§ 2.2.3.5 Lunar Polar Volatiles
 In 1998, the Lunar Prospector (LP) probe discovered hydrogen at the lunar poles in large quantities and high concentrations, presumably in the form of water ice, located in craters that are permanently shadowed from the sun and extremely cold. Existence of carbon and other volatiles is unknown, as LP sensors are designed to detect hydrogen. Volatiles will be useful for rocket fuel propellant, industry, and life support. The challenge will be mining this extremely cold material, at -220 Celsius / -370 Fahrenheit / 50 Kelvin.
Origins of lunar ices
Craters exist at the lunar poles which are never sunlit in their interior. This is because the Moon's axis of rotation is nearly the same as its orbit around the sun -- 1.6 degrees -- so that the Moon doesn't have "seasons". The insides of these craters are extremely cold, at roughly -220 Centigrade (50 K), or -370 Fahrenheit.
Over the eons, comets and asteroids rich in volatiles such as water, hydrogen, carbon and nitrogen have bombarded the Moon. Each impact resulted in vaporization of these volatiles and a temporary, extremely thin gas around the Moon. Some of this gas would have impinged on these polar craters, which served as "cold traps". The amount of vapor captured per impact is small, but it has added up over the eons and countless comet and asteroid impacts.
History of discovery
The US Department of Defense's probe Clementine, funded by the Strategic Defense Initiative Organization (SDIO, aka "Star Wars") in conjunction with NASA, sent a probe into a lunar polar orbit in 1994 to map the Moon before heading off for an asteroid near Earth. Data from Clementine's radar indicated the existence of concentrations of water ice at the lunar south pole, though other interpretations were possible albeit less likely. (Mission cost was $75 million, with multiple other purposes as well.)
In response, NASA funded the Lunar Prospector spacecraft, based on a preliminary design due to SSI’s previous private sector support, and built by Lockheed Martin Astronautics in Denver, Colorado. Data from Lunar Prospector confirmed the existence of hydrogen and gave a vastly better measurement, using a neutron spectrometer. The hydrogen is inferred to be in the form of water ice. (Total mission cost: $63 million.)
Quantities and concentrations of lunar ices
Hydrogen exists in craters at both the north and south poles. Total ice at the north pole totals about 50% more than ice at the south pole. The sensor on Lunar Prospector can detect water up to a depth of 0.5 meters, so that's what the data reports on. It's possible that there's more water, since the lunar surface has been covered with layers of crater ejecta to an average depth of 2 meters, though the depth varies by location. Practically all the water ice is thought to come from comet and asteroid impacts.
There's little mention of hydrocarbons in the NASA analyses released to the public, as the equipment was not designed to measure carbon abundance or certain other volatiles.
The location of the ice has not yet been pinpointed. This is because the LP spacecraft's sensors have a wide field of view, or "footprint" -- roughly 150 kilometers by 175 kilometers. At any good viewing time, there are several permanently shadowed craters in the sensor's field of view, as illustrated in the first figure following. The second figure shows the footprint of the prospector against a real image of the lunar poles and their larger craters.
Source: here at NASA's Ames Research Center.
As of the time of this writing, in late 1998, the Lunar Prospector spacecraft is planning to lower its orbit to get closer to the target areas, thereby getting more spatially detailed information.
The initial estimates of water ice in March, 1998, were awesomely high. However, upon further collection and analysis of data, these estimates were dramatically increased by a factor of about 10 times by the time the next major report was published in September, 1998. It was initially thought that the hydrogen exists in the form of small crystals of water ice in concentrations of 0.3% to 1%, dispersed over a large surface area of 5,000 to 20,000 square kilometers at the south pole and 10,000 to 50,000 square kilometers at the north pole. However, as of the time of this writing in December 1998, the most recent data, analysis and predominating theory suggests a total of six billion metric tons of water are concentrated in a small number of lunar polar craters. As explained by Dr. Alan Binder, the Lunar Prospector principal investigator, "if the main source is cometary impacts, as most scientists believe, our expectation is that we have areas at both poles with layers of near-pure water ice" in the form of "discrete, confined, near-pure water ice deposits buried beneath as much as 18 inches (40 centimeters) of dry regolith", which is around the 50 centimeter maximum depth that Lunar Prospector can detect water.
Mining Challenges
The main challenge in recovering these volatiles is handling the extremely cold material, at -220 Celsius / -370 Fahrenheit / 50 Kelvin. We've never mined anything near that temperature. Thus, a valid question is "What's the scoop?"
To successfully scoop up material without breaking our machinery, we would probably warm the surface material to be mined, e.g., using microwaves or infra-red heaters in front of the mining equipment and scraping up the warmed surface. We would mine the material slowly, limited by the heat transfer properties of the material. The vehicle and any appendages would also need to be warmed.
Another option is to put a hood over an area and heat the material underneath, bringing in the water as vapor through pipes and into tanks.
On the plus side is the likely prospect that some places on the rims of polar craters may be permanently sunlit. If so, they could provide continuous solar power. This is one geographical issue that has yet to be resolved at the time of this writing.
The south pole has bigger, permanently shadowed depressions. The north pole has a larger number and more interspersed permanently shadowed areas, which may give more choices of places to mine, e.g., near a crater rim that is always sunlit to produce electrical power. However, the exact layout in detail has yet to be determined.
More information on the Lunar Prospector discovery of water ice can be found here at the NASA Ames Research Center.
|
Back to 
Forward to