Moon crash reveals crater held water

Plume of lunar material contained roughly 25 gallons of vapor and ice

A small NASA spacecraft and its companion rocket did indeed strike water when they slammed into a permanently shadowed crater at the moon’s south pole, NASA announced November 13.

PLUME HAD WATER The plume (inset) generated by a Centaur rocket crashing into a crater at the moon’s south pole is seen about 20 seconds after impact in this visible-light image taken by LCROSS, the rocket’s mother craft. New analyses reveal that the plume contained vapor and ice. NASA
EVIDENCE FOR HYDROXYL This spectrum shows that ultraviolet emission (y axis) from hydroxyl, a molecular fragment of water vapor, dramatically increased just after the’ rocket struck Cabeus. The emission remained strong for several minutes after the impact (x axis is time). NASA

The LCROSS craft and its spent rocket were deliberately crashed into the Cabeus crater October 9 as a way to detect whether the crater contained water or signs of water. The rocket collision generated only a modest plume, barely visible by telescopes from Earth. Whether the plume was large enough to analyze for evidence of water was not immediately evident.

But analyses since then reveal that the impact kicked up at least 100 kilograms of water vapor and ice, or 25 gallons. LCROSS project scientist Anthony Colaprete of the NASA Ames Research Center in Mountain View, Calif., reported the finding during a briefing at Ames.

Data recorded by two spectrometers on LCROSS minutes before its own death plunge into the crater independently revealed the presence of water, Colaprete said, in analyses completed about two weeks ago. Notches in the spectrum of light seen by the craft’s infrared spectrometer revealed water molecules absorbing light within the cloud of material kicked up by the rocket. In addition, the visible-light/ultraviolet spectrometer on LCROSS showed the presence of hydroxyl, a molecular fragment of water vapor broken down by sunlight.

That each instrument recorded evidence of water “made us really confident” of the findings, Colaprete said.

It’s not known how much of the water was in vapor form and how much in ice, nor whether the water was interspersed with dirt on the crater floor or existed as icy chunks that might adhere to the soil, he said.

While water on the moon could be a vital resource for human explorers, it’s unclear whether the abundance in Cabeus is typical of other permanently shadowed craters, whose cold temperatures could attract and retain frozen water.

More than a decade ago, a lunar-orbiting craft called Lunar Prospector recorded an abundance of hydrogen at the moon’s poles, but researchers have debated whether or not the hydrogen was an indicator of water there, noted planetary scientist Greg Delory of the University of California, Berkeley. “LCROSS has now made the definitive discovery” that a lot of that hydrogen is in fact in the form of water, he said.

The findings, he added, have whet the appetites of researchers who now want to determine when the water arrived at the poles, and how. For example, it could have been delivered by comets or generated by chemical reactions with the charged particles that make up the solar wind. Answering these questions could reveal much not only about the history of the moon but also about the early days of the inner solar system. 

Temperature measurements of the crash site taken by the infrared spectrometer indicate the crater floor had a temperature of about -220 degrees Celsius. The crash forged a hole within Cabeus about 20 meters wide, Colaprete said.

The LCROSS spectrometer also revealed several other compounds, including the possible presence of carbon dioxide, methanol and sulfur dioxide. In general, the spectra suggest a composition similar to that of icy bodies like the Centaurs, cometlike objects that lie between the orbits of Jupiter and Neptune, Delory said.

Ground-based observations, still under analysis, were made more difficult because the high-density material from the crashes spread out laterally instead of rising above the crater’s rim. It was primarily lower density material that rose high enough above the crater rim to be seen from Earth, Colaprete said.

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