Moon Water Still a Mystery

The crash of Lunar Prospector into the Moon did not liberate detectable signatures of water ice.

Oct 13, 1999: The controlled crash of NASA's Lunar Prospector spacecraft into a crater near the south pole of the Moon on July 31 produced no observable signature of water, according to scientists digging through data from Earth- based observatories and spacecraft such as the Hubble Space Telescope.

This lack of physical evidence leaves open the question of whether ancient cometary impacts delivered ice that remains buried in permanently shadowed regions of the Moon, as suggested by the large amounts of hydrogen measured indirectly from lunar orbit by Lunar Prospector during its main mapping mission.

Above: The estimated trajectory of Lunar Prospector over local topography near the moon's south pole. The blue lines indicate uncertainties in the path followed by the spacecraft. Estimated errors in the radar-derived topography (by Margot et al, 1999) was 300m. [more information]

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Research group leaders from the University of Texas at Austin announced their results today at the annual meeting of the American Astronomical Society's Division for Planetary Sciences meeting in Padua, Italy.

In a low-budget attempt to wring one last bit of scientific productivity from the low-cost Lunar Prospector mission, NASA worked with engineers and astronomers at the University of Texas to precisely crash the barrel-shaped spacecraft into a specific shadowed crater. NASA accepted the team's proposal based on successful scientific peer review of the idea and the pending end of the spacecraft's useful life, although the chances of positive detection of water were judged to be less than 10 percent.

Worldwide observations of the crash were focused primarily on using sensitive spectrometers tuned to look for the ultraviolet emission lines expected from the hydroxyl (OH) molecules that should be a by-product of any icy rock and dust kicked up by the impact of the 354- pound spacecraft.

"There are several possible explanations why we did not detect any water signature, and none of them can really be discounted at this time," said Dr. Ed Barker, assistant director of the university's McDonald Observatory at UT Austin, who coordinated the observing campaign. These explanations include:

• the spacecraft might have missed the target area;
• the spacecraft might have hit a rock or dry soil at the target site;
• water molecules may have been firmly bound in rocks as hydrated mineral as opposed to existing as free ice crystals, and the crash lacked enough energy to separate water from hydrated minerals;
• no water exists in the crater and the hydrogen detected by the Lunar Prospector spacecraft earlier is simply pure hydrogen;
• studies of the impact's physical outcome were inadequate;
• the parameters used to model the plume that resulted from the impact were inappropriate;
• the telescopes used to observe the crash, which have a very small field of view, may not have been pointed correctly;
• water and other materials may not have risen above the crater wall or otherwise were directed away from the telescopes' view.

Although the crash did not confirm the existence of water ice on the Moon, "this high-risk, potentially high- payoff experiment did produce several benefits," said Dr. David Goldstein, the aerospace engineer who led the UT Austin team. "We now have experience building a remarkably complex, coordinated observing program with astronomers across the world, we established useful upper limits on the properties of the Moon's natural atmosphere, and we tested a possible means of true 'lunar prospecting' using direct impacts."

Above: Artist's concept of ice in a crater on the Moon. Is water-ice really there? The Lunar Prospector impact experiment did not result in the detection of water, and it leaves the question open. Image credit NASA/Ames.

Lunar Prospector was launched on Jan. 6, 1998, from Cape Canaveral Air Station, FL, aboard an Athena 2 rocket. In March 1998, mission scientists announced their first tentative findings of the presence of water ice in shadowed craters near the Moon's south and north poles. They estimated later that up to six billion metric tons of water ice might be buried in these craters under about 18 inches of soil, in more concentrated deposits than originally thought. However, the evidence was indirect, they cautioned, based on reasonable scientific assumptions given the levels of hydrogen detected, from which water ice is inferred.

Since then, Prospector data have also been used to develop the first precise gravity map of the entire lunar surface. While the Moon's magnetic field is relatively weak, Prospector has confirmed the presence of local magnetic fields that create the two smallest magnetospheres in the Solar System. Another scientific landmark is the assembly of the first global maps of the Moon's elemental composition.

The $63 million Lunar Prospector mission was led by Dr. Alan Binder of the Lunar Research Institute, Tucson, AZ, and managed by NASA's Ames Research Center in Moffett Field, CA. It was built by Lockheed Martin Missiles & Space, Sunnyvale, CA. Other participating organizations included the Department of Energy's Los Alamos National Laboratory, NM, and NASA's Goddard Space Flight Center, Greenbelt, MD, and Jet Propulsion Laboratory, Pasadena, CA.

Left: During its 18-month mission orbiting the Moon, Lunar Prospector used a method called neutron spectroscopy to look for lunar water. 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) are a possible signature of water. Scientists hoped that by crashing the spacecraft into a shadowed crater they would be able to find direct evidence of water-ice. More information about lunar water. Credit NASA/Ames.

Lunar Prospector in Eclipse -- The July 28, 1999 partial lunar eclipse poses a last-minute threat to Lunar Prospector. , July 28, 1999, NASA Space Science News

Bracing for Impact -- Astronomers prepare to observe the crash of Lunar Prospector on July 31, 1999. Includes observing hints for amateurs, July 21, 1999, NASA Science News

Destined for a Watery Grave -- NASA scientists have decided to send Lunar Prospector crashing into the Moon's south pole in search of water, June 4, 1999, NASA Science News

Zeroing in on Lunar Ice -- Astronomers explore the Lunar Prospector crash site using radar, June 4, 1999, NASA Space Science News

Lunar Prospector set to make science "splash" -- NASA/Ames press release

NASA Press Release (3 September 1998) -- announcing enhanced estimate of quantity of water on the Moon

NASA Press Release (5 March 1998) -- announcing the detection of ice on the Moon

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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