Ice, probably frozen nitrogen, that accumulated on the highlands (right) is draining from Pluto’s mountains into the informally named Sputnik Planum through a series of valleys (red arrows). The flow front of the ice moving into Sputnik Planum is outlined by the blue arrows. The setting sun illuminates a fog or near-surface haze, cut by the parallel shadows of many local hills and small mountains on Pluto Article: http://phenomena.nationalgeographic.com/2015/09/17/pluto-at-dusk-spectacular-and-spookily-earthlike/
