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Leaping Lunar Dust
September 4, 2013
Quite a few things have been associated with the
Moon in the centuries before the
space age. The
romantic aspects of the Moon have root in its association with the
Greek goddess,
Selene (Σεληνη), who was known for having several lovers; and, for having fallen in love with
Endymion (Ενδυμιων), a
mortal man,
at first sight.
Werewolves,
fictional humans who transform into
wolf-like creatures, are also associated with the Moon, since their transformation occurs at
full moon.
Authors of
young adult novels have combined the romantic and werewolf aspects of the Moon into many popular titles, including the
Twilight novels. Lest we think this idea is too strange, we are reminded that the Moon is also associated with
lunacy ("Luna" is the
Latin word for Moon).
The genesis of Selene, from Hesiod's Theogony, ll.371-374 (Scan of author's copy of ref. 1.)[1)]
Since the
orbital period of the Moon (27.32 days) is slightly smaller than the length of an average
month, it's possible to have two full moons in the same month. This second full moon is called a
blue moon, whence we get the expression, "Once in a blue moon."
The surface features of the Moon, evident in the contrast between the darker
maria and other terrain, are quite visible to the unaided eye.
Humans, who try to make sense out of every observation, tend to see familiar images in unfamiliar objects. This
psychological phenomenon, called
pareidolia (from παρα = "false" plus ειδωλον = "image"), gives rise to the
Man in the Moon, an example of which can be seen in the opening credits of
The Honeymooners.
Men first walked on the Moon on July 21, 1969, as part of
NASA's Apollo 11 mission. The last human presence on the Moon was on December 14, 1972, during the
Apollo 17 mission, more than forty years ago. When planning the first manned exploration of the Moon,
scientists and
engineers weren't that worried about
Selenites or the
Cat-Women of the Moon. They were worried about something much more down to Earth (down to Moon?); namely, the
lunar soil.
Apollo 17 astronaut Buzz Aldrin's bootprint on the Moon.
This photograph was taken on July 20, 1969, and it demonstrates some of the mechanical properties of the lunar soil
(Via Wikimedia Commons.)
The
mechanical properties of lunar soil (formally, the upper layer of the lunar
regolith) were mostly unknown before
spacecraft landed on the lunar surface. It was always expected that lunar soil would be quite different from that on
Earth. The Moon is
dry, so the
cohesive forces offered by
water are absent, and the soil was expected to be very fine material. Lunar rocks, pummeled by
solar radiation for millions of years and subjected to extreme
thermal cycling, would have transformed to a very fine dust.
Without data, one might expect a
worst-case possibility of having a
lunar lander sink into the ground. One other strange possibility involves the
electrostatics of dust. A 1956
science fiction short story by
Hal Clement features lunar astronauts blinded by dust sticking to their faceplates.[2-3] As I wrote in a
previous article (Scientist Novelists, June 15, 2012), my novels are written in the same "hard" science fiction style as Clement's.
Astronauts found that lunar soil caused a myriad of problems. These are listed in a NASA report; viz.,[4]
• Vision obscuration.
• False instrument readings.
• Dust coating and contamination.
• Loss of traction.
• Clogging of mechanisms.
• Abrasion.
• Thermal management problems.
• Seal failures.
• Inhalation and irritation.
In the
Apollo program, the dust problem was significantly underestimated in ground tests.[4-6] After the Apollo missions, a 62 page report on the toxicity of lunar dust concluded that the only viable studies would need to be conducted
in situ; that is, on the Moon itself.[7]
The
anecdotal evidence of the lunar astronauts is considerable. Their
space suits were covered in dust, so they tracked it into the landing module, and it scratched
camera lenses and corroded seals.[6]. They reported that it smelled like burnt
gunpowder when it reacted with
oxygen in the lander.[5-6] Apollo 17 astronaut
Harrison Schmitt had a severe allergic reaction to the dust.[5-6] The dust compacts easily, like
talcum powder.[5]
Apollo 17 astronauts Eugene Cernan and Harrison Schmitt were able to roam the Moon in their Lunar Roving Vehicle (a.k.a., "moonbuggy"). This vehicle kicked up a lot of dust, so the accidental loss of a portion of one of its fenders was a big problem.
