Recently the ESA Lunar Probe Smart 1 impacted on the Lunar surface, raising a plume of dust high into the atmosphere... Atmosphere? 

The event was captured on film by several sources and does indeed clearly show a plume of dust, and both NASA and ESA call it a "plume", despite what we have been told about the density of the atmosphere. {Note: NASA has never stated that there is NO atmosphere - see previous page)

This mosaic was built with infrared images taken by the Canada-France-Hawaii Telescope (CFHT) 3 September 2006, and shows the flash and the dust cloud that followed the SMART-1 impact. The 15 exposures that make up the mosaic start with the one taken at the time of the flash. Courtesy of ESA/CFHT. (Click on image for hires version) ID number: SEM3353VRRE
 

"Analysis of images obtained at the CFHT by Christian Veillet have revealed a plume of debris thrown up when SMART-1 impacted the lunar surface."
Image Source: ESA

What happened? Dust after the flash

To determine what part of the flash comes from the lunar rock heated at impact or from the volatile substances released by the probe, it is important to obtain measurements in several optical and infrared wavelengths, in addition to the CFHT observations (2.12 microns).

From a detailed analysis of the CFHT infrared movie of the variations after the flash, a cloud of ejected material or debris travelling some 80 kilometres in about 130 seconds has been detected by observer Christian Veillet, Principal Investigator for the SMART-1 impact observations at CFHT. - SOURCE and More Details

Title: 
 The Lunar Sodium Atmosphere: A Study as Observed Through Four Lunar Eclipses
Authors: 
 Morrill, A. L.; Mendillo, M.; Baumgardner, J.
Affiliation: 
 AA(Boston University), AB(Boston University), AC(Boston University)
Publication: 
 American Astronomical Society, DPS meeting #29, #13.10; Bulletin of the American Astronomical Society, Vol. 29, p.987
Publication Date: 
 07/1997
Origin: 
 AAS
Abstract Copyright: 
 (c) 1997: American Astronomical Society
Bibliographic Code: 
 1997DPS....29.1310M

Abstract

The Moon's sodium atmosphere has been imaged during four lunar eclipses: November 29, 1993, April 2, 1996, September 27, 1996, and March 24, 1997, using a coronagraph type system at the Boston University four inch telescope located at the McDonald Observatory, TX, and at La Palma, Canary Islands. The Moon is imaged with a 5893A filter with a FWHP of 16A to include the sodium D1 and D2 lines. The eclipse condition provides the opportunity to observe the faint lunar atmosphere when the bright disk of the Moon is within the umbra and penumbra greatly reducing the scattered light in the system. In all four cases, the sodium atmosphere was imaged out to radial distances of 10 lunar radii. The brightness patterns were essentially uniform in azimuth and exhibited a radial decay far more gradual than seen at sub-solar radial distances at quarter Moon. While some variability appears among the four data sets, the large scale morphology under eclipse conditions was remarkably constant during the 1993 to 1997 events. This implies a steady source of sodium at times of full Moon.

Smithsonian Reports

Alan Bean describes the Lunar Sky...

"...as black as a pair of black patent leather shoes!"

The color we used in the image above came from a color chart that John sent to Howard Menger... he picked the color as he remembered it to be when he was taken to the Moon...

See Howard Menger's Story
See Opinion by John Lear

NASA Space Science
Moon Fountains
March 30, 2005

Excerpt #1

In the early 1960s before Apollo 11, several early Surveyor spacecraft that soft-landed on the Moon returned photographs showing an unmistakable twilight glow low over the lunar horizon persisting after the sun had set. Moreover, the distant horizon between land and sky did not look razor-sharp, as would have been expected in a vacuum where there was no atmospheric haze.

But most amazing of all, Apollo 17 astronauts orbiting the Moon in 1972 repeatedly saw and sketched what they variously called "bands," "streamers" or "twilight rays" for about 10 seconds before lunar sunrise or lunar sunset. Such rays were also reported by astronauts aboard Apollo 8, 10, and 15.
 

Here on Earth we see something similar: crepuscular rays. These are shafts of light and shadow cast by mountain ridges at sunrise or sunset. We see the shafts when they pass through dusty air. Perhaps the Moon's "twilight rays" are caused, likewise, by mountain shadows passing through levitating moondust. Many planetary scientists in the 1970s thought so, and some of them wrote papers to that effect (see the "more information" box at the end of this story for references).

But without an atmosphere, how could dust hover far above the Moon's surface? Even if temporarily kicked up by, say, a meteorite impact, wouldn't dust particles rapidly settle back onto the ground?

Well, no--at least not according to the "dynamic fountain model" for lunar dust recently proposed by Timothy J. Stubbs, Richard R. Vondrak, and William M. Farrell of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center.

"The Moon seems to have a tenuous atmosphere of moving dust particles," Stubbs explains. "We use the word 'fountain' to evoke the idea of a drinking fountain: the arc of water coming out of the spout looks static, but we know the water molecules are in motion." In the same way, individual bits of moondust are constantly leaping up from and falling back to the Moon's surface, giving rise to a "dust atmosphere" that looks static but is composed of dust particles in constant motion. 
 

Excerpt #2

On the Moon, there is no rubbing. The dust is electrostatically charged by the Sun in two different ways: by sunlight itself and by charged particles flowing out from the Sun (the solar wind).

On the daylit side of the Moon, solar ultraviolet and X-ray radiation is so energetic that it knocks electrons out of atoms and molecules in the lunar soil. Positive charges build up until the tiniest particles of lunar dust (measuring 1 micron and smaller) are repelled from the surface and lofted anywhere from meters to kilometers high, with the smallest particles reaching the highest altitudes, Stubbs explains. Eventually they fall back toward the surface where the process is repeated over and over again. 

