Tikalon Blog by Dev Gualtieri

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 moonbuggy fender repaired with duct tape.

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.

Leaping Lunar Dust

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