When astronauts return to the Moon
in the years ahead, they might encounter electrified fountains and
other strange things.
by Trudy E. Bell
It's astounding how
prophetic some science fiction has been.
Back in 1956, two years
before NASA was even created, Hal Clement wrote a short story called
"Dust Rag" published in Astounding Science Fiction,
about two astronauts descending into a crater on the Moon to investigate
a mysterious haze dimming stars near the lunar horizon. After discarding
a wild guess that they were seeing traces of a lunar atmosphere - "gases
don't behave that way" - they figured it had to be dust somehow
suspended above the ground. In a conversation remarkable for its
scientific prescience, one of the astronauts explains:
[Moon's] surface material is one of the lousiest imaginable electrical
conductors, so the dust normally on the surface picks up and keeps
a charge. And what, dear student, happens to particles carrying
like electrical charges?"
repelled from each other."
"Head of the
class. And if a hundred-kilometer circle with a rim a couple of
[kilometers] high is charged all over, what happens to the dust
lying on it?"
The answer, given only
by narrative description, is that electrostatic charging caused
the dust to levitate.
Well, guess what? Writer
Clement was righter than he knew. It appears lunar dust does levitate
above the Moon's surface because of electrostatic charging. And
the first evidence came almost the way Clement had described.
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.
the left are lunar "twilight rays" sketched
by Apollo 17 astronauts; on the right are terrestrial
crepuscular rays photographed by author Trudy E. Bell.
You can get some hands-on
experience with the fountain model ... on top of your head.
Rub an inflated balloon
on your hair, and then hold the balloon a few inches away. Your
hair will rise of its own accord to reach out toward the balloon.
Rubbing the balloon removes some of the electrons from your hair,
leaving your hair with a net positive charge. Your positively charged
hair is attracted to the negatively charged balloon.
Now watch what happens
when you hold the balloon far away. This is key: Your individual
hairs spread out from one another and do not immediately fall back
to lie flat on your head. What's happened? When the balloon was
removed, each positively charged hair repels its positively charged
neighbor and some of your hair remains suspended - just like dust
on the Moon.
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
Stubbs and his colleagues
are now hard at work on a host of fascinating questions. For example,
there are deep craters at the lunar poles that never see sunlight.
Would these craters have a strong surplus of negative charge? 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?
It's not science fiction