Amateur astronomers are puzzling over a seemingly anomalous cloud that has shown up on images of Mars taken over the past few days. Is it really a cloud, or a trick of the eye? Does it really extend 150 miles up from the surface, as some of the observers suggest? And what churned up all that stuff, anyway? The amateurs and the pros will be trying to resolve those questions before the phenomenon fades away."It's not completely unexpected," Jonathon Hill, a member of the team at the Mars Space Flight Facility at Arizona State University, told me today. "But it's bigger than we would expect, and it's definitely something that our atmosphere guys want to take a look at."