                        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 July 31
   A dark irregularly-shaped moon is seen in front of the red planet Mars.
     Craters are visible in the foreground and the edge of the planet is
    just visible at the top of the image. Please see the explanation for
                         more detailed information.

                              Phobos over Mars
   Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0
                            License: Andrea Luck

   Explanation: Why is Phobos so dark? Phobos, the largest and innermost
   of the two Martian moons, is the darkest moon in the entire Solar
   System. Its unusual orbit and color indicate that it may be a captured
   asteroid composed of a mixture of ice and dark rock. The featured
   assigned-color picture of Phobos near the edge of Mars was captured in
   late 2021 by ESA's robot spacecraft Mars Express, currently orbiting
   Mars. Phobos is a heavily cratered and barren moon, with its largest
   crater located on the far side. From images like this, Phobos has been
   determined to be covered by perhaps a meter of loose dust. Phobos
   orbits so close to Mars that from some places it would appear to rise
   and set twice a day, while from other places it would not be visible at
   all. Phobos' orbit around Mars is continually decaying -- it will
   likely break up with pieces crashing to the Martian surface in about 50
   million years.

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                                 (post 1995)
                Tomorrow's picture: monster at the Sun's edge
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       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.

