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"Would I bet my house, the answer is 'yes.' We are certain that water is there. The uncertainty we have is how much." - Principal Investigator, Alan Binder

The Prospector Mission team announced in a press conference on March 5th that the tiny, low budget craft has found the answer to one of the most hotly debated questions in lunar science. Prospector HAS found somewhere between 10 to 300 million tons of water-ice scattered inside the craters of the lunar poles. Not only was ice found--as expected--in the Aitken Basin of the lunar South Pole, but also in the craters of the North. To many's surprise, Prospector detected nearly 50% more water ice in the North than in the South.

Photo Mosaic of lunar poles

This figure shows Lunar Prospector's "footprint," or mapping area, at both poles. This mapping area is roughly 150 kilometers by 175 kilometers. The footprint is round because Prospector is a spinning spacecraft and slightly elliptical because of the speed at which the rotating vehicle circles the Moon in its north-south polar orbit. In addition, the precise mapping area for each instrument varies slightly, due to size and distance factors peculiar to the positioning of each instrument.

Graphic of ice on moon

Prospector's neutron spectrometer can sense water ice (hydrogen) down to a depth of a half-meter (a couple of feet). However, according to previous estimates predicted by the scientific community, since the lunar soil has been effectively "gardened" to a depth of 2 meters by meteoritic impacts over the past 2 billion years. Thus, water could theoretically be present that deep (2 meters). What Lunar Prospector scientists can't yet determine is exactly how many craters at the north and south poles contain the 10 to 300 million tons of water ice measured by the neutron spectrometer. Further data analyses, as well as data from another of Prospector's instruments, the gamma ray spectrometer, will help mission scientists sort out the precise distribution of lunar ice. The most informative information is expected to be gleaned in just under a year, when the spacecraft begins its extended mission and dips down into a very low orbit of 10 kilometers above the lunar surface. This will enable the instruments to gather extremely high resolution data.

Neutron spectroscopy, the method which Lunar Prospector mission scientists are using to search for water ice on the Moon, hinges upon the detection of -- not surprisingly -- small particles of energy called neutrons which continually emanate from the lunar surface. Actually, there are three energy ranges for such neutrons which the neutron spectrometer can detect: low-energy "thermal" neutrons, medium-energy "epithermal" neutrons and high-energy "fast" neutrons.

water ice signature

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The key to finding evidence of water with this technique is how each neutron type interacts with wet lunar soil vs. dry lunar soil. Lunar soil containing water (and therefore an abundance of hydrogen ions) is much better at "moderating" (slowing down) epithermal and fast neutrons. Put another way, collisions between hydrogen ions and neutrons very much resembles ping-pong balls bumping into each other -- after the collision, each neutron loses energy and travels more slowly. In the graphic above, note the coincident dips in medium-energy neutrons at both lunar poles (see arrows). This is a definitive signature for water. Based on the extent of the dips, mission scientists estimate that the total amount of water on the Moon could be anywhere from 10 to 300 million metric tons (2.6 to 26 billion gallons).

At this early point in the mission and data analyses, this range could possibly be as much as an order of magnitude (factor of 10) off, because Lunar Prospector is the first interplanetary mission to use neutron spectroscopy to measure water, and thus there exist no precise models describing exactly how neutrons on the lunar surface actually behave. Further extensive analysis of Prospector's reams of data, with the help of newly crafted computer algorithms, with time will allow mission scientists to pinpoint the actual amount of water much more accurately. As put by Alan Binder, the mission's principal investigator, "The answer is in the data ... it's just a matter of finding out what it is."

water ice crystal in regolith

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While changes in the flux of medium-energy neutrons serve as a distinct signature for the presence of water, they say nothing about in what form that water (ice), is present in the lunar soil. Other types of neutrons, called "fast neutrons," indicate to scientists the actual concentration of water, or mixing ratio, in the lunar soil. Large dips in the neutron flux of high-energy "fast" neutrons are a telltale signature of water ice in the form of chunks of solid ice. Lunar Prospector's data, in contrast, does not reveal such signature dips in the high-energy neutron flux, meaning that water is instead present in the form of small crystals at a very low mixing ratio: ranging from 0.3% to 1%. From this data, mission scientists also can infer that the ice crystals must be dispersed over a large surface area: 5,000 to 20,000 square kilometers at the south pole and 10,000 to 50,000 square kilometers at the north pole.