                        Astronomy Picture of the Day

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                                2021 July 13

                    Saturn's Iapetus: Painted Moon in 3D
   Image Credit: NASA, ESA, JPL, SSI, Cassini Imaging Team; 3D Rendering:
                                 NASA's VTAD

   Explanation: What has happened to Saturn's moon Iapetus? Vast sections
   of this strange world are dark brown, while others are as bright white.
   The composition of the dark material is unknown, but infrared spectra
   indicate that it possibly contains some dark form of carbon. Iapetus
   also has an unusual equatorial ridge that makes it appear like a
   walnut. To help better understand this seemingly painted moon, NASA
   directed the robotic Cassini spacecraft orbiting Saturn to swoop within
   2,000 kilometers in 2007. Iapetus is pictured here in 3D. A huge impact
   crater seen in the south spans a tremendous 450 kilometers and appears
   superposed on an older crater of similar size. The dark material is
   seen increasingly coating the easternmost part of Iapetus, darkening
   craters and highlands alike. Close inspection indicates that the dark
   coating typically faces the moon's equator and is less than a meter
   thick. A leading hypothesis is that the dark material is mostly dirt
   leftover when relatively warm but dirty ice sublimates. An initial
   coating of dark material may have been effectively painted on by the
   accretion of meteor-liberated debris from other moons.

                     Tomorrow's picture: black hole eats
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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.


