[mikeash]

It's interesting to consider the numbers from the opposite direction as well. If this stuff is real, where are all the non-face artifacts? If you plopped a rover down in, say, the ruins of Troy, you'd expect to have a bazillion photos of pottery and arrowheads and boring walls for every photo of a statue of a face. Why aren't the rovers finding anything like that? Humans have no mental bias for seeing such artifacts in random noise, so of course "we find faces, faces, and more faces, and nothing else" fits the pareidolia explanation much better than the actual-alien-artifacts explanation.