The target of Lcross was Cabeus crater, about 60 miles wide near the south pole.

Dr. Colaprete said the Lcross spacecraft captured a flash of light as the upper stage hit the bottom of Cabeus and then captured a thermal image of the resulting crater, about 60 to 65 feet wide, close to what had been predicted.

What was missing was the plume of debris that was knocked out by the impact. “We saw a crater. We saw a flash,” Dr. Colaprete said. “Something had to happen in between.”

Ground-based telescopes that had been pointed to that crater at the bottom of the Moon also failed to spot the theatrics, at least at first glance of their images.

“As far as I can tell from our quick processing, we did not see any plume,” said William C. Keel, a professor of astronomy at the University of Alabama who was operating a telescope at the Kitt Peak National Observatory in Arizona.

If the expended rocket stage hit a rocky area or a slope, the debris may not have been tossed high enough to reach sunlight and thus not have been seen.

But Lcross’s spectrometers  instruments that break down light into wavelengths and detect subtle changes, perhaps from vapor or fine particles not visible to the eye  did observe changes before and after the impact. Dr. Colaprete said the spectrometer data could identify the water and other molecules. “When I saw actually the spectra, I was like, “We got something,’ ” he said.

The analysis will take at least a few days and maybe weeks.

“We’re going to take our time” Dr. Colaprete said, “and build up a case for water and the ejecta, if it’s there or the case against it if it’s not there.”