Sunlight illuminates the scene from the lower left.
Sunlight illuminates the heart from the lower left, thus the lower left wall of the heart looks bright because it is reflecting this sunlight. The problem here is frost. MOC images in the martian polar regions can sometimes fool the eye because a frosted slope might at first glance seem to be a sunlit slope. This picture was taken in late southern spring in a region that spends each winter and most of each spring completely buried in bright frost. This valentine from Mars is actually a pit formed by collapse within a straight-walled trough known in geological terms as a graben.