                        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.

                                 2022 May 3

                            Mercury's Sodium Tail
                 Image Credit & Copyright: Sebastian Voltmer

   Explanation: That's no comet. Below the Pleiades star cluster is
   actually a planet: 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 last week from La Palma, Spain
   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, of course, are usually associated
   with comets.

                  Tomorrow's picture: planet pyramid parade
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       Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (UMCP)
            NASA Official: Phillip Newman Specific rights apply.
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