ITHACA, N.Y. -- Detailed radar images of the north and south poles of the moon show no evidence of ice in areas of permanent shadow that are observable with earth-based radars, researchers said today (June 6).
The data appear to contradict the conclusions of scientists, based on data from the Clementine spacecraft and reported in the journal Science (29 November 1996), that there may be ice deposits at the South Pole of the moon.
"We don't see anything that suggests ice. We don't think there is any obvious evidence from the Arecibo radar images for the presence of water-ice at the poles of the moon," said Donald B. Campbell, Cornell University professor of astronomy and associate director of the National Astronomy and Ionosphere Center (NAIC), which operates the Arecibo Observatory under a cooperative agreement with the National Science Foundation (NSF).
The observations, taken at Arecibo Observatory in Puerto Rico in 1992 and released in the journal Science this week (6 June 1997), do indicate a number of small areas less than a half-square-mile in size throughout the polar regions whose radar reflection could be interpreted as ice. However, some of these features occur in areas near the poles that are sunlit and similar features are seen in radar images taken of clearly sunlit areas much closer to the moon's equator. It is much more likely that their radar reflection properties are due to very rough surfaces associated with the steep slopes of impact craters than with ice deposits, the researchers said.
The Arecibo work was done by Nicholas J. S. Stacy, a former Cornell graduate student now at the Defense Science and Technology Organization, Australia; Campbell and Peter G. Ford, a principal research scientist at the Center for Space Research at the Massachusetts Institute of Technology. The studies initially constituted Stacy's thesis for a Cornell doctoral degree, which he received in 1993. The researchers have presented their findings at several scientific conferences since then.
Researchers analyzing Clementine data suggested that ice deposits may cover an area totaling 90 to 135 square kilometers (35 to 50 square miles) at the south pole of the moon.
But, "We see no real evidence for anything like that," Campbell said, adding that the Arecibo data studied the moon polar regions at the same radio wavelength -- 13 centimeters -- as Clementine and at a similar viewing angle, but with a resolution of 125 meters, or 400 feet, much greater than the resolution used in the Clementine studies.
It is thought that areas of the moon in permanent shadow from the sun, at the north and south poles, would be cold enough to harbor frozen water over geologic time. There have been suggestions for more than 30 years that ice may be found at the lunar poles.
In 1991 radar observations of Mercury using the NASA/JPL Goldstone antenna and the Arecibo telescope found unusual radar reflections from the bottom of several impact craters in that planet's polar regions. These reflections are thought to indicate the presence of water-ice. The Mercury results provided a strong incentive to do high-resolution radar imaging of the lunar poles.
"We were hoping to see a similar situation for the moon, with unambiguous, bright spots at the poles," Campbell said. "With Mercury it's pretty clear. But with the moon we didn't see anything like that. Our contention is that the surface roughness is a much better candidate for the signatures we're seeing. However, neither Arecibo nor Clementine observed all the areas that are in permanent shadow and there is still the possibility that there are ice deposits in the bottoms of deep craters."
The studies were funded by the National Science Foundation (NSF) and NASA. Arecibo Observatory, which recently completed a $27 million upgrade that will be dedicated on June 14, is operated by Cornell University's National Astronomy and Ionosphere Center under a cooperative agreement with the NSF. It is the world's largest single dish radio telescope, with a 1,000-foot antenna and, when combined with a new one-megawatt transmitter, forms the world's most powerful radar.