April 27, 2008
Zapped by Earth?
Surveyor 7 image from SCIENCE@NASA
Recently a NASA-funded scientist has suggested that a previously unknown electrostatic phenomena effects the Moon every month. SCIENCE@NASA reports that Tim Stubbs, a University of Maryland scientist, proposes that during the 5-6 (Earth) days centered on Full Moon, the Moon passes through the Earth's magnetotail, whose charged particles build up a negative charge on the lunar surface. If you have walked across carpet with rubber soled shoes you have probably experienced the effect of a charge build up - you get a static shock when touching a doorknob. The same thing may happen to an astronaut walking across the charged lunar surface who then touches a spacecraft or an electronic instrument. Stubbs proposes that magentotail-related electrostatic charge could be responsible for two mysterious phenomena seen from near the lunar surface. The Surveyor 7 spacecraft imaged a bright glow on the horizon during lunar night, widely thought to be charged dust that levitated above the surface. Apollo astronauts in orbit around the Moon sometimes observed twilight rays just before sunrise or after sunset. This would be sun reflected off of dust above the surface. Levitating dust may cause problems when humans return to the Moon and stay there during Full Moon or during Sunrise or Sunset. Or it may be a non-issue: I am unaware that any of the Apollo and Lunakhod instruments on the surface of the Moon suffered electronic glitches during Full Moon or Sunrise or set. Electrostatic movement of lunar dust has a bad reputation, for Tommy Gold, of Cornell University, believed that suchmovement would concentrate dust in deep piles that Apollo landers might sink into and disappear - he was wrong. Back to the SCIENCE@NASA story which says that during its passage through Earth's magnetotail the electrons build up on night side of the Moon (the farside) but then that side loses the excess charge during the day when solar radiation liberates electrons from the surface. This suggests something not mentioned in the story. Do the permanently shadowed crater bottoms at the lunar poles have huge negative charges because the magnetotail electrons have never leaked away?
Yesterday's LPOD: Another Gift From LPI
Tomorrow's LPOD: Changes?
(0) I think this one definitely answers the question about whether your description of the LPOD photos is necessary. I had no clue what I was looking at. Never heard of it. Very interesting. Thanks.
-- Kay Meyer
(1) See also pages 175, 176, and 177 of the book MYSTERIOUS UNIVERSE; A HANDBOOK OF ASTRONOMICAL ANOMALIES by W.R.Corliss (The Sourcebook Project, 1979). And also THE LUNAR SOURCEBOOK; A USER'S GUIDE TO THE MOON.
The caption to a later photo in this same sequence in the excellent review article of Surveyor findings by Leonard Jaffee (Science, May 16, 1969, Vol. 164, pp. 775-788) identifies it as a "Brightness along western horizon 90 minutes after upper limb of the sun had set. Exposure 1.2 seconds at f/10 (portion of Surveyor 7 narrow-angle picture, width 6.4 degrees, taken 23 January 1968, 07:32:49 UT)". The accompanying text (p. 784) says: "After local sunset, when the center of the sun was about 1 degree below the horizon, a bright band was visible very close to the horizon near the sunset azimuth, extending up to 5 degrees horizontally. The light from this band is apparently not polarized. The effect is presumably due to diffraction, refraction, or scattering by the surface material." (the LPOD photos look like they show the center part of the Science photo up to 10 minutes earlier)
This is interesting to me, because an extremely similar looking "burning bush" phenomenon can be seen on Earth if one examines, through a telescope, the point at which the Sun is about to rise. It does not happen all the time, or with equal intensity, but I have seen it hundreds of times while waiting for the Sun to rise over scrub-covered hills anywhere from 5 to 25 miles distant. For anywhere from 10 seconds to 3 minutes before sunrise a striking line of brightness is seen along the ridge top, in which bushes and tree branches (if present) look on fire as in a "solarized" print. It can extend for a couple of degrees around the point where the Sun is about to rise. I have heard this effect attributed to diffraction of the otherwise straight rays of sunlight by the sharp edge of the ridge, but to my mind it is simply the sunlight grazing along the unseen far slope of the ridge and scattering off every favorably inclined surface (in such matters, the distinction between "diffraction" and "scattering" is a bit muddled). On Earth, it may also be enhanced by refraction if the branches are coated with dew.
Given the Moon's smaller radius of curvature, and that the sunrise/sunset is 30 times slower than on Earth, I find the similarity in scale, duration and appearance quite striking. Of course there are no bushes on the Moon (and certainly no dew-covered ones), but I think this effect can be seen over flat and relatively smooth surfaces (like rooftops) as well. There may be no relationship between the two phenomena, but as an alternative to a levitating dust layer, scattering by the faces of rocks and boulders reflecting light from the sunlit part of the Moon that we can't see over the horizon would seem to be a possible explanation (as is probably explained in Jaffee's several references on the subject, which I haven't had a chance to read).
The odd thing to me about the lunar photos shown in the SCIENCE@NASA article is that they seem to show the phenomenon spreading out horizontally from its initial bright core as it fades. Possibly this is just an artifact of the way the photos were exposed or processed. The phenomenon seen on Earth does have a bright core, but the more distant points brighten/fade with it as the Sun approaches/recedes from the horizon. It's also unclear to me if the present photos are trying to show that the lunar phenomenon peaks in brightness some time after sunset. The Earthly phenomenon gets brighter and brighter the closer the Sun is to the horizon.
Having seen the overexposed Surveyor 3 photos of the Earth during eclipse (which make the Earth's extremely thin refracting atmosphere look many times thicker than it actually is), it is less than "compelling" to me that the present photos actually show material floating above the lunar surface. Also, since these photos were taken near Last Quarter, when ultraviolet light striking the Surveyor 7 landing site had had a week or more to discharge it from the last magnetotail passage, I'm not sure I entirely understand Stubbs' phenomenon that would ignite a dust storm as night falls. But if there are actually Surveyor photos showing a dark post-sunset horizon that suddenly becomes luminous as a cloud of migrating nighttime dust particles develops, that would be most interesting. Also, I believe that under Stubbs' hypothesis lunar sunrises would look different from sunsets, since the charge situation would be quite different then. I wonder if that's true?
-- Jim Mosher
(3) Thoughtful comments. It seems there is a lot to explore about these images and the levitating dust hypothesis or speculation. I wonder if the lunar equivalent of the scrub branches is the blocky lunar surface? I also, hope someone will comment on my wondering if there might be a huge charge buildup in permanently shadowed craters.