Ice at the Lunar South Pole

Brant Sponberg

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 support.

• 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 available.

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.