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.

                              2019 September 24

                         Sand Dunes Thawing on Mars
               Image Credit & License: ESA, Roscosmos, CaSSIS

   Explanation: What are these strange shapes on Mars? Defrosting sand
   dunes. As spring dawned on the Northern Hemisphere of Mars, dunes of
   sand near the pole, as pictured here in late May by ESA's ExoMars Trace
   Gas Orbiter, began to thaw. The carbon dioxide and water ice actually
   sublime in the thin atmosphere directly to gas. Thinner regions of ice
   typically defrost first revealing sand whose darkness soaks in sunlight
   and accelerates the thaw. The process might even involve sandy jets
   exploding through the thinning ice. By summer, spots will expand to
   encompass the entire dunes. The Martian North Pole is ringed by many
   similar fields of barchan sand dunes, whose strange, smooth arcs are
   shaped by persistent Martian winds.

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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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                             & Michigan Tech. U.

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