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June 4, 1999: Now that NASA has decided to
fly Lunar
Prospector kamakazee-style into the Moon's south pole in search of water,
scientists are working to pinpoint the best crash site.
"In order to impact the spacecraft at the desired location, very accurate knowledge of the topography is needed," says Don Campbell, associate director of the National Astronomy and Ionosphere Center in Ithaca, NY. Thanks to Campbell and his collaborators at Cornell University and the Jet Propulsion Laboratory, the complicated job of selecting a crash site just got easier. The scientists used radio antennas at NASA's Deep Space Network facility in Goldstone, CA to construct detailed radar maps of the hard-to-see lunar poles. The south pole image, in particular, reveals a chaotic surface, with deep craters that are in permanent shadow from the sun and which are potential repositories for water ice.
The new images, obtained through a technique called radar interferometry, are published in the latest edition (June 4) of Science magazine. They are a leap forward in settling the "significant argument" about the existence of water ice on the moon, says Donald Campbell, professor of astronomy at Cornell, and one of the paper's authors. In the solar system, ice has a unique, and not totally understood, "signature" when probed by radar beams. This was first discovered by Campbell and others when they used the radar system of the Cornell-operated Arecibo Observatory in Puerto Rico to get radar echoes from the icy Galilean satellites of Jupiter.  Left: The
70-meter (230-foot) diameter antenna in Goldstone California
is part of NASA's Deep Space Network (DSN). It is the largest
of the DSN antennas, and is capable of tracking a spacecraft
travelling more than 16 billion kilometers (10 billion miles)
from Earth. Scientists used the Goldstone radar to map
locations for possible ice deposits at the lunar poles.
In 1996, researchers working with radar data from the orbiting lunar spacecraft Clementine reported indications of ice at the south pole of the moon. However, in 1997 researchers, Campbell among them, published a paper in Science reporting on Arecibo's radar imaging of the lunar poles that showed no evidence of ice. Both the Clementine and Arecibo radars would only have detected ice if it had been in the form of large chunks or slabs. The absence of an Arecibo radar detection did not preclude ice being present in small chunks or crystals mixed in with the lunar "soil." Last year, the neutron spectrometer aboard the Lunar Prospector orbiter, launched in January 1998, detected significant deposits of hydrogen at the moon's north and south poles. This was interpreted as indicating the presence of water ice, since hydrogen in water molecules is thought to be the most likely source of the element at the poles. However, without detailed topographic maps of the poles, it was not possible to identify potential ice-containing regions -- so-called cold traps, or areas where the sun never shines and the temperature hovers around 100 degrees Kelvin (minus 280 degrees Fahrenheit). 
Left: Lunar Prospector uses a method
called neutron spectroscopy to look for water on the
Moon. Neutrons are subatomic particles that are continually ejected
from
the lunar soil by cosmic rays.
In this graphic, the coincident dips in medium-energy neutrons at both lunar poles (see
arrows) 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 million to 6 billion metric tons (2.6 to 26 billion gallons).
More information.
Credit NASA/Ames.
With these first detailed altimetry data for the polar regions, the Cornell researchers have been able to map out the positions of most of these cold traps. Because the sun's limb rises less than two degrees above the horizon at the south pole, the floors of impact craters and other low areas can be in permanent shadow. In contrast, the radar beam from Goldstone reaches up 6 to 7 degrees above the horizon, allowing many polar features hidden from the sun to be imaged by radar. The floors of five large craters in the south polar region are hidden from the sun, the researchers say. These five crater floors constitute the largest potential deposits of water ice at the south pole and would be expected to display an excess of hydrogen if they contained ice, says Jean-Luc Margot, who carried out the observations for his doctoral research at Cornell. He is now a research associate at Arecibo Observatory. To obtain the topographic features of the hidden lunar polar regions, Margot, Campbell and Martin Slade and Raymond Jurgens of JPL used the Goldstone 70-meter antenna to transmit the radar signals. Two separate 34-meter antennas, 20 kilometers (12 miles) apart at the Goldstone site, received the echoes. By comparing the images from the two antennas, Margot derived a three-dimensional digital elevation model of the lunar poles, with measurements every 150 meters (500 feet) over the imaged area and a height accuracy of 50 meters (165 feet). To calculate which areas were in permanent shadow, Margot wrote a computer program that calculated whether each point in the three-dimensional image would be in shadow for any allowed position of the sun. "The program simulated light rays from the sun to each point on the map and tested to see if the ray was intercepted by the surrounding topography," Margot says. "If a single light ray was received, that point was in sunlight. If not, it was in shadow." This detailed topography, says Margot, also has applications in cratering and other, studies. "The data is of such fine resolution that we can find out much about crater shape and impact mechanics," he says. Lunar Prospector was the first of NASA's Discovery class of "faster, better, cheaper" space exploration missions. The $63 million mission is managed by NASA Ames Research Center, Moffett Field, CA. |
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The Deep Space Network -- information about the Goldstone radio telescope The Arecibo Observatory -- home page Lunar Prospector set to make science "splash" -- NASA/Ames press release Lunar Prospector Home Page -- from NASA/Ames Ice on the Moon -- informative article about lunar water -- where it is and how to find it. Lunar Prospects -- Astronomy Picture of the Day, Sep. 18, 1998 Impact Moon -- Astronomy Picture of the Day, Mar. 26, 1999 The Nine Planets: the Moon -- from SEDS |
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