NASA Science News
10.23.2009
http://science.nasa.gov/headlines/y2009/23oct_ladee.htm
Oct. 23, 2009: Right now, the Moon is a ghost town. Nothing stirs.
Here and there, an abandoned Apollo rover - or the dusty base of a lunar
lander - linger as silent testimony to past human activity. But these
days, only occasional asteroid impacts disrupt the decades-long spell of
profound stillness.
And this stillness presents scientists with an important opportunity.
Currently, the Moon's tenuous atmosphere is relatively undisturbed. But
that won't be true for long. NASA is planning to return people to the
Moon, and human activity will kick up dust, expel rocket exhaust, and
release other gaseous emissions into the lunar atmosphere. Because the
atmosphere is so thin, these disturbances could quickly swamp its
natural composition.
If scientists are ever to know the lunar atmosphere in a relatively
natural state, now is the time to look. So researchers are building a
probe called the Lunar Atmosphere and Dust Environment Explorer (LADEE)
that will orbit the Moon and measure its wispy atmosphere better than
ever before.
"It's important that we understand it in its pristine state before
there's much perturbation," says Anthony Colaprete of NASA's Ames
Research Center in Moffett Field, California. "It's such a fragile
system. It's possible that it will be hard to study once humans are once
more living and working on the Moon."
*Thinner than thin*
Right about now, you might be thinking to yourself: "Hold on a second. I
thought the Moon doesn't have an atmosphere!" And you would be almost
correct. The Moon's "atmosphere" is so tenuous that it's technically
considered an exosphere, not an atmosphere.
"It's not anything like an atmosphere we would think of," Colaprete
says. For example, a cubic centimeter of Earth's atmosphere at sea level
contains about 100 billion /billion/ molecules. That same volume of the
Moon's exosphere contains only about 100 molecules.
In fact, that's so thin that molecules in the lunar exosphere almost
never collide with each other. Rather than constantly ricocheting off
each other to create a cohesive, swarming mass of molecules as happens
in Earth's atmosphere, molecules in the lunar exosphere fly unimpeded,
like microscopic cannon balls following curved, ballistic trajectories.
And the weirdness of the exosphere doesn't stop there. During the lunar
night, the Moon's exosphere mostly falls to the ground. (Just imagine if
our atmosphere fell to the ground at night!) When sunlight returns, the
solar wind kicks up new particles to replenish the exosphere.
Also, intense ultraviolet sunlight kicks electrons off particles in the
lunar soil, giving those particles an electric charge that can cause
them to levitate. Ambient electric fields lift these charged dust
particles as high as kilometers above the surface, forming an important
part of the exosphere.
Lunar astronauts will have to live and work in this bizarre environment,
so scientists want a better picture of the exosphere and its odd
behaviors. Levitating dust can get into equipment, spacesuits, and
computers, causing damage and shortening the hardware's useful life. In
fact, moondust wrecked havoc with the Apollo spacesuits, which were
nearly threadbare by the time they returned to Earth. Knowing how much
dust is floating around in the exosphere and how it behaves will help
engineers design next-generation lunar hardware.
After it launches in 2012, LADEE's spectrometers and dust detectors will
measure the concentrations of 18 different chemicals in the exosphere,
including methane and water vapor. These sensors will document how those
chemicals vary, both from place to place and over time.
Beyond the inherent scientific value of understanding the
chemical makeup of the Moon's exosphere, knowing how chemicals move
within the exosphere could help answer a question of keen interest to
future human habitants: How could the Moon have frozen reserves of water?
Evidence suggests that the Moon might harbor stores of ice in deep, dark
polar craters. On the lunar surface, fierce sunlight would quickly
sublimate any ice and the vapors would drift off into space. But a deep
dark crater, combining unimaginable cold with an absence of sunlight,
could provide a safe-haven for frozen water.
A popular idea is that icy comets brought water to the Moon in a series
of ancient impacts. But there's a problem: Even if a comet landed in one
of those dark polar craters by sheer luck, the heat of impact would
evaporate most of the ice. So how could significant amounts of ice
accumulate?
The Moon's exosphere could help.
Suppose a comet hits the Moon and leaves some H_2 O molecules on the
exposed surface. That water could survive by, essentially, leaping to
safety. Water molecules could "jump" across the lunar surface by
escaping into the exosphere and later be recaptured by the surface as
the exosphere breathes in and out. Individual water molecules could move
around in this way until they land in one of the dark polar craters,
where they would accumulate as solid ice.
Data from LADEE should show whether this "jumping" process works in a
way that could explain how cometary ice could have found its way into
those craters. "We can estimate the likelihood that the water on the
Moon is cometary in origin," Colaprete says.
So much information from such a trifling amount of atmosphere! Stay
tuned for results from LADEE.
Author: Patrick Barry | Editor: Dr. Tony Phillips
Credit: Science@NASA
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