Winter on Mars comes with a blanket of carbon dioxide snow. During the spring “thaw,” this snow evaporates into the atmosphere, lingering longest in the shallow depressions such as the troughs of polygon patterned ground.
Enhanced color shows the carbon dioxide snow as bluish-white patches among areas of rusty red bare ground. We took this image in 2008 as a possible landing site for the Phoenix Lander that arrived on Mars later that same year. [More at link]