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                        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.

                               2021 January 6
    [2]See Explanation. Clicking on the picture will download the highest
                        resolution version available.

                         Striped Sand Dunes on Mars
       Image Credit: [3]HiRISE, [4]MRO, [5]LPL (U. Arizona), [6]NASA;
                     Processing: [7]Wl/odek Gl/azewski;
   Text: [8]Alex R. Howe ([9]NASA/[10]USRA, [11]Reader's History of SciFi
                                  Podcast)

   Explanation: Why are these sand dunes on Mars striped? No one is sure.
   The [12]featured image shows striped dunes in [13]Kunowsky Crater on
   Mars, photographed recently with the [14]Mars Reconnaissance Orbiter's
   HiRISE Camera. Many Martian dunes are known to be covered unevenly with
   carbon dioxide ([15]dry ice) frost, creating patterns of light and dark
   areas. Carbon dioxide doesn't melt, but [16]sublimates, turning
   directly into a gas. Carbon dioxide is also a [17]greenhouse material
   even as a solid, so it can trap heat under the ice and [18]sublimate
   from the bottom up, causing [19]geyser-like eruptions. During Martian
   spring, these eruptions can cause a pattern of dark [20]defrosting
   spots, where the [21]darker sand is exposed. The [22]featured image,
   though, was taken during Martian autumn, when the weather is getting
   colder - making these stripes particularly puzzling. One hypothesis is
   that they are caused by cracks in the ice that form from weaker
   eruptions or [23]thermal stress as part of the day-night cycle, but
   research continues. Watching these dunes and others through more
   [24]Martian seasons may give us more clues to solve this mystery.

                       Tomorrow's picture: open space
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    Authors & editors: [36]Robert Nemiroff ([37]MTU) & [38]Jerry Bonnell
                                 ([39]UMCP)
          NASA Official: Phillip Newman [40]Specific rights apply.
              [41]NASA Web Privacy Policy and Important Notices
                A service of: [42]ASD at [43]NASA / [44]GSFC
                           & [45]Michigan Tech. U.

References

   1. https://apod.nasa.gov/apod/archivepix.html
   2. https://apod.nasa.gov/apod/image/2101/StripedDunes_HiRISE_1182.jpg
   3. https://hirise.lpl.arizona.edu/
   4. https://mars.jpl.nasa.gov/mro/
   5. https://www.lpl.arizona.edu/
   6. https://www.nasa.gov/
   7. https://www.flickr.com/people/158004304@N05/
   8. https://sciencemeetsfiction.com/about/
   9. https://www.nasa.gov/
  10. https://www.usra.edu/
  11. http://readershistoryofscifi.libsyn.com/
  12. https://www.uahirise.org/ESP_066476_2370
  13. https://en.wikipedia.org/wiki/Kunowsky_(Martian_crater)
  14. https://mars.nasa.gov/mro/
  15. https://en.wikipedia.org/wiki/Dry_ice
  16. https://en.wikipedia.org/wiki/Sublimation_(phase_transition)
  17. https://en.wikipedia.org/wiki/Greenhouse_gas
  18. https://youtu.be/L_gnxWUuLm0
  19. https://en.wikipedia.org/wiki/Geysers_on_Mars
  20. https://apod.nasa.gov/apod/ap070805.html
  21. https://apod.nasa.gov/apod/ap160119.html
  22. https://www.flickr.com/photos/186557130@N06/50642211592/in/pool-apods/
  23. https://en.wikipedia.org/wiki/Thermal_stress
  24. https://www.msss.com/http/ps/seasons/seasons.html
  25. https://apod.nasa.gov/apod/ap210105.html
  26. https://apod.nasa.gov/apod/archivepix.html
  27. https://apod.nasa.gov/apod/lib/apsubmit2015.html
  28. https://apod.nasa.gov/apod/lib/aptree.html
  29. https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  30. https://apod.nasa.gov/apod/calendar/allyears.html
  31. https://apod.nasa.gov/apod.rss
  32. https://apod.nasa.gov/apod/lib/edlinks.html
  33. https://apod.nasa.gov/apod/lib/about_apod.html
  34. http://asterisk.apod.com/discuss_apod.php?date=210106
  35. https://apod.nasa.gov/apod/ap210107.html
  36. http://www.phy.mtu.edu/faculty/Nemiroff.html
  37. http://www.phy.mtu.edu/
  38. https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  39. http://www.astro.umd.edu/
  40. https://apod.nasa.gov/apod/lib/about_apod.html#srapply
  41. https://www.nasa.gov/about/highlights/HP_Privacy.html
  42. https://astrophysics.gsfc.nasa.gov/
  43. https://www.nasa.gov/
  44. https://www.nasa.gov/centers/goddard/
  45. http://www.mtu.edu/