The north and south poles of the Moon may contain up to six billion metric tons of water ice, a more than ten-fold increase over previous estimates, according to scientists working with data from NASA's Lunar Prospector mission.
Growing evidence now suggests that water ice deposits of relatively high concentration are trapped beneath the soil in the permanently shadowed craters of both lunar polar regions. The researchers believe that alternative explanations, such as concentrations of hydrogen from the solar wind, are unlikely.
Mission scientists also report the detection of strong, localized magnetic fields; delineation of new mass concentrations on the surface; and the mapping of the global distribution of major rock types, key resources and trace elements. In addition, there are strong suggestions that the Moon has a small, iron-rich core. The new findings are published in the Sept. 4 issue of Science magazine.
"The Apollo program gave us an excellent picture of the Moon's basic structure and its regional composition, along with some hints about its origin and evolution," said Dr. Carl Pilcher, science director for Solar System exploration in NASA's Office of Space Science, Washington, DC. "Lunar Prospector is now expanding that knowledge into a global perspective. The indications of water ice at the poles are tantalizing and likely to spark spirited debate among lunar scientists."
In March, mission scientists reported a water signal with a minimum abundance of one percent by weight of water ice in rocky lunar soil (regolith) corresponding to an estimated total of 300 million metric tons of ice at the Moon's poles. "We based those earlier, conscientiously conservative estimates on graphs of neutron spectrometer data, which showed distinctive dips over the lunar polar regions," said Dr. Alan Binder of the Lunar Research Institute, Gilroy, CA, the Lunar Prospector principal investigator. "This indicated significant hydrogen enrichment, a telltale signature of the presence of water ice.
"Subsequent analysis, combined with improved lunar models, shows conclusively that there is hydrogen at the Moon's poles," Binder said. "Though other explanations are possible, we interpret the data to mean that significant quantities of water ice are located in permanently shadowed craters in both lunar polar regions.
"The data do not tell us definitively the form of the water ice," Binder added. "However, 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 fact, the new analysis "indicates the presence of discrete, confined, near-pure water ice deposits buried beneath as much as 18 inches (40 centimeters) of dry regolith, with the water signature being 15 percent stronger at the Moon's north pole than at the south."
How much water do scientists believe they have found? "It is difficult to develop a numerical estimate," said Dr. William Feldman, co-investigator and spectrometer specialist at the Department of Energy's Los Alamos National Laboratory, NM. "However, we calculate that each polar region may contain as much as three billion metric tons of water ice."
Feldman noted he had cautioned that earlier estimates "could be off by a factor of ten," due to the inadequacy of existing lunar models. The new estimate is well within reason, he added, since it is still "one to two orders of magnitude less than the amount of water predicted as possibly delivered to, and retained on, the Moon by comets," according to earlier projections by Dr. Jim Arnold of the University of California at San Diego.
In other results, data from Lunar Prospector's gamma ray spectrometer have been used to develop the first global maps of the Moon's elemental composition. The maps delineate large compositional variations of thorium, potassium and iron over the lunar surface, providing insights into the Moon's crust as it was formed. The distribution of thorium and potassium on the Moon's near side supports the idea that some portion of materials rich in these trace elements was scattered over a large area as a result of ejection by asteroid and comet impacts.
While its magnetic field is relatively weak and not global in nature like those of most planets, the Moon does contain magnetized rocks on its upper surface, according to data from Lunar Prospector's magnetometer and electron reflectometer. The resultant strong, local magnetic fields create the two smallest known magnetospheres in the Solar System.
"The Moon was previously interpreted as just an unmagnetized body without a major effect on what is going on in the solar wind," explained Dr. Mario Acuna, a member of the team located at NASA's Goddard Space Flight Center, Greenbelt, MD. "We are discovering that there is nothing simple about the Moon as an obstacle to this continuous flow of electrically charged gas from the Sun."
These mini-magnetospheres are located diametrically opposite to large impact basins on the lunar surface, leading scientists to conclude that the magnetic regions formed as the result of these titanic impacts. One theory is that these impacts produced a cloud of electrically charged gas that expanded around the Moon in about five minutes, compressing and amplifying the pre-existing, primitive ambient magnetic field on the opposite side. This field was then "frozen" into the surface crust and retained as the Moon's then-molten core solidified and the global field vanished.
Using data from Prospector's doppler gravity experiment, scientists have developed the first precise gravity map of the entire lunar surface. In the process, they have discovered seven previously unknown mass concentrations, lava-filled craters on the lunar surface known to cause gravitational anomalies. Three are located on the Moon's near side and four on its far side. This new, high-quality information will help engineers determine the long-term, altitude-related behavior of lunar-orbiting spacecraft, and more accurately assess fuel needs for possible future Moon missions.
Finally, Lunar Prospector data suggests that the Moon has a small, iron-rich core approximately 186 miles (300 kilometers) in radius, which is toward the smaller end of the range predicted by most current theories. "This theory seems to best fit the available data and models, but it is not a unique fit," cautioned Binder. "We will be able to say much more about this when we get magnetic data related to core size later in the mission." Ultimately, a precise figure for the core size will help constrain models of how the Moon originally formed.
Lunar Prospector was launched on Jan. 6, 1998, aboard a Lockheed Martin Athena 2 solid-fuel rocket and entered lunar orbit on Jan. 11. After a one-year primary mission orbiting the Moon at a height of approximately 63 miles (100 kilometers), mission controllers plan to the lower the spacecraft's orbit substantially to obtain detailed measurements. The $63 million mission is managed by NASA's Ames Research Center, Moffett Field, CA.
Further information about Lunar Prospector, its science data return, and relevant charts and graphics can be found on the project website at:
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