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Ice on the Moon: Blazing a Trail: The Clementine Mission
I. Background The $75 million Pentagon spacecraft, Clementine, was brought from conception
to lift off in only two years. Designed to track missiles and test "Star Wars"
sensors, it 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 It was on a single pass in April 1994, while Clementine was directly in line with Earth, that the spacecraft's radio swept over and area 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 One of the major discoveries of the Clementine mission was simply
that conditions that could support 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. The animated image you see here was compiled
from images returned by Clementine and represents the course of one lunar day. It
reveals an Many smaller craters exist on the floor of this basin and are never exposed to sunlight. 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. 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 in the past. 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 and was left as debris in orbit around our planet." III. The Debate With respect to ice however, many are still unconvinced. A skeptical 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 are encouraged because the ratio rises to a maximum exactly where it should-over the polar darkness. One independent check was made from Earth with the giant Arecibo radio dish in Puerto Rico. Donald B. Campbell of Cornell conducted the study with Nicholas J. S. Stacy. He 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. "Clementine's evidence is indirect, provided by bistatic radar data that measures the reflectance of polarized radio waves," said principal investigator Alan Binder. "Lunar Prospector will use a complementary approach based upon a different technique. 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. "The instruments on Lunar Prospector are more sensitive and less ambiguous than those on the earlier Clementine mission," said Hubbard. "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 will greatly extend the quality and quantity of data on lunar surface composition collected during the two-month Clementine orbiting mission. It also will provide data on lunar gravity and magnetic field characteristics. Moreover, while Pentagon officials had to wait two years to analyze and interpret the data from Clementine, Prospector's scientists should know in a little over a month's time. ICE ON THE MOON |
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technology to to make a reconnaissance
of the Moon. He proposed using the craft's transmitter in an investigation, known
as the 
archipelago of craters in the dusty basin, some 15,000 square kilometers
of crater floors and valleys never touched by sunlight. The north pole, however,
has much less area in continual shadow, totaling perhaps several hundred square kilometers.
Although this basin is centered at 50° south latitude, it is so large that it
encompasses the south pole of the Moon.