rotate the view):
A wider view of the area: target.lroc.asu.edu...
edit on 25-6-2013 by wildespace because: (no reason given)