Ice at the Lunar South Pole
In early December 1996, the press announced that water had been found on the
Moon. For the purposes of clarification, as this is an issue directly
related the Aremis Project, water has not been proven to exist on
the moon. For many years, especially following the science data returned
by the Clementine probe, scientists have
hypothesised that ice may have frozen onto the areas of the Moon especially
in the sourth polar basin, which were in permanent shadow. Evidence has
been inconclusive, although encouraging. There are several points to make
with regards to this issue:
- There is no "lake" of water ice at the lunar south pole
Only a small portion of the umbra surrounding the lunar south pole is
estimated to be composed of ices. Scientists theorize that this ice
is found as small, individual crystals in the interstices of regolith
grains spread over 15,500 square kilometers. If these ice crystals
were gathered together in one region, they are estimated to have the
volume of a small lake on Earth (60-120 thousand cubic meters or four
football fields sixteen feet deep). Not an easily obtained resource
to say the least.
- The evidence is still pretty shaky. The instrument (see
next bullet point) and technique used to detect the ice is not a finely
calibrated feat of engineering (it took a year to tease the data out).
The ice could be primarily composed of a frozen substance other than
water, or it might also be sulfur given the lack of clear
identification provided by the observational technique. More
importantly, Clementine employed the technique in question four times
(twice over each lunar pole). Only one pass over the lunar south pole
provided evidence of ice; the other pass over the lunar south pole did
not and neither did the two north pole passes.
- Clementine did not use any of its advanced Strategic Defence
Initiative Office/Ballistic Missile Defense Office
technologies to make this particular discovery (except in a supporting
role for GNC or computing). The
"instrument" in question was
Clementine's own communications antenna which "probed" the polar
regions to a depth of roughly 50 feet. The reflected X-band
transmissions were received by the Deep Space Network. Researchers
were able to make the ice claim because dry rock bounces radio waves
back with a 180 degree change in polarization whereas radio waves that
hit ices or sulfur have their polarization changed by amounts
significantly smaller or greater than 180 degrees. (The
term "bi-static radar" is used to describe this technique, which refers to
the fact that the radio transmitter and receiver were physically removed
from each other by some distance.) The experiment
as a whole was an afterthought once other mission objectives were
achieved and was probably made possible by Clementine 's adaptable
architecture. There's an important lesson here about adaptability
versus using the latest, specialized technologies.
- It is unlikely that this Clementine "I" discovery will bode
well for Clementine II funding because Clementine I was an SDIO/BMDO
project whereas Clementine II is USAF's baby. A reporter asked this
question directly and this was the answer from Stu Nozette,
Clementine's Deputy Mission Director.
- Although most of the region surrounding the lunar south
pole is covered by shadow, a small region (maybe tens of square
kilometers in area) directly on top of the lunar south pole is not in
the umbra (the so-called Mt. Clementine). Presumably, this region
would be an ideal spot for collecting solar energy to mine and process
the lunar ice into hydrogen for shipping to other stations (such as Angus Bay) to be used for life
- The lunar ice, if it exists, is probably not indigenous to
the Moon since Apollo samples indicate that the Moon never outgassed
any volatiles like water. The origin of the theorized water would be comets
depositing volatiles on cold traps, such as the permanently shaded regions
of the south polar basin and lava tubes.
- It has often been reported in the media that a possible use for ice
would be electrolysing it into return propellant. However, the Artemis
Project has an official policy that rare and nonrenewable resources such as
hydrogen and other volatiles are too valuable to shoot out of a
rocket engine when there are other in-situ propellant combinations
In summary, the claim of ice is made on the assumption that some
radio waves from an antenna during one pass over the lunar south pole
came back with polarizations different from the 180 degrees typical of
dry rock. Although it's an indication of possible water ice on the
lunar south pole, the evidence is far from conclusive (we still don't
have spectra of the lunar south pole) and the water ice, if it exists,
is not waiting in a readily usable form. Confirmation will have to
wait until Lunar Prospector is launched.
Content by Brant Sponberg <email@example.com>
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