              APOD: 2018 June 3 - Saturns Iapetus: Painted Moon

                         Astronomy Picture of the Day

    Discover the cosmos! [1] Each day a different image or photograph of our
 fascinating universe is featured, along with a brief explanation written by a
                           professional astronomer.

                                  2018 June 3
                                      [2]
                        Saturn's Iapetus: Painted Moon
  Image Credit: NASA [3] , ESA [4] , JPL [5] , SSI [6] , Cassini Imaging Team
                                      [7]

Explanation: What has happened to Saturn's moon Iapetus? Vast sections of this
strange world [8]  are dark as coal [9] , while others are as bright as ice.
The composition of the dark material is unknown, but infrared [10]  spectra
[11]  indicate that it possibly contains some dark form of carbon [12] .
Iapetus also has an unusual equatorial ridge [13] that makes it appear like a
walnut [14] . To help better understand this seemingly painted moon, NASA [15]
 directed the robotic Cassini spacecraft [16]  orbiting Saturn to swoop within
2,000 kilometers [17]  in 2007. Pictured here [18] , from about 75,000
kilometers out, Cassini's trajectory allowed unprecedented imaging of the
hemisphere of Iapetus that is always trailing [19] . A huge impact crater seen
in the south spans a tremendous 450 kilometers and appears superposed on an
older crater [20]  of similar size. The dark material [21] is seen
increasingly coating the easternmost part of Iapetus [22] , darkening craters
and highlands alike. Close inspection [23]  indicates that the dark coating
typically faces the moon's equator and is less than a meter thick. A leading
hypothesis [24]  is that the dark material is mostly dirt leftover when
relatively warm but dirty ice sublimates [25] . An initial coating of dark
material [26]  may have been effectively painted on by the accretion of
meteor-liberated debris from other moon [27] s.

                      Tomorrow's picture: moon zoom [28]

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    Authors & editors: Robert Nemiroff [40] (MTU [41] ) & Jerry Bonnell [42]
                                  (UMCP [43] )
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Site notes:
  [1] archivepix.html
  [2] image/1806/iapetus3_cassini_4100.jpg
  [3] https://www.nasa.gov/
  [4] http://www.esa.int/
  [5] http://www.jpl.nasa.gov/
  [6] http://www.spacescience.org/
  [7] http://ciclops.org/
  [8] http://en.wikipedia.org/wiki/Iapetus_%28moon%29
  [9] http://en.wikipedia.org/wiki/Coal
  [10] https://science.nasa.gov/ems/07_infraredwaves
  [11] https://science.nasa.gov/ems/01_intro
  [12] http://periodic.lanl.gov/6.shtml
  [13] ap050201.html
  [14] http://www.whfoods.com/genpage.php?tname=foodspice&dbid=99
  [15] https://www.nasa.gov/about/index.html
  [16] http://en.wikipedia.org/wiki/Cassini_spacecraft
  [17] ap070919.html
  [18] https://photojournal.jpl.nasa.gov/catalog/PIA08384
  [19] http://en.wikipedia.org/wiki/Tidal_locking
  [20] ap060530.html
  [21] ap070914.html
  [22] ap060103.html
  [23] http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/
  [24] http://adsabs.harvard.edu/abs/2005DPS....37.3908S
  [25] http://en.wikipedia.org/wiki/Sublimation_(chemistry)
  [26] http://www.youtube.com/watch?v=DYvITG_TDfE
  [27] ap180514.html
  [28] ap180604.html
  [29] ap180602.html
  [30] archivepix.html
  [31] lib/apsubmit2015.html
  [32] lib/aptree.html
  [33] https://antwrp.gsfc.nasa.gov/cgi-bin/apod/apod_search
  [34] calendar/allyears.html
  [35] /apod.rss
  [36] lib/edlinks.html
  [37] lib/about_apod.html
  [38] http://asterisk.apod.com/discuss_apod.php?date=180603
  [39] ap180604.html
  [40] http://www.phy.mtu.edu/faculty/Nemiroff.html
  [41] http://www.phy.mtu.edu/
  [42] https://antwrp.gsfc.nasa.gov/htmltest/jbonnell/www/bonnell.html
  [43] http://www.astro.umd.edu/
  [44] lib/about_apod.html#srapply
  [45] https://www.nasa.gov/about/highlights/HP_Privacy.html
  [46] https://astrophysics.gsfc.nasa.gov/
  [47] https://www.nasa.gov/
  [48] https://www.nasa.gov/centers/goddard/
  [49] http://www.mtu.edu/
