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The Case of the Missing Moon Water

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The Case of the Missing Moon Water

Lunar Prospector failed to kick up a visible dust cloud when it crashed into the Moon, but researchers are still sifting through their data for elusive signatures of water.

image credit: Myron E. WasiutaSeptember 3, 1999: On July 31, 1999, NASA's Lunar Prospector spacecraft dived into a permanently shadowed crater near the Moon's south pole. Scientists hoped that the crash might liberate up to 40 lb of water vapor along with a plume of dusty impact debris, proving once and for all that water exists on the moon. Hundreds of amateur and professional astronomers watched for signs of the impact using everything from home-built telescopes to the world's most powerful observatories. The chances of seeing a cloud of dust kicked up by the crash were slim and, true to expectations, no one saw or photographed clear evidence of a dust plume.

Above: Amateur astronomer Myron E. Wasiuta captured this CCD image of the lunar south pole on July 31, 1999 minutes after Lunar Prospector hit the moon. He used a 6 inch refractor telescope equipped with a Tiffen 87C Series 6 infra-red filter. A series of similar pictures by Wasiuta covering the entire impact window showed no indication of a visible debris plume.

Despite the widespread interest in seeing the ejecta from Prospector's crash, scientists say that detecting a visible plume of mainly lunar dust and spacecraft debris was not critical to the hunt for water. Instead, the most powerful observatories were searching for a nearly transparent haze of water vapor and photodissociated hydroxyl (OH) that might have drifted above the lunar limb for hours after the dusty ejecta from the crash fell back to the lunar surface. The vaporous haze wouldn't have been visible to the naked eye or even through the eyepiece of a large telescope. The only way to detect water or OH was by means of sensitive spectrometers tuned to the ultraviolet and infrared spectral lines expected from those molecules.

"There was no huge water signal or we would have seen that easily," says Dr. David Goldstein, who led the University of Texas team that proposed the south polar crash. "But we weren't expecting a huge signal. In the best case scenario, there would have been weak spectral lines from OH or water. Right now we don't know that water ice was not observed -- it might be in the data somewhere. We're still looking closely at spectra from the Hubble Space telescope, the McDonald Observatory, and Keck I."

Ironically, the absence of a visible plume just after the crash may be hopeful news.

"The only way we could have seen dust would be if it had risen above the bright lunar surface and we might have seen it against the black of the sky," continued Goldstein. "Since we didn't see a plume, it probably means we landed in the hole. On the other hand, if we had seen a dust plume it might have meant that the spacecraft hit the near rim or the far rim of the crater."

Hitting the crater's rim was a real possibility. The impact target site was chosen based on Arecibo Observatory and Goldstone radar maps of the lunar south pole. The topographical data were accurate to plus or minus 300 meters. Prospector's trajectory took it a relatively safe 900 meters above the rim, but there were uncertainties.

Click for animation.  Credit UT Austin and Analytical Graphics "The Cornell radar data were excellent and we have a lot of confidence in their topography," says Goldstein, "but there's always the unexpected.... For example, what if there were a narrow, high peak jutting above the crater's rim that the radar couldn't resolve? The spacecraft might have plowed into that before hitting the crater floor. We didn't have enough fuel to come in more vertical, so it was a tight maneuver."

Left: A frame from a 2MB MPG animation of the simulated Lunar Prospector crash. Credit: the UT Austin Lunar Prospector Impact Page

Nevertheless, mission controllers at NASA's Ames Research Center express confidence that the diminutive spacecraft hit its intended target precisely. The failure to reacquire a signal from the vehicle at the time it would have emerged from the dark side of the moon is proof that impact occurred, and all indications are that it crashed as planned.

Even if a water signal is never found in the data from Prospector's crash, it doesn't mean that water is absent. Lunar water in the form of hydrated minerals would be difficult to extract. Prospector's impact would have had enough energy to vaporize water ice, and possibly create a detectable vaporous cloud, but not enough energy to separate the water from minerals. Also, even if Prospector hit the crater as precisely as thought, ice deposits inside the crater could be patchy. The spacecraft might have simply struck a dry spot.

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The University of Texas team expects to announce its findings about hydroxyl and water in the next several weeks.

Whether or not water is eventually found on the moon, scientists agree that the July 31st impact was a fittingly creative finish to a low-cost Discovery Program mission that exceeded all expectations after more than 6,800 lunar orbits in 18 months. Lunar Prospector's data gathering resulted in a series of discoveries and new scientific tools, including:
  • Tentative evidence that water ice exists in shadowed craters near the moon's south and north poles,
  • The first precise gravity map of the entire lunar surface,
  • Confirmation of the presence of local magnetic fields that create the two smallest magnetospheres in the Solar System, and
  • The first global maps of the moon's elemental composition.

Please visit the Lunar Prospector project web site from NASA/Ames and the Lunar Prospector impact page from UT Austin for more information about Lunar Prospector. See also: for more science news and images.


Web Links

Farewell, Lunar Prospector -- Lunar Prospector crashes into the Moon, July 31, 1999, NASA Space Science News

A Stay of Execution for Lunar Prospector -- The spacecraft has survived the July 28 lunar eclipse and is on track for a scheduled July 31 impact with the Moon , July 28, 1999, NASA Science News

Lunar Prospector in Eclipse -- The July 28, 1999 partial lunar eclipse poses a last-minute threat to Lunar Prospector, July 26, 1999, NASA Science News







Lunar Prospector Home Page -- from NASA/Ames




The Nine Planets: the Moon -- from SEDS

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