Astronomy Picture of the Day

   [1]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
    [2]See Explanation. Clicking on the picture will download the highest
                        resolution version available.

                         Sand Dunes Thawing on Mars
         Image Credit & [3]License: [4]ESA, [5]Roscosmos, [6]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 [7]pictured here in late May by ESA's [8]ExoMars
   Trace Gas Orbiter, began to thaw. The [9]carbon dioxide and [10]water
   ice actually [11]sublime in the thin atmosphere directly to gas.
   Thinner regions of ice typically [12]defrost first revealing sand whose
   darkness soaks in sunlight and accelerates the thaw. The process might
   even involve [13]sandy jets exploding through the thinning ice. By
   summer, [14]spots will expand to encompass the entire dunes. The
   [15]Martian North Pole is ringed by many similar fields of [16]barchan
   sand dunes, whose strange, smooth arcs are shaped by persistent
   [17]Martian winds.

            Create a Distant Legacy: [18]Send your name to Mars.
                     Tomorrow's picture: really big bird
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    Authors & editors: [30]Robert Nemiroff ([31]MTU) & [32]Jerry Bonnell
                                 ([33]UMCP)
          NASA Official: Phillip Newman [34]Specific rights apply.
              [35]NASA Web Privacy Policy and Important Notices
                A service of: [36]ASD at [37]NASA / [38]GSFC
                           & [39]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/1909/DunesThawingOnMars_ExoMars_1400.jpg
   3. https://creativecommons.org/licenses/by-sa/3.0/igo/
   4. http://www.esa.int/
   5. http://en.roscosmos.ru/
   6. https://www.cassis.unibe.ch/
   7. http://www.esa.int/spaceinimages/Images/2019/09/North_polar_dunes_on_Mars
   8. https://exploration.esa.int/web/mars/-/46475-trace-gas-orbiter
   9. https://en.wikipedia.org/wiki/Carbon_dioxide
  10. https://apod.nasa.gov/apod/ap050720.html
  11. http://www.youtube.com/watch?v=J8mDGwf-5x0
  12. https://www.youtube.com/watch?v=GcwttjZOMPY
  13. https://apod.nasa.gov/apod/ap060823.html
  14. https://apod.nasa.gov/apod/ap040831.html
  15. https://apod.nasa.gov/apod/ap981216.html
  16. http://en.wikipedia.org/wiki/Barchan
  17. https://apod.nasa.gov/apod/ap990112.html
  18. https://mars.nasa.gov/participate/send-your-name/mars2020/
  19. https://apod.nasa.gov/apod/ap190923.html
  20. https://apod.nasa.gov/apod/archivepix.html
  21. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  22. https://apod.nasa.gov/apod/lib/aptree.html
  23. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  24. https://apod.nasa.gov/apod/calendar/allyears.html
  25. https://apod.nasa.gov/apod.rss
  26. https://apod.nasa.gov/apod/lib/edlinks.html
  27. https://apod.nasa.gov/apod/lib/about_apod.html
  28. http://asterisk.apod.com/discuss_apod.php?date=190924
  29. https://apod.nasa.gov/apod/ap190925.html
  30. http://www.phy.mtu.edu/faculty/Nemiroff.html
  31. http://www.phy.mtu.edu/
  32. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  33. http://www.astro.umd.edu/
  34. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  35. https://www.nasa.gov/about/highlights/HP_Privacy.html
  36. https://astrophysics.gsfc.nasa.gov/
  37. https://www.nasa.gov/
  38. https://www.nasa.gov/centers/goddard/
  39. http://www.mtu.edu/