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Lunar ice is water ice that is hypothesised to exist on the surface of the Moon, delivered over geological timescales by the regular bombardment of the Moon by comets, asteroids and meteoroids. Energy from sunlight usually splits this water into its constituent elements hydrogen and oxygen, which generally escape to space. Attesting to the dryness of lunar rocks, the samples collected by Apollo astronauts near the equator have been found to contain no trace of water.
However, because of the only very slight axial tilt of the Moon's spin axis to the ecliptic plane (only 1.5°), some deep craters near the poles never receive any light from the Sun, and are permanently shadowed (see, for example, Shackleton crater). The temperature in these regions never rises above about 100 K (about −170 degrees Celsius), and any water that eventually ended up in these craters could remain frozen and stable for extremely long periods of time — probably billions of years. The quantities (if any) and concentrations of this water ice are at present unknown, but it has been suggested that, at the south pole at least, any lunar ice is more likely to exist as small grains widely dispersed in the regolith rather than as thick deposits.
The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water (or hydrogen and oxygen) from Earth would be prohibitively expensive. Water ice could be mined to provide liquid water for drinking and plant propagation, and the water could also be split into hydrogen and oxygen by solar panel-equipped electric power stations or a nuclear generator, providing breathable oxygen as well as the components of rocket fuel.
Analysis of lunar ice, should it be found, will also provide scientific information about the the impact history of the Moon and the abundance of comets and asteroids in the early inner solar system.
The Neutron Spectrometer looks for so-called "slow" (or
thermal) and "intermediate" (or epithermal) 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 show a distinctive 4.6 percent signature over the
north polar region and a 3.0 percent signature over the south, a strong
indication that water is present in both these areas. The instrument
can detect water to a depth of about half a meter.
The Moon's surface is continuously bombarded by meteorites and micrometeorites. Many, if not most, of these impactors contain water ice, and the lunar craters show that many of these were very large objects. Any ice which survived impact would be scattered over the lunar surface. Most would be quickly vaporized by sunlight and lost to space, but some would end up inside the permanently shadowed craters, either by directly entering the crater or migrating over the surface as randomly moving individual molecules which would reach the craters and freeze there. Once inside the crater, the ice would be relatively stable, so over time the ice would collect in these "cold traps", and be buried to some extent by meteoritic gardening. Such a possibility was suggested as early as 1961 (3). However, loss of ice due to photodissociation, solar wind sputtering, and micrometeoroid gardening is not well quantified
Recent radar maps of the Moon's southern pole revealed a dramatic,
jagged landscape that astronauts could someday call home. But unfortunately,
these radar images didn't provide any new information about something
that would make living at the lunar pole much easier: frozen water.
New evidence on whether water ice exists at the Moon's poles will
have to wait for a robotic probe called Lunar Reconnaissance Orbiter
or "LRO." Currently, engineers at NASA's Goddard Space Flight
Center are receiving the individual, hand-delivered scientific instruments
and integrating them into the satellite, which is scheduled to launch
by the end of the year.
Temperatures across the lunar surface are extreme. When shined by the light coming from the Sun, the surface can experience temperatures more than 100 degrees Celsius, while locations in constant darkness such as the beds of craters can cool down to temperatures of -240 degrees Celsius, cold enough to keep water in a frozen state.
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.
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.
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.
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."
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.
Expert response from Jennifer Heldmann:
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