EUREKA! ICE FOUND AT LUNAR POLES
"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.
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.