If that's what happens on the day side of the Moon, the natural question then becomes, what happens on the night side? The dust there, Stubbs believes, is negatively charged. This charge comes from electrons in the solar wind, which flows around the Moon onto the night side. Indeed, the fountain model suggests that the night side would charge up to higher voltages than the day side, possibly launching dust particles to higher velocities and altitudes.

Day side: positive. Night side: negative. What, then, happens at the Moon's terminator--the moving line of sunrise or sunset between day and night?

There could be "significant horizontal electric fields forming between the day and night areas, so there might be horizontal dust transport," Stubbs speculates. "Dust would get sucked across the terminator sideways." Because the biggest flows would involve microscopic particles too small to see with the naked eye, an astronaut would not notice dust speeding past. Still, if he or she were on the Moon's dark side alert for lunar sunrise, the astronaut "might see a weird, shifting glow extending along the horizon, almost like a dancing curtain of light." Such a display might resemble pale auroras on Earth. 

Astronauts need to know, because in the years ahead NASA plans to send people back to the Moon, and deep dark craters are places where they might find pockets of frozen water--a crucial resource for any colony. Will they also encounter swarms of electric dust?

Credits:
Author: Trudy E. Bell 
NASA Science Reports - Rest of Article - Moon Fountains

During the Apollo era of exploration it was discovered that sunlight was scattered at the terminators giving rise to “horizon glow” and “streamers” above the lunar surface. This was observed from the dark side of the Moon during
sunset and sunrise by both surface landers and astronauts in orbit. These observations were quite unexpected, as the Moon was thought to be a pristine environment with a negligible atmosphere or exosphere.

LPI-1899 PDF

Just one of several papers from the early 1970s hypothesizing that the twilight glows photographed by the Surveyor landers and the "lunar rays" seen by the Apollo 17 astronauts were due to suspended lunar dust was "Evidence for a Lunar Dust Atmosphere from Apollo Orbital Observations" by J. E. McCoy and D. R. Criswell, Abstracts of the Lunar and Planetary Science Conference, volume 5, page 475, (1974). Another was "Surveyor Observations of Lunar Horizon-Glow," by J. J Rennilson and D. R. Criswell, The Moon 10: 121--142 (1974).

Electrostatic levitation of dust is also being studied by the Dusty Plasma Group in the physics department at the University of Colorado.

J. J. Rennilson1 and D. R. Criswell2
(1)  Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif., USA
(2)  The Lunar Science Institute, Houston, Tex., USA

Received: 13 August 1973 
Abstract  Each of the Surveyor 7, 6, and 5 spacecraft observed a line of light along its western lunar horizon following local sunset. It has been suggested that this horizon-glow (HG) is sunlight, which is forward-scattered by dust grains (~ 10µ in diam, ~ 50 grains cm–2) present in a tenuous cloud formed temporarily (lap 3 h duration) just above sharp sunlight/shadow boundaries in the terminator zone. Electrically charged grains could be levitated into the cloud by intense electrostatic fields (> 500 V cm–1) extending across the sunlight/shadow boundaries. Detailed analysis of the HG absolute luminance, temporal decay, and morphology confirm the cloud model. The levitation mechanism must eject 107 more particles per unit time into the cloud than could micro meteorites. Electrostatic transport is probably the dominant local transport mechanism of lunar surface fines.
This work was supported in part by the California Institute of Technology under Grant NGR 05-002-158, and in part by the Lunar Science Institute, which is operated by the Universities Space Research Association under Contract No. NSR-09-051-001 with the National Aeronautics and Space Administration. This paper is Lunar Science Institute Contribution No. 163.

Surveyor Observations of Lunar Horizon-Glow
 

These events occur when the sun, at a low lunar altitude, projects a ray or spike of light, through a broken wall feature of a crater. Although many of these events may be visible on the surface of the moon, these are a listing of the more common ray events which have been reported in astronomical magazines, publications, or from observers who may have detected a ray for the first time, and reported it. Although not of any scientific value, the allusiveness of these events, coupled with the short time frame they are visible, make these real challenges for the avid lunar observer!

If you observe any of these events, and would like to have your observations placed in the reports, or if you think you have discovered another notable ray events, let me know and I will get it published here.

Report #1

Date: 1997/5/29
Location: ASH Naylor Observatory, Lewisberry, Pa. 
          76d53'4" west, 40d8'54" north; elevation 570 feet 
Seeing: good 
Transparency: good 
Dome Temperature: 52 d F at session's end 
Instrument: 17" f/15 classical Cassegrain 
Ocular: 26mm Tele Vue Ploessl (249x) 
Time: 07:10 UT 

After I finished up a successful Herschel 400 globular cluster hunt on Thursday morning (I bagged 9 new H400 objects) I took a quick look at Jupiter and then the rising Moon as it obliterated the summer Milky Way. Although it was getting very late and quite chilly I was very happy to chance upon what just might be a new "lunar ray". I was scanning along the terminator at 249x when I noticed a triangular ray of sunlight streaming through a break in the western crater wall of Walter (at approximately 2 degrees west, 33 degrees south - Rukl chart 65). The ray illuminated Walter's western floor and the lower part of its central peak (the upper part was in direct sunlight, I believe). At approximately 07:42 UT I spotted a "reverse" triangular shadow being cast from an object on the western wall onto the illuminated crater floor. I could not stay any longer and by the time I had returned to my residence and set up my C4.5 (about 08:30 UT) the phenomenon was over and the crater floor was in darkness. 

Dave Mitsky
Harrisburg, PA
ASH, DVAA

The Robinson Lunar Observatory

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