Curiosity used its Mast Camera (Mastcam) to record the sunset during an evening of skywatching on April 15, 2015. The imaging was done between dust storms, but some dust remained suspended high in the atmosphere. The sunset observations help researchers assess the vertical distribution of dust in the atmosphere.
“The colors come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently,” said Mark Lemmon of Texas A&M University, College Station, the Curiosity science-team member who planned the observations. “When the blue light scatters off the dust, it stays closer to the direction of the sun than light of other colors does. The rest of the sky is yellow to orange, as yellow and red light scatter all over the sky instead of being absorbed or staying close to the sun.” [More at link]