October 26, 2006

Scientists Find Limited Evidence for Lunar Ice

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Those who hope for smooth sailing for proposed human settlements on the moon may find the way a bit rockier than they expected. People living on a permanent lunar base would need an accessible source of water not just for drinking but also as a source of breathable oxygen and hydrogen for fuel. However, any ice on the lunar South Pole is probably in the form of tiny crystals mixed in with the dirt, rather than in a more easily collected form, according to a team of Cornell and Smithsonian astronomers led by Prof. Don Campbell, astronomy.

Scientists have debated the question of whether or not there is ice on the moon since the early 1990s, when radar observations of Mercury indicated that there was probably ice at its poles. The same techniques used with Mercury were used to observe the moon. Astronomers used to associate a phenomenon known as a high circular polarization ratio (CPR) with the presence of ice. Normally, when an electromagnetic wave reflects off a surface, it displays a polarization entirely opposite to that of the incoming wave; in much the same way as your reflection in a mirror moves its right arm when you move your left. The CPR is the amount of the returning wave that is polarized the same way as the original wave. A mirror would have a CPR of zero.

When astronomers bounced radio waves off Mercury, they found that a lot of the returning signal was polarized in the same way as the outgoing signal, meaning that these areas exhibited a high CPR. This unusual polarization had previously been noticed in radio echoes from the moons of Jupiter, where we know ice exists. In addition, high CPR values were found in parts of craters on Mercury that never saw direct sunlight, known as “cold traps” because of the extremely low temperatures possible there — down to almost -200° Celsius. Ice could remain in these extremely cold areas for eons without evaporating. This led astronomers to conclude that a high CPR value is correlated with ice.

This discovery gave a great push to the search for ice on the moon, where there are also areas of permanent shadow that could have held onto ice that has long since evaporated from sunnier regions. The Lunar Prospector Orbiter provided further supporting evidence in 1998, when it found a higher level of hydrogen at the lunar poles than elsewhere on the moon. High concentrations of hydrogen could indicate the presence of water.

“The problem with the moon,” said Prof. Jean-Luc Margot, astronomy, “is that we see nothing like [what was seen on Mercury].” Margot, Campbell and a team of scientists from the Smithsonian Institute sent circularly polarized radio waves at the moon from the National Science Foundation’s (NSF) Arecibo telescope in Puerto Rico. They collected the reflected waves seconds later with the NSF’s Robert C. Byrd Green Bank Telescope in West Virginia.

However, according to Campbell’s study, a high CPR value does not necessarily indicate the presence of ice. High CPR values were found in areas of the moon that are in direct sunlight for a large part of the lunar cycle and therefore are too warm to plausibly have ice. Campbell pointed out that high CPR values have also been observed for near-Earth asteroids, a mountain on Venus and for lava flows near Flagstaff, Ariz., where there is clearly no frozen water. In the case of the moon, the high CPR values can be explained by the fact that they came from young, fresh impact craters, and are probably due to the original wave bouncing between the loose rocks that abound in these craters, like a pinball in a pinball machine.

This doesn’t rule out the possibility of finding any ice on the moon at all, however. If ice were present as little grains mixed in with the lunar dirt, radio signals would not be able to detect it. It’s also possible that thick deposits of ice exist deep under the surface of the moon, buried by at least a meter of soil. This would make collecting the ice for human consumption much more complicated, however.

Campbell’s findings can be found in the Oct. 19 issue of the journal Nature.