An improvised fender was made from four laminated maps attached with duct tape.
(NASA Image.)
The Moon doesn't have an
atmosphere with
winds to levitate surface dust, but there was still a background level of dust near the lunar surface in the absence of astronauts. The dust is
electrostatically levitated, and variations of this phenomenon are being modeled by the
DREAM Center (Dynamic Response of the Environment At the Moon) at the NASA Goddard Space Flight Center.[8-9]
It's easy to see how levitation of fine particles can occur.
Ultraviolet radiation and
X-rays from the
Sun will
ionize these particles; and, since they're similarly
charged, they will repel each other. Particles on the night side of the Moon will experience a similar ionization from
electrons in the
solar wind.
What's more interesting is what might happen in adjacent light/dark regions, such as those near shadowed
craters. According to calculations by NASA scientists, lunar dust can bounce back and forth over such shadowed regions from one lighted region to another.[8] Such leaping lunar dust would be most prominent during lunar dusk and dawn, when mountains and crater rims cast long shadows.[8]
Michael Collier of NASA's
Goddard Space Flight Center,
Greenbelt, Maryland, explains it this way.[8]
"The motion of an individual dust particle is like a pendulum or a swing... We predict dust can swarm like bees around a hive over partially shaded regions on the moon and other airless objects in the solar system, such as asteroids. [It] stays locally trapped, executing oscillations over a shaded region of 1 to 10 meters in size."
Lunar horizon glow, caused by dust particles above the Moon's surface, as observed by the Surveyor 7 lunar lander. This is early evidence of leaping lunar dust. (NASA Image.)
As the positively-charged dust rises from the sunlit regions, its
trajectory is altered by the negative charge of the shadowed regions. It jumps over the shadow to an adjacent sunlit region, and the process repeats.[8] Some of the dust will settle into darkened craters, and this process may explain the observation of dust ponds inside craters on the asteroid,
Eros.[8]
NASA's
Lunar Atmosphere and Dust Environment Explorer (LADEE), a space probe directed at the Moon, is
scheduled for launch this Friday, September 6, 2013.[9-10] As its name indicates, it's purpose is to study lunar dust, and one thing it will observe are the sites where astronauts saw "air" glows and rays at lunar sunrise and sunset.
References:
- Hugh G. Evelyn-White, "Hesiod, The Homeric Hymns and Homerica," The Macmillan Co. (New York, 1914), pp. 106-107 (via Amazon). English translation: "And Theia was subject in love to Hyperion and bare great Helius and clear Selene and Eos, who shines upon all that are on earth and upon the deathless gods, who live in the wide heaven."
- Hal Clement, "Dust Rag," Astounding Science Fiction, September, 1956 (via Amazon).
- Hal Clement's birth name was Harry Clement Stubbs. It's possible that he didn't think such an earthy name was appropriate for a science fiction author. I'm reminded of the classical pianist, Byron Janis, who was born Byron Yanks. Even then, Yanks was a shortened form of his traditional family name.
- James R. Gaier, "The Effects of Lunar Dust on EVA Systems During the Apollo Missions, NASA Report NASA/TM—2005-213610, March 2005 (PDF File).
- Jonathan O'Callaghan, "How hazardous was lunar dust for Apollo astronauts on the Moon?" Space Answers, January 25, 2013.
- Amit Asaravala, "What a Little Moon Dust Can Do," Wired, April 4, 2005.
- Dag Linnarsson, James Carpenter, Bice Fubini, Per Gerde, Lars L. Karlsson, David J. Loftus, G. Kim Prisk, Urs Staufer, Erin M. Tranfield and Wim van Westrenen, "Toxicity of lunar dust," ArXiv Preprint Server, June 27, 2012.
- Nancy Neal-Jones and Bill Steigerwald, "Leaping Lunar Dust," NASA, March 15, 2013.
- Leonard David, "Does the Moon Have Levitating Lunar Dust?" Scientific American, November 20, 2012.
- NASA LADEE Mission Page.
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