... Mars . ... An artificial comet holds secrets to Earths life . ... This HiRISE image of the Holden crater shows the megabreccia and the possible ancient lake. ... The megabreccia is topped by layers of fine sediments that formed in what apparently was a long-lived, calm lake that filled Holden crater on early Mars, HiRISE scientists say. ... HiRISE images show megabreccia outcrops in Holden crater walls. ... The megabreccia excavated when Holden crater formed is the first found on Mars, Grant says. ...