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                           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)   
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