Moon's river

This little instrument reads the energy of incoming neutrons -- giving hints about what the neutron has collided with on the moon.Courtesy NASA.

  It's not the frigidity -- it's the humidity
Let's say we start explaining the discovery of ice on the moon by looking at the neutron spectrometer. After all, this is the instrument that sent those intriguing signals in the first place. This instrument analyzes neutrons -- the uncharged particles, neutron spectrometerthat, with positively charged protons, comprise atomic nuclei. The spectrometer measures neutrons that are formed when the moon is bombarded by cosmic rays -- energetic gamma rays from deep space (see "Cosmic Bullets" in the bibliography).

When a ray strikes moon dirt, it kicks loose high-energy neutrons. If these neutrons collide with other atoms in the soil, they may be bounced into space. A few of those will enter Prospector's neutron spectrometer, which measures their energy.

Each time a neutron bounces against something, it loses some energy. Thus, its energy -- or spectrum -- depends on its history of bounces. (This sounds complicated, but it's simple. Bounce a basketball against a concrete driveway, and then against a pan of lime Jell-O. Doesn't the nature of the target affect the ball's energy after the bounce?)

How does the neutron spectrometer read the moon's surface? First cosmic rays from outer space hit the moon's surface causing neutrons to collide with other atoms. Moist areas emit cool, slower neutrons while dry areas emit hot, fast neutrons.   howitworks
When Prospector's neutron spectrometer captures these bounced-and-flung-into-space neutrons, it reads their spectrum and determines what kinds of atoms they hit while in the lunar soil. Again, this goes better with Jell-O: The spectrometer works something like the nerve cells inside your eye. They're designed to read the spectrum of incoming light so you can distinguish lime green Jello from banana yellow Jell-O.

"I see hydrogen!"
The signals Prospector has been reading in lunar orbit signify hydrogen, a Shaker-simple atom built of just one proton and one electron. The obvious explanation for hydrogen is water -- or ice -- in the moon's soil.

Hydrogen shouldn't exist on the moon. None was detected in samples from the Apollo mission. Gravity holds it weakly, and individual hydrogen atoms or molecules would likely be lost to the vacuum of space. That's true even at the frigid poles where cold hydrogen has little of the thermal energy that could bounce it into space. Hydrogen is more likely to stick around when bonded with other atoms into a chemical like water, methane or ammonia. But methane and ammonia would be gases in those cold lunar shadows, and thus would probably evaporate and disappear.

That leaves water. Feldman, who designed the neutron spectrometer, says, "There is water on the moon -- I'm certain of that. It's a question of how much."

Others are less convinced. The sun, for example, emits a stream of hydrogen atoms called the solar wind that could get implanted in the lunar soil. Thus, James Garvin of NASA Goddard Space Flight Center says the "very preliminary" data from Lunar Prospector leave "room for doubt" about water on the moon.

Just getting started...
Garvin notes that the scientific process of looking at multiple hypotheses has just begun. "Yes, water is a simple molecule well suited for holding the hydrogen, but so are methane, ammonia, and hygroscopic [water-holding] minerals that could be left behind by the cometary collisions that made these craters. Thus, requiring the hydrogen to be in the form of frozen water in the uppermost 'soil' of the moon is premature, but nonetheless exciting."

His excitement does not concern cut-rate Lunar Scooter rocket fuel or Luniere Light bottled water. It also concerns pure science -- analyzing different forms of elements gives clues to the origin of that water. "If there is frozen water, then the recovery and laboratory analysis of its isotopic signatures would be a fantastic way to understand its origin," Garvin says. "It would be like having a fingerprint of the earlier history of the solar system."

More results from Prospector may help settle the question. In one year, after the el-cheapo, $65-million satellite completes its primary mission 100 kilometers above the moon, scientists plan to slow it down so it skims just 10 to 20 kilometers high. That should allow more accurate observations.

To really nail down the water issue, Garvin suggests sending another satellite with a passive microwave radiometer. Similar devices, which read a low-energy part of the electromagnetic spectrum, are used by Earth-observing satellites to measure soil moisture.

Let's think deeply. If there's water, it must have a source.


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