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