Kim wrote:
At Bryce Canyon NP in June I had an interesting conversation with John Dobson regarding the gegenschein. I've always understood that the source of both the zodiacal light and gegenschein is sunlight reflected from interplanetary comet dust. In his characteristic way, John said, "NO!" According to John, the source is actually reflected sunlight from dust-size fused silica (glass) beads blasted from the surface of the Moon by impacts.
It is possible that I've missed an article, there doesn't seem to be much support for this statement in the journal literature. There are two possible causes: (1) dust in the interplanetary medium from comets and asteriod collisions and (2) dust from lunar impacts. Supporting evidence is stronger for a cometary and asteriod source - graphically illustrated by this year's earlier apparition of the once-in-forty year "great" Comet McNaught. http://antwrp.gsfc.nasa.gov/apod/ap070122.html. I. Possible Cause A: Kordylewski clouds That lunar libration clouds (Kordylewski clouds) at the Earth-Lunar L4 (leading) and L5 (trailing) points might exist have been speculated for some time. ( http://en.wikipedia.org/wiki/Image:Lagrange_points_Earth_vs_Moon.jpg ) The Earth-lunar L4 and L5 points are in the moving reference frame of the Moon's orbit. In the Earth-Moon libration system, points L2 (behind the Moon) and L1 (between the Earth and the Moon) would not be expected to be visible. I understand L3 is too unstable to be expected to hold dust particles. Kordylewski clouds are elevated areas of brightness either within the gegenschien or adjacent to it. See: Kordylewski 1961 http://adsabs.harvard.edu/abs/1961AcA....11..165K Krumm 1968 AAVSO http://www.homestead.com/cleopanther/files/AAVSO.pdf Recent dynamical modeling of the L4 and L5 points (Jorba 2000, A&A 364:327-338) indicates that such stable orbital regions can persist for up to 1,000 years. Freitas's studies related to a SETI search are often cited as failing to physically confirm these credible lunar libration cloud sightings. See - Freitas 1980 (in context of SETI search) http://www.rfreitas.com/Astro/SearchIcarus1980.htm Freitas 1983 (in context of SETI search) http://www.rfreitas.com/Astro/SearchIcarus1983.htm Freitas stated that his work could not rule-out the existence of micrometeorite clouds. In summary, there is no recent visible, radar, radio or infra-red confirmation of the existence of Kordylewski lunar libration dust clouds. II. Potential source B: Comets and asteriods Satellites with dust detectors including Helios, Pioneer 9, Galileo, Ulysses and Cassini have been used to trace the interplanetary dust medium out to about 3 A.U. It is not a local Earth-Moon phenomena. The dust disk has been imaged by Skylab in the 1970s, IRAS in the 1980s and by Clementine in the 1990s. IRAS satellite images from the 1980s revealed subbands of heightened illumination within the smooth zodicial light disk. ( Skyes (2005?), Fig. 3, http://www.lpi.usra.edu/books/CometsII/7024.pdf ). Through modeling, these bands are associated with the orbital paths of: a) Jupiter family comets with an inclination of 7 degs to the ecliptic. b) Halley family comets with an inclination of 33 degs to the ecliplitc. c) Istropically distributed Oort comet clouds. Hahn 2002 ( http://arxiv.org/abs/astro-ph/0204111 ). Hahn's modelling attributes 89% of the volume of interplantary dust within 1 AU to cometary sources. High altitude capture of interplanetary medium dust indicates their structure and composition is consistent with cometary or asteriod origin. See Sykes as Fig. 4. III. Contra-evidence: Lunar sodium atmosphere Spectroscopic studies of the Moon and anti-lunar point made since the mid-1990s indicates that there is a significant sodium atmosphere surrounding the Moon out to 2 or 3 lunar radii. The density of the thin sodium lunar atmosphere varies with the intensity of meteor showers such as Leonid storms in 1998 and 2001. See - Potter 2001 http://adsabs.harvard.edu/abs/2000JGR...10515073P Smith, SM 1999 http://adsabs.harvard.edu/abs/1999GeoRL..26.1649S When the Moon is between the Sun and the Earth, this sodium atmosphere is blown away from the Moon and around the Earth. The sodium wind "tail" has been imaged spectroscoptically at the anti-lunar point by professionals using a 1.2 meter telescope. Smith 1998. (It is not clear whether this sodium atmosphere could be detected by amateurs using modern CCD equipment.) The sodium gas is simply easier to detect - as compared to lunar micrometeorites or dust. Presumably, the impacts that increase the density of the sodium lunar atmosphere also replenish any lunar micrometeor dust in the Earth-Moon system. But the extent and volume of such replenishment is not known. Kim, in conclusion, there is little support in the journal literature that gegenschien is principally caused by reflection from lunar meteor dust. The weight of the literature is that the dust off of which gegenschien reflects has its source in comets, asteriods and the Oort cloud. This does not rule out contributions to gegenschien from lunar micrometeor dust or Kordylewski lunar libration dust clouds. Kordylewski clouds are probably a transient phenomena and without more continuous monitoring, it is unlikely their frequency and causes will be found in the near future. Part of NASA's return-to-the-Moon project is a professional program, using two dedicated RCT 14" robo-scopes to continuously monitor lunar meteor impacts on the Moon. ( http://www.nasa.gov/centers/marshall/news/lunar/photos.html ) Amateur members of the ALPO has been reporting such impacts for years. ( http://www.lpl.arizona.edu/~rhill/alpo/lunarstuff/lunimpacts.html ) The NASA program site listed above provides instructions for amateurs on how to image for lunar meteor impacts and software to automatically scan AVI files for meteor impact flashes. The ALPO site ( http://www.lpl.arizona.edu/~rhill/alpo/lunarstuff/AnnualShowers.doc ) includes a list of favorable meteor showers that might produce impact flashes. The next favorable dates for 2007 are Nov. 6, Nov. 18 (Leonids) and Dec. 14 (Geminids). - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net