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   New Evidence for Ice on Mercury        
      
   Nov. 29, 2012:  Observations by NASA's MESSENGER spacecraft provide compelling   
   support for the long-held hypothesis that Mercury harbors abundant water ice   
   and other frozen materials in permanently shadowed polar craters.    
      
   "The new data indicate the water ice in Mercury's polar regions, if spread   
   over an area the size of Washington, D.C., would be more than 2 miles thick,"   
   said David Lawrence, a MESSENGER participating scientist at the Johns Hopkins   
   University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of   
   one of three papers describing the findings in the online edition of Science   
   Express.    
      
   http://www.nasa.gov/mission_pages/messenger/multimedia/PressConf20121126_3.html   
      
   Mercury's north pole. Red denotes areas that are in shadow in all images   
   acquired by MESSENGER to date. (The mapping of shadows is still incomplete   
   near the pole.) Yellow shows the locations of bright polar deposits imaged by   
   Earth-based radar. Updated from N. L. Chabot et al., Journal of Geophysical   
   Research, 117, doi: 10.1029/2012JE004172 (2012). [more]    
      
   Given its proximity to the Sun, Mercury would seem to be an unlikely place to   
   find ice. But the tilt of Mercury's rotational axis is almost zero -- less   
   than one degree -- so there are pockets at the planet's poles that never see   
   sunlight. Scientists suggested decades ago that  water ice might be trapped in   
   those shadowed areas at Mercury's poles.   
      
   The idea received a boost in 1991, when the Arecibo radio telescope in Puerto   
   Rico detected unusually radar-bright patches at Mercury's poles, spots that   
   reflected radio waves in the way one would expect if there were water ice.   
   Many of these patches corresponded to the location of large impact craters   
   mapped by the Mariner 10 spacecraft in the 1970s. But researchers weren't sure   
   if the radar-bright patches detected by Arecibo corresponded to shadowly   
   places in the craters.    
      
   MESSENGER's arrival at Mercury last year changed that. Images from the   
   spacecraft's Mercury Dual Imaging System taken in 2011 and earlier this year   
   show that radar-bright features at Mercury's north and south poles are within   
   shadowed regions on Mercury's surface.   
      
   Now, the newest data from MESSENGER confirm that water ice is the major   
   constituent of Mercury's north polar deposits.  In the coldest places, the ice   
   is exposed on the surface.  In slightly warmer spots, some kind of dark   
   insulating material appears to cover the ice.   
      
   MESSENGER uses neutron spectroscopy to measure average hydrogen concentrations   
   within Mercury's radar-bright regions. Ice concentrations are derived, in   
   turn, from the hydrogen measurements.  This is possible because water, or H2O,   
   is two parts hydrogen.    
      
   "The neutron data indicate that Mercury's radar-bright polar deposits contain,   
   on average, a hydrogen-rich layer more than tens of centimeters thick beneath   
   a surficial layer 10 to 20 centimeters thick that is less rich in hydrogen,"   
   says Lawrence. "The buried layer has a hydrogen content consistent with nearly   
   pure water ice."   
      
   Data from MESSENGER's Mercury Laser Altimeter (MLA) -- which has fired more   
   than 10 million laser pulses at Mercury to make detailed maps of the planet's   
   topography -- corroborate the ice hypothesis, writes Gregory Neumann of the   
   NASA Goddard Flight Center. In a second paper, Neumann and his colleagues   
   report that the first laser measurements of the shadowed north polar regions   
   reveal irregular dark and bright deposits near Mercury's north pole.    
      
   "Nobody had seen these dark regions on Mercury before, so they were mysterious   
   at first," Neumann says.    
      
   Neumann suggests that both the dark and bright materials were brought to   
   Mercury by comets or asteroids, a finding corroborated in a third paper led by   
   David Paige of the University of California, Los Angeles.    
      
   "The dark material is likely a mix of complex organic compounds delivered to   
   Mercury by the impacts of comets and volatile-rich asteroids, the same objects   
   that likely delivered water to the innermost planet," Paige says.   This dark   
   insulating material is a new wrinkle to the story, adds Sean Solomon of the   
   Columbia University's Lamont-Doherty Earth Observatory, principal investigator   
   of the MESSENGER mission. "For more than 20 years the jury has been   
   deliberating on whether the planet closest to the Sun hosts abundant water ice   
   in its permanently shadowed polar regions. MESSENGER has now supplied a   
   unanimous affirmative verdict."   "But the new observations have also raised   
   new questions," adds Solomon. "Do the dark materials in the polar deposits   
   consist mostly of organic compounds? What kind of chemical reactions has that   
   material experienced? Are there any regions on or within Mercury that might   
   have both liquid water and organic compounds? Only with the continued   
   exploration of Mercury can we hope to make progress on these new questions."    
   Stay tuned to Science@NASA for answers.    
      
      
   Production editor: Dr. Tony Phillips | Credit: Science@NASA   
      
   More Information    
   MESSENGER Home Page -- NASA    
   MESSENGER Home Page -- Johns Hopkins Applied Physics Lab    
      
   MESSENGER was designed and built by APL. The lab manages and operates the   
   mission for NASA's Science Mission Directorate in Washington. The mission is   
   part of NASA's Discovery Program, managed for the directorate by the agency's   
   Marshall Space Flight Center in Huntsville,    
      
      
   Regards,   
      
   Roger    
   --- timEd/386 1.10.y2k+   
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