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                           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.   
      
                                 2023 February 26   
      An unusual two-toned ball is pictured. The ball, Saturn's moon Iapetus,   
       has many craters and an unusual ridge running along its equator that   
         makes it look like a walnut. Please see the explanation for more   
                               detailed information.   
      
                   Saturn's Iapetus: Moon with a Strange Surface   
              Image Credit: NASA, ESA, JPL, SSI, Cassini Imaging Team   
      
      Explanation: What would make a moon look like a walnut? A strange ridge   
      that circles Saturn's moon Iapetus's equator, visible near the bottom   
      of the featured image, makes it appear similar to a popular edible nut.   
      The origin of the ridge remains unknown, though, with hypotheses   
      including ice that welled up from below, a ring that crashed down from   
      above, and structure left over from its formation perhaps 100 million   
      years ago. Also strange is that about half of Iapetus is so dark that   
      it can nearly disappear when viewed from Earth, while the rest is,   
      reflectively, quite bright. Observations show that the degree of   
      darkness of the terrain is strangely uniform, as if a dark coating was   
      somehow recently applied to an ancient and highly cratered surface.   
      Last, several large impact basins occur around Iapetus, with a   
      400-kilometer wide crater visible near the image center, surrounded by   
      deep cliffs that drop sharply to the crater floor. The featured image   
      was taken by the Saturn-orbiting Cassini spacecraft during a flyby of   
      Iapetus at the end of 2004.   
      
                       Tomorrow's picture: dawn before dawn   
        __________________________________________________________________   
      
          Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)   
               NASA Official: Phillip Newman Specific rights apply.   
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                              NASA Science Activation   
                                & Michigan Tech. U.   
      
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