Mystery of the Lunar Ionosphere   
       
   Nov. 14, 2011: How can a world without air have an ionosphere? Somehow the   
   Moon has done it.   
       
   Lunar researchers have been struggling with the mystery for years, and they   
   may have finally found a solution.   
       
   But first, what is an ionosphere?   
       
   Every terrestrial planet with an atmosphere has one.  High above the planet's   
   rocky surface where the atmosphere meets the vacuum of space, ultraviolet rays   
   from the sun break apart atoms of air.  This creates a layer of ionized   
   gas--an "ionosphere."   
       
   Here on Earth, the ionosphere has a big impact on communications and   
   navigation.  For instance, it reflects radio waves, allowing shortwave radio   
   operators to bounce transmissions over the horizon for long-range   
   communications.  The ionosphere also bends and scatters signals from GPS   
   satellites, sometimes causing your GPS tracker to mis-read your position.   
       
   Go to the link below to view a ScienceCast video about the Moon's puzzling   
   ionosphere. [video]   
       
   http://www.youtube.com/watch?v=_zSrP4MacFE   
       
   The first convincing evidence for an ionosphere around the Moon came in the   
   1970s from the Soviet probes Luna 19 and 22.  Circling the Moon at close   
   range, the orbiters sensed a layer of charged material extending a few tens of   
   km above the lunar surface containing as many as 1000 electrons per cubic   
   centimeter-a thousand times more than any theory could explain.  Radio   
   astronomers also found hints of the lunar ionosphere when distant radio   
   sources passed behind the Moon's limb.   
       
   The idea of an "airless Moon" having an ionosphere didn't make much sense, but   
   the evidence seemed compelling.   
       
   As a matter of fact, the Moon isn't quite as airless as most people think.   
   Small amounts of gas created by radioactive decay seep out of the lunar   
   interior; meteoroids and the solar wind also blast atoms off the Moon's   
   surface. The resulting shroud of gas is so thin, however, that many   
   researchers refuse to call it an atmosphere, preferring instead the term   
   "exosphere." The density of the lunar exosphere is about a hundred million   
   billion times less than that of air on Earth-not enough to support an   
   ionosphere as dense as the ones the Luna probes sensed.   
       
   For 40 years, the Moon's ionosphere remained a mystery until Tim Stubbs of the   
   Goddard Space Flight Center published a possible solution earlier this year.    
   The answer, he proposes, is moondust.   
       
   Stubbs--a 30-something scientist who wasn't even born when the Moon's   
   ionosphere was discovered-read the accounts of Apollo 15 astronauts who   
   reported seeing a strange glow over the Moon's horizon.  Many researchers   
   believe the astronauts were seeing moondust. The Moon is an extremely dusty   
   place, naturally surrounded by a swarm of dust grains--think PigPen in Charlie   
   Brown.  When these floating grains catch the light of the rising or setting   
   sun, they create a glow along the horizon.   
       
   http://science.nasa.gov/media/medialibrary/2011/11/14/chargeddust.jpg   
       
   Dust grains floating above the lunar surface are ionized by solar UV   
   radiation. [larger images: #1, #2] Stubbs and colleagues realized that   
   floating dust could provide the answer.  UV rays from the sun hit the grains   
   and ionize them.   According to their calculations, this process produces   
   enough charge (positive grains surrounded by negative electrons) to create the   
   observed ionosphere.   
       
   An ionosphere made of dust instead of gas is new to planetary science.  No one   
   knows how it will behave at different times of night and day or at different   
   phases of the solar cycle, or how it might affect future radio communications   
   and navigation on the Moon.  NASA's ARTEMIS probes (orbiting the Moon now) and   
   the LADEE spacecraft (scheduled to launch in 2013 specifically for the purpose   
   of studying the lunar exosphere) may yet reveal its habits.   
       
   Updates may be expected in less than 40 years.   
       
       
   Author:Dr. Tony Phillips| Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   More Information   
       
   NASA Mission to Study the Moon's Fragile Atmosphere -- Science@NASA   
       
   Dead Spacecraft Walking -- Science@NASA   
       
   Earth's Magnetotail Lashes the Moon -- (Science@NASA)   
       
   Moon Storms -- (Science@NASA)   
       
   Moon Fountains -- (Science@NASA)   
       
   ARTEMIS -- mission home page   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.64   
    * Origin: NCS BBS (1:3828/7)