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Compared to Earth, the moon appears to be perfectly dry
and airless, but in fact it possesses an extremely sparse
atmosphere, barely thicker than a vacuum.
From late 2013 to early 2014, a NASA mission called LADEE
explored the lunar atmosphere and dust environment.
Now, LADEE's observations have led to a new discovery about the
moon's water.
During the initial exploration of the moon, and the analysis of
all the returned samples from the Apollo and the Luna
missions, we thought that the surface of the moon was dry.
But more recent missions, like Lunar Prospector, LCROSS, and
Lunar Reconnaissance Orbiter, have not only shown that the
surface of the moon has a global hydration, but there are
actually high concentrations of ice water in the permanently
shadowed regions of the lunar poles.
In the decades following the Apollo program, a series of
robotic explorers revealed tantalizing hints of water on
the moon, challenging the conclusion that the moon was
dry.
The first definitive discovery of water was made in 2008 by the
Indian mission Chandrayaan-1, which detected hydroxyl
molecules spread across the lunar surface and concentrated
at the poles.
The following year, NASA's LCROSS mission deliberately
impacted part of its launch vehicle into the southern crater
Cabeus, ejecting a plume that contained water ice.
These discoveries showed that the moon harbors water, and that
the highest concentrations occur within darkened craters at the
poles.
But questions remained about the abundance of water at the moon's
mid latitudes.
Now, data from LADEE are beginning provide answers.
What we discovered is that the surface releases its water when
the moon is bombarded by micrometeoroids.
This is especially noticeable during meteor showers.
What we also found is that the surface that's releasing the
water is being protected by a layer, a few centimeters of dry
soil that can only be breached by large micrometeoroids.
When micrometeoroids impact the surface of the moon, most of the
material in the crater is vaporized.
There is also a shock wave that propagates outward.
That shock wave carries enough energy to release the water
that's coating the grains of the soil.
Most of that water will get released into space, and that's
the signature that LADEE detects with its instrument from its
orbit.
LADEE observed water being released from within the moon,
but the micrometeoroids impacting the moon's surface
have a more exotic origin: comets.
When Earth passes through the leftover debris trail of a
comet, small particles of rock and dust burn up in our
atmosphere, creating a meteor shower.
On the moon, these micrometeoroids impact the
surface, releasing water at the same time that meteor showers
are occuring on Earth.
By analyzing the data returned by the neutral mass
spectrometer, we found that the intensity and the frequencies of
the fluctuations of signals from the water to be perfectly
correlated with known meteor streams.
For example, we were able to detect a big spike of water
during the Geminid meteor shower that occurred in December of
2013.
Knowing how much water is available at the moon's mid
latitudes is important for future exploration.
LADEE's observations show that beneath three inches of dry
lunar soil is a wet layer ten feet deep.
But on the moon, "wet" is a relative term.
The concentration of water in the wet layer is about 200-500
part per million per weight.
So, to fill an eight-ounce bottle with lunar water, you
would need to squeeze water out of about one to two thousand
pounds of lunar soil.
So, while the wet surface is wet, it's drier than you think.
The search for water on the moon spans five decades, and is of
great interest to future exploration.
Thanks to LADEE, we now know that trace amounts of water are
widely distributed across the lunar surface, improving our
understanding the moon's geologic past and its continued
evolution.
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