The $75 million Pentagon spacecraft, Clementine, designed to track missiles
and test "Star Wars" sensors, was an unlikely candidate to prospect
for water on the moon. However, early in its four-month lunar run, Deputy
Project Manager Stewart Nozette talked the Ballistic Missile Defense Organization
into using its Star Wars technology to make a new reconnaissance of the Moon. He
proposed using the craft's transmitter (in an investigation known as the
Bistatic Radar Experiment) to beam radio waves into the dark regions
of the south pole of the Moon, into lighted regions of the south
pole, and elsewhere on the Moon for comparison. Echoes of these waves
were obtained from the large dish antennas of the Deep Space Network
on Earth. On a single pass in April 1994, while Clementine was directly
in line with Earth, the spacecraft swept over and area of the South Pole
about 200 kilometers across. As many had hoped, the radar echo for that
orbit, orbit 234, was altered in ways consistent with ice, rather than
the ground-up rock powder characteristic of the rest of the Moon's surface.
Additional data taken on orbits where the reflected radio spots were
not over these south polar regions and at the north pole do not show this
ice signature. Paul Spudis of the Lunar and Planetary Institute
proclaimed the finding to be "an amazing discovery."
II. Data Return
Although the interpretation of the data from Clementine's makeshift radar
experiment has been the source of much debate, one incontrovertible discovery
of the Clementine mission was simply that conditions that could support
lunar ice existed. The size and depth of the South Pole-Aitken basin,
an impact crater over 2 billion years old, was found to be over 2500 kilometers
(km) in diameter and up to 13 km deep, the largest and deepest crater
in the Solar System. Although this basin is centered at 50° south
latitude, it is so large that it encompasses the south pole. Many smaller
craters exist on the floor of this basin and are never exposed to sunlight.
The animated image you see here was compiled from images returned by
Clementine and shows a portion of the Aitken basin over the course of
a lunar day. It reveals only a small sampling of the large archipelago
of craters and valleys never touched by sunlight which total some 15,000
square kilometers. Within these 'cold traps' the temperatures would never
rise above about 100 degrees K (280 degrees below zero F). Any water ice
at the bottom of the crater could probably exist for billions of years
at these temperatures, and the verification of these findings offers the
prospect of deciphering two billion years of comet impact. The north pole,
in contrast, has much less area in continual shadow, totaling perhaps
several hundred square kilometers. This suggested that any ice would likely
be found in the South, however to many's surprise, Prospector found 50
to 100 percent more ice on the North Pole.
Reknown geologist and astronomer Eugene Shoemaker said the spacecraft
also has provided a wealth of data on the moon's composition useful in
determining its origin. He said information from Clementine argues that
the moon is not a piece of the primitive Earth as some scientists have
theorized. More likely, the moon is leftover material from a very large
body that hit the Earth in the past. "Something about the size of
Mars hit the Earth," he said, "and was left as debris in orbit
around our planet."
III. The Debate
With respect to ice however, many are still unconvinced Steven J. Ostro
of the Jet Propulsion Laboratory didn't find the single blip in the polarization
ratio from a single orbit particularly convincing, and radar veteran Gordon
Pettengill (MIT) worries that since the peak appears in just one pass
it could be merely a statistical fluke. Proponents, however, are encouraged
because the ratio rises to a maximum exactly where it should-over the
One independent check was made from Earth with the giant Arecibo
radio dish in Puerto Rico. Donald B. Campbell of Cornell who conducted
the study with Nicholas J. S. Stacy said Arecibo's radar system also found
high polarization ratios near the south pole in several craters. However,
he warns, some of these areas are in sunlight at least part of the time,
and he doubts that the observed polarization effects seen by Arecibo are
due to ice.
Despite two years of tedious analysis and critical appraisal, the Clementine
team still concedes that other interpretations (like a rough, ice-free
surface) are possible. "Even after Clementine, we don't know for
sure if ice is there," Spudis admitted in 1994, "You need to
follow that up with another mission to really confirm if it's there and
how much of it is there."
Enter Lunar Prospector.
"Lunar Prospector will use a complementary approach based upon a
different technique," said Prospector's Principal Investigator, Dr.
Alan Binder. "It will employ a neutron spectrometer to
measure the amount of hydrogen on the lunar surface. It will determine
hydrogen abundance and location to within 50 parts-per-million. From this,
we can map the distribution of hydrogen over the entire lunar surface.
This will permit us to directly infer the presence or absence of ice in
the critical polar regions," he explained.
Mission Manager, Scott Hubbard added, "The instruments on
Lunar Prospector are more sensitive and less ambiguous than those on the
earlier Clementine mission. Lunar Prospector will fly at an altitude of
100 km above the lunar surface, and may approach as close as 10 km after
its initial 12-month mission. This is four times closer than Clementine,
using more finely tuned, complementary instruments. We are very excited
about the prospects of finding definite confirmation of the existence
of ice in the shaded lunar polar regions."
With its suite of five instruments and its low-altitude orbit, Lunar
Prospector has already greatly extended the quality and quantity of data
on lunar surface composition collected during the two-month Clementine
orbiting mission. It also has provided data on lunar gravity and,
as the mission continues will add to our current understanding of the
moonÕs magnetic field characteristics. Moreover, while Pentagon officials
had to wait two years to analyze and interpret the data from Clementine,
has located water in a little over a month's time.