On March 5, l998, it was announced that data returned by the Lunar Prospector spacecraft indicated that water ice is present both the north and south lunar poles.
This confirms earlier reports in November l996 by the Clementine probe that ice was present at the south pole of the Moon.
The Lunar Prospector indicates that ice appears to be mixed in with the lunar regolith (surface rocks, soil, and dust) at low concentrations of 0.3 to 1 percent. The ice appears to be spread over 3,600 to 18,000 square miles of area near the north pole and 1,800 to 7,200 square miles around the south pole. It is estimated that the ice is distributed in a layer from 0.5 to 2 meters deep, giving an estimated total volume of ice of 11 to 1300 million tons although these models can be off considerably.
How Was The Ice Detected?
In those areas that are permanently shadowed pictures cannot be obtained. The Clementine spacecraft searched for ice using an investigation known as the
Bistatic Radar Experiment.
The Lunar Prospector, a NASA Discovery mission, was launched into lunar orbit in January l998. The Lunar Prospector carried equipment called the Neutron Spectrometer designed to detect minute amounts of water ice at a level of less than 0.01%. the Neutron Spectrometer can detect water to a depth of about half a meter. The instrument concentrated on areas near the lunar poles where it was thought water ice deposits might be found.
The Neutron spectrometer looks for so-called "slow neutrons" which result from collisions of normal "fast" neutrons with hydrogen atoms. A significant amount of hydrogen would indicate the existence of water.
The data from the first two months of orbit show a distinctive 3.4 percent signature over the north polar region and a 2.2 percent signature over the south pole. This is a strong indication that water is present in both these areas.
How Can Ice Survive on the Moon?
The Moon has no atmosphere, so any substance on the lunar surface is exposed directly to vacuum. Thus the Moon's low gravity cannot hold gas for any length of time. This means that ice will rapidly turn directly into water vapor and escape into space.
During the course of a lunar day, all regions of the Moon are exposed to sunlight. The temperature on the Moon in direct sunlight reaches about 395 degrees K or 250 degrees above zero F. Therefore, any ice exposed to sunlight for even a short time would be lost. The only possible way for ice to exist on the Moon would be in a permanently shadowed area.
The Clementine imaging experiment showed that such permanently shadowed areas do exist in the bottom of deep craters near the Moon's south pole. It appears that approximately 2300 to 5800 square miles of area around the south pole is permanently shadowed. The permanently shadowed area near the north pole appears on Clementine images to be considerably less, but the Lunar Prospector results show much larger water-bearing area at the north pole.
Much of the area around the south pole is within the South-Pole-Aitken Basin, a giant impact crater 1550 miles in diameter and 12 km deep at its lowest point. Many smaller craters also exist on the floor of this basin. Since they are down in this basin, the floors of many of these craters are never exposed to sunlight. Within these craters the temperatures would never rise above 280 degrees below zero F. Due to these temperatures any water ice at the bottom of the crater could probably exist for billions of years.
Where Did The Ice Come From?
The Moon's surface is continuously bombarded by meteorites and micro-meteorites and as indicated by the size of the craters any of these were very large objects. Many contain water-ice. Any ice which survived impact is scattered over the lunar surface. Most of the ice is quickly vaporized by sunlight and lost to space, but some ends up inside the permanently shadowed craters and remains frozen there.
Why Is Ice On the Moon Important?
The ice could represent relatively pristine asteroid material may have existed on the Moon for millions or billions of years. The simple fact that the ice is there will help scientists construct models of impacts on the lunar surface and the effects of meteorite gardening, photodissociation, and solar wind sputtering on the Moon.
To scientists these finds are intriguing. However, deposits of ice on the Moon could have many practical aspects for future manned lunar exploration. There is no other source of water on the Moon, and shipping water to the moon for use by humans would be extremely expensive possibly $2,000 to $20,000 per kg.
The lunar water could also serve as a source of oxygen, another vital material not readily found on the Moon. Lunar water might also serve as a source of hydrogen which could be used as rocket fuel. Paul Spudis, one of the scientists who took part in the Clementine study, referred to the lunar ice deposits as possibly "the most valuable piece of real estate in the solar system." (1)
What We Know About the Moon
Exploration of the Moon continues to provide us with knowledge about our universe both past and present. In addition, it supports the development of new technologies and scientific discoveries. Although manned flights to the Moon have been discontinued, NASA continues to explore the lunar surface such as the Clementine and Lunar Prospector spacecraft which mapped the Moon extensively in l994.
Clementine Mission
The Clementine mission suggested that small, frozen pockets of water ice (remnants of water-rich comet impacts) may be embedded unmelted in the permanently shadowed regions of the lunar crust. Although the pockets are thought to be small, the overall amount of water may be quite significant--one billion cubic meters, or an amount the size of Lake Erie.
The Lunar Prospector
Unmanned and manned missions have changed our knowledge of the Moon. Lunar Prospector used its Neutron Spectrometer and Gamma Ray Spectrometer to determine the bulk elemental composition of the Moon as well as to identify potential lunar resources, including water ice (in the permanently shadowed poles.)
Primary Elements
The lunar crust is composed of a variety of primary elements including: uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminum and hydrogen. When bombarded by cosmic rays, each element bounces back into space its own radiation, in the form of gamma rays.
Some elements, such as uranium, thorium and potassium, are radioactive and emit gamma rays on their own However, regardless of what causes them, gamma rays for each element are all different from one another---each produce a unique spectral "signature" detectable by an instrument called a spectrometer.
Lunar Crystal Magnetism
Using data obtained from both its Magnetometer and Electron reflectometer instruments, Lunar Prospector will correlate magnetic anomalies with lunar surface geology. By mapping global locations, strengths and orientations of lunar crystal magnetic fields, scientists can learn more about the relationship between such magnetic fields and the surface selenology.
Magnetic measurements can also supply information about the size and electrical conductivity of the lunar core---evidence that will help scientists better understand the Moon's origins.
Crystal Structure
The crust ranges from 38 miles on the near side to 63 miles on the far side. Blanketed atop the Moon's crust is a dusty outer rock layer called regolith. Both the crust and regolith are unevenly distributed over the entire Moon. The regolith varies from 10 to 16 feet in the maria to 33 to 66 feet in the highlands.
Scientists think that such asymmetry of the lunar crust most likely accounts for the Moon's off-set center of mass. Crystal asymmetry may also explain difference in lunar terrain, such as the dominance of smooth rock (maria) and the near side of the Moon.