XPost: sci.astro   
      
   In sci.astro Wirt Atmar  wrote on 26 Jan 2004 22:49:46 GMT:   
   :WA Mark asks:   
      
   >>In 1 of the 3 DIMES images taken by Opportunity on its approach to the   
   >>surface (at 1404m), there is a brightly ringed crater and a distinct,   
   >>slightly diffuse dark circular shadow about half a crater width to its   
   >>left.  Is this the shadow of the spacecraft?   
      
   :WA JPL says that it's the shadow of the parachute. The bright ring around the   
   :WA shadow is of course caused by the diffraction of sunlight around the   
   parachute.   
      
   Yes, that is the shadow of the parachute/spacecraft, but the bright   
   halo around it is caused by the opposition affect, not by diffraction   
   of sunlight around the parachute.  It is the backscattering of   
   sunlight near the zero phase angle that can also be seen in the   
   abnormal brightening of asteroids near opposition, or even of the   
   full moon.  The full moon, for example, is much more than twice as   
   bright as the first quarter moon appears.  You can also see the same   
   affect from an airplane if you look on the ground opposite the sun   
   where the shadow of the plane should be - you'll see a bright area   
   on the ground.   
      
   Jim.   
      
   Jim Scotti   
   Lunar & Planetary Laboratory   
   University of Arizona   
   Tucson, AZ 85721 USA                 http://www.lpl.arizona.edu/~jscotti/   
      
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