Lunar Polar Water Ice and Volatiles in Shadowed Craters

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

Clementine 1

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.)

This website has a separate page on Clementine 1 in the probes section.

In response, NASA funded the Lunar Prospector spacecraft, based on a preliminary design by Dr. Alan Binder which had been promoted by others as a potential private sector project.

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.

Source: here at NASA's Ames Research Center.

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. This was later presented as too conservative. As more data came in, the estimates went up and the concentrations were better resolved, but there was still considerable debate about concentrations.

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.

This website has a separate page on Lunar Prospector in the probes section.

Lunar Crater Observations and Sensing Satellite (LCROSS)

In 2009, the Lunar Reconnaissance Orbiter (LRO) and the Lunar Crater Observation and Sensing Satellite (LCROSS) were launched together. The LCROSS mission had its upper stage tank impact a lunar crater, whereby the LCROSS spacecraft flew thru the plume to measure the volatiles within it, and send the data back to Earth before LCROSS would impact 6 minutes later. The mission was a success.

The result is that the crater had approximately 5.6% water, with an error margin of + / - 2.3%.

The plume was a mix of surface and fairly deep subsurface regolith.

This website has a separate page on the Lunar Crater Observations and Sensing Satellite (LCROSS) and Lunar Reconnaissance Orbiter (LRO) in the probes section.

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.

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