GSFC: Model Helps Search for Moon Dust Fountains

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GSFC: Model Helps Search for Moon Dust Fountains

Post by bystander » Fri Jun 11, 2010 3:48 am

Model Helps Search for Moon Dust Fountains
NASA GSFC NSSDC LADEE - 10 June 2010
In exploration, sometimes you find more than what you're looking for, including things that shouldn’t be there. As the Apollo 17 astronauts orbited over the night side of the moon, with the sun just beneath the horizon right before orbital "sunrise," Eugene Cernan prepared to make observations of sunlight scattered by the sun's thin outer atmosphere and interplanetary dust from comets and collisions between asteroids. The idea was to have the moon block the brilliant direct sunlight so this faint glow, called Coronal and Zodiacal Light (CZL), could be seen. They should have seen a dim "hump" of light in the middle of the horizon that gradually grew in size and intensity until it was overwhelmed by sunrise. What came next was not supposed to happen.

Picture of coronal light taken with Clementine spacecraftThis is a picture of coronal and zodiacal light (CZL) taken with the Clementine spacecraft, when the sun was behind the moon. The white area on the edge of the moon is the CZL, and the bright dot at the top is the planet Venus. Credit: NASA
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Cernan did see the CZL glow, but it had a strange companion. A slim crescent of light appeared all along the horizon, and just before sunrise, faint rays appeared, similar to the columns of light seen on Earth when sunlight pokes through a hole in a layer of clouds. On Earth, the horizon glow seen at sunrise and sunset, and the rays, are created when sunlight scatters off atmospheric moisture and dust. But the moon has almost no atmosphere -- its atmosphere is so thin, atoms and molecules there rarely collide with each other and it's technically referred to as an "exosphere". This thin atmosphere should not have produced the horizon glow seen.
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While dust can produce a LHG by scattering sunlight, the presence of even an intermittent high-altitude dust atmosphere was unexpected -- the moon's exosphere is far too thin for wind to blow and suspend dust. Although the moon is constantly bombarded by meteorites (mostly microscopic) that kick up dust, "the dust concentrations inferred from LHG are much higher than expected from debris ejected by meteorite impacts alone," adds Stubbs.

It has long been suggested that lunar dust gets transported electrically. For example, on the day side of the moon, solar ultraviolet radiation is strong enough to kick electrons from dust particles in the lunar soil. Removal of electrons, which have a negative electric charge, leaves the dust with a positive electric charge. Since like charges repel, the positively charged dust particles get pushed away from each other, and the only direction not blocked by more dust is up.

The smaller particles likely get ejected higher because they are lighter. This might explain the different LHG observed by Surveyor and Apollo. The low-altitude glow seen by Surveyor appeared to be from larger, relatively heavy particles, while the high-altitude glow seen by Apollo astronauts was likely from the smallest particles. Small particles can get such a boost from the surface charge that they are lofted high above the surface and follow ballistic trajectories, returning to the surface under the influence of the moon's gravity. This movement of the smallest particles could make fountains of moon dust.
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As early as 2012, NASA could launch the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft that will orbit the moon and look for the LHG and exospheric dust hinted at in the Apollo observations. LADEE will look for LHG using the Ultraviolet Spectrometer (UVS) instrument, which will measure the intensity of light at different wavelengths at a point above the lunar horizon. Each potential source – sodium or dust, for example -- will produce a unique "signature" glow in ultraviolet and/or visible light.

LADEE may also be able to use its star-tracker navigation cameras to observe LHG, which would provide details on the shape of the visible light scattering sources, according to Stubbs. LADEE also has a dust detector instrument to record any hits from high-altitude dust.

To give the LADEE mission an idea of the best techniques and places to look for the LHG, Stubbs and his team recently created computer simulations of dust and sodium-generated glows. "The simulations show that if LHG is produced by dust, then it will be brightest in the forward scattering direction; that is, with the dust between the sun and the observer," says Dr. David Glenar of New Mexico State University. "However, for this viewing geometry, the disk of the sun needs to be blocked (e.g., by the edge of the moon), so as not to be overwhelmed by its brightness. This is why LHG has previously only been observed from the shadow of the moon close to the terminator, and this is the initial approach that is planned for LADEE/UVS."

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