$$\require{cancel}$$

# 5.30 Ice on the Moon

The Moon is a very dry place. Rocks and soil samples returned by the Apollo mission, admittedly collected from particularly arid areas, contain no water of hydration (water molecules trapped in the minerals), which is found in many minerals formed on the wet Earth. Without laboratory measurements of lunar rock, only secondary evidence could be sought. In 1996, radar equipment on a small spacecraft orbiting the Moon returned exactly the signals that would be expected from ice deposits. They emanated from crater floors at the Moon’s south pole, in an area near Aitken Basin. The water ice was purported to be mixed in with the regolith at low concentrations. The initial results were controversial, but they were supported in 1998 by a low-cost NASA spacecraft, the Lunar Prospector orbiter. This second mission inferred the presence of ice deposits near both lunar poles, based on the amounts of hydrogen gas detected. In 2009, the LCROSS mission dropped an empty Centaur rocket stage into a polar crater and detected water in the resulting plume of material.

Earth's Moon. Click here for original source URL.

The geography of the lunar pole makes ice and other volatiles possible. In the early 1960s, however, a few scientists pointed out that craters at the poles of the Moon could be in permanent shadow, and would therefore be extremely cold. These researchers speculated that if water molecules had been released near the Moon (for example, by impacts of ice-bearing comets over the eons), they could then be trapped in the cold pockets of these craters. Accumulations of ice would form, hidden in the permanent shadows. Because the Moon has no atmosphere, any exposed water ice would quickly sublime into gas and escape the weak gravity of the Moon. So for it to survive for long periods, the ice would have to be buried under the lunar regolith.

This is an artist's illustration of the Lunar Crater Observation and Sensing Satellite (LCROSS) Centaur rocket stage and shepherding spacecraft as they approach impact with the lunar south pole on October 9, 2009. Click here for original source URL

Imagine what it would be like to visit one of these craters at the south pole of the Moon. Unlike the Earth with its 23.5° axial tilt, the Moon's rotation axis is only tilted by about 7°. This means that if you stand at the south pole of Moon, the Sun is near the horizon all year long. (As seen from Earth's south pole, in contrast, the Sun stays above the horizon 24 hours a day throughout the autumn and winter, and then drops below the horizon for the entire spring and summer.) If you stood in a depressed crater floor at the Moons pole, the crater walls would always hide the Sun, and you would be in perpetual shadow.

It is now estimated that the soil in some lunar craters is several percent water. Unfortunately, the water is mixed in with the soil and will not be easily accessible to future astronauts. Nonetheless, this is exciting news, and tells us that the moon still has interesting problems for us to try and solve. NASA has tested portable processing plants that could extract about a liter of water from a metric ton of lunar soil (a volume of about one cubic meter). Other processes can extract oxygen from that same soil, to breathe or to liquefy and use as rocket fuel. A third process could melt the soil and turn it into slump blocks of building material. So while the Moon appears barren, there is enough there to support an inhabited base some day.

", the Lunar Prospector orbiter. This second mission inferred the presence of ice deposits near both lunar poles, based on the amounts of hydrogen gas detected. In 2009, the LCROSS mission dropped an empty centaur rocket stage into a polar crater and detected water in the resulting plume of material.

Lunar Prospector spacecraft. One of the most significant finds by Lunar Prospector was confirmation that there could be as much as 10 billion tons of subsurface frozen water near the Moon's polar region. The Lunar Prospector mission came to a creative and daring conclusion when on July 31, 1999 at 2:52:00.8 a.m. PDT Mission Control Ames directed the spacecraft to a crash landing into a deep crater near the Moon's South pole. The hope was that the impact might release trapped water vapor. However no visible debris plume was detected by numerous observatories monitoring the event. This lack of direct evidence has not diminished the hope or belief that subsurface frozen water does exist. Click here for original source URL

The geography of the lunar pole makes ice and other volatiles possible. In the early 1960s, however, a few scientists pointed out that craters at the poles of the Moon could be in permanent shadow, and would therefore be extremely cold. These researchers speculated that if water molecules had been released near the Moon (for example, by impacts of ice-bearing comets over the eons), they could then be trapped in the cold pockets of these craters. Accumulations of ice would form, hidden in the permanent shadows. Because the Moon has no atmosphere, any exposed water ice would quickly sublime into gas and escape the weak gravity of the Moon. So for it to survive for long periods, the ice would have to be buried under the lunar regolith.

Schematic of the Earth?Moon system (without a consistent scale). Note the differences in the Earth and Moon axial tilts. Click here for original source URL.

Imagine what it would be like to visit one of these craters at the south pole of the Moon. Unlike the Earth with its 23.5° axial tilt, the Moon’s rotation axis is only tilted by about 7°. This means that if you stand at the south pole of Moon, the Sun is near the horizon all year long. (As seen from Earth’s south pole, in contrast, the Sun stays above the horizon 24 hours a day throughout the autumn and winter, and then drops below the horizon for the entire spring and summer.) If you stood in a depressed crater floor at the Moon’s pole, the crater walls would always hide the Sun, and you would be in perpetual shadow.

It is now estimated that the soil in some lunar craters is several percent water. Unfortunately, the water is mixed in with the soil and will not be easily accessible to future astronauts. Nonetheless, this is exciting news," and tells us that the moon still has interesting problems for us to try and solve.