Photometric changes on Saturn's Titan: Evidence for active cryovolcanism - Nelson, Robert; Kamp, Lucas; Lopes, Rosaly; Matson, Dennis; Kirk, Randolph; Hapke, Bruce; Wall, Stephen; Boryta, Mark; Leader, Frank; Smythe, William; Mitchell, Karl; Baines, Kevin; et al. (Geophysical Research Letters, 2009)