Astronomy Picture of the Day Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. 2013 June 17 Dry Ice Sled Streaks on Mars Image Credit: HiRISE, MRO, LPL (U. Arizona), NASA Explanation: What creates these long and nearly straight grooves on Mars? Dubbed linear gullies, they app..