NASA Mission To Study Mysterious Lunar Twilight Rays   
       
   September 3, 2013:  Back in the 60s and 70s, Apollo astronauts circling the   
   Moon saw something that still puzzles researchers today. About 10 seconds   
   before lunar sunrise or lunar sunset, pale luminous streamers would pop up   
   over the gray horizon. These "twilight rays" were witnessed by crewmembers of   
   Apollo 8, 10, 15 and 17.   
       
   Back on Earth, we see twilight rays all the time as shafts of sunlight   
   penetrate evening clouds and haze.  The "airless Moon" shouldn't have such   
   rays, yet the men of Apollo clearly saw them.   
       
   Later this week a NASA spacecraft is going back to the Moon to investigate.   
   Slated for launch on Sept 6, 2013, the Lunar Atmosphere and Dust Environment   
   Explorer ("LADEE" for short) will seek out twilight rays and other mysteries   
   of the lunar atmosphere.   
       
   http://www.youtube.com/watch?v=2z_oi36O8tw   
   A new ScienceCast video explores the mysteries of the lunar atmosphere. Play it   
       
   "Yes, the Moon does have an atmosphere," says Richard Elphic, the project   
   scientist for LADEE at NASA Ames.  "It's just much more tenuous than ours."   
       
   The Moon's atmosphere is so flimsy-about ten thousand billion times less dense   
   than Earth's-that a good sneeze would rip through it like a hurricane. "Lunar   
   air" is a gossamer mix of argon-40, which seeps out of the ground due to   
   radioactive decay in the lunar interior, plus elements such as helium, sodium,   
   and potassium, sputtered off the lunar surface by solar wind and   
   micrometeoroids.   
       
   None of these gases appear in sufficient quantities, however, to explain the   
   twilight rays.   
       
   "We're missing something," says Elphic.   
       
   The missing piece might be dust. When sunlight falls on the Moon, solar UV   
   radiation electrifies the unprotected topsoil, possibly causing lightweight   
   grains of moondust to rise off the ground, joining the gases already there.   
       
   "This electrically charged dust may be what the astronauts saw," says Elphic.   
   LADEE's Lunar Dust Experiment will collect and analyze dust in the Moon's   
   atmosphere to test this hypothesis.   
       
   http://tinyurl.com/oqjwnvt   
   Lunar twilight rays sketched by Apollo 17 astronauts. Researchers have a   
   special name for atmospheres as fantastically thin as the Moon's: an   
   exosphere. On Earth, molecules in the thick air are constantly bumping into   
   each other, spreading pressure and heat in all directions. In an exosphere,   
   however, molecules are so far apart they rarely collide.   
       
   "Instead of bumping into each other," says Elphic, "they bump into the lunar   
   surface."   
       
   Air molecules coming into contact with moondust are expected to stick,   
   briefly, before moving on again. Hop and stick, hop and stick.  At any given   
   moment millions of molecules could be hopping like bunnies across every square   
   inch of lunar terrain.  Ultraviolet, visible light, and mass spectrometers on   
   board LADEE will inventory the molecules present and determine how they behave.   
       
   "The dusty, flimsy mix of atoms and molecules in the lunar atmosphere is sure   
   to have alien properties that our experience on Earth has not prepared us to   
   anticipate," says Elphic.   
       
   To find out, LADEE will be working on a deadline.  On April 15th of next year,   
   the sunset-colored shadow of Earth will envelop the Moon for a lunar eclipse.   
   It will be a grand sight from Earth, but bad news for LADEE.  The spacecraft   
   is solar powered and requires sunlight to charge its batteries. An eclipse   
   could end the mission.   
       
   "The current plan," says Elphic, "is, before the eclipse, to guide the   
   spacecraft into the surface of the moon for a final impact that we can study.    
   We'll be taking data until the very end."   
       
   Credits:   
       
   Author: Dr. Tony Phillips |Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   Related Links:   
       
   Lunar Atmosphere and Dust Environment Explorer  -- home page   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.94   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)