"It's puzzling," said Michael Carr of the US Geological Survey. "I looked at a few pictures around [the area] and couldn't find anything to explain it. Very puzzling! These are huge boulders. There are no indications of any outcrops that could shed such boulders."
How huge are they? It is estimated that they are between 50 and 80 feet in diameter! Those are big rocks! "Wow! These have me totally stumped," commented Ron Greeley of Arizona State University. "Not only is the dark color of the boulders a surprise, but they appear totally out of context in the surrounding terrain. There is nothing in the rest of the image to suggest a source for such large boulders, nor their arrangement on the surface."
One proposed theory is that the boulders are the remains of a meteor that shattered on impact. Yet there is no impact crater; the meteor would have had to have been moving quite slowly to make no crater and keep its fragments in such a close grouping. The meteor theory is highly unlikely. Planetary scientists have yet to come up with a plausible, satisfactory explanation for the boulders.