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
5that 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.
Follow the events and ideas that led up to this amazing
discovery in:
Ice on the Moon?
