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.

                                 2020 July 8

                            Mercury's Sodium Tail
                Image Credit & Copyright: Andrea Alessandrini

   Explanation: What is that fuzzy streak extending from Mercury? Long
   exposures of our Solar System's innermost planet may reveal something
   unexpected: a tail. Mercury's thin atmosphere contains small amounts of
   sodium that glow when excited by light from the Sun. Sunlight also
   liberates these molecules from Mercury's surface and pushes them away.
   The yellow glow from sodium, in particular, is relatively bright.
   Pictured, Mercury and its sodium tail are visible in a deep image taken
   in late May from Italy through a filter that primarily transmits yellow
   light emitted by sodium. First predicted in the 1980s, Mercury's tail
   was first discovered in 2001. Many tail details were revealed in
   multiple observations by NASA's robotic MESSENGER spacecraft that
   orbited Mercury between 2011 and 2015. Tails are usually associated
   with comets. The tails of Comet NEOWISE are currently visible with the
   unaided eye in the morning sky.

    Comet NEOWISE from Around the Globe: Notable Images Submitted to APOD
                       Tomorrow's picture: open space
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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.

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