A head-on collision by a NASA spacecraft last year has confirmed the presence of ice and other frozen compounds on the surface on the Moon, according to scientists.
The findings, reported in a series of papers appearing torday in the journal Science, come from the Lunar Crater Observation and Sensing Satellite (LCROSS) mission which collided with the Moon in October last year.
Both LCROSS and its Centaur rocket upper stage booster were deliberately crashed into the Cabeus crater near the Moon's South Pole on 9 October 2009.
The event was monitored by NASA's Lunar Reconnaissance Orbiter (LRO) which was launched along with LCROSS to map of the Moon's surface, as well as assist scientists in selecting the LCROSS impact site. Cabeus was chosen because its floor is permanently shadowed making it one of the coldest places on the Moon.
The Centaur rocket, which was first to impact, left a crater about 30 metres wide, and ejected up to 6000 kilograms of debris, dust and vapour.
Dr Anthony Colaprete and colleagues from NASA's Ames Research Centre using spectrometers on board LCROSS, estimate the Centaur impact blew up 155 kilograms of water vapour and water ice.
Colaprete says around 5% of that was thought to be water ice.
"Seeing mostly pure water ice grains in the plume means water ice was somehow delivered to the Moon in the past, or chemical processes have been causing ice to accumulate in large quantities," he says.
"Also, the diversity and abundance of certain materials called volatiles in the plume, suggest a variety of sources, like comets and asteroids, and an active water cycle within the lunar shadows."
Scientists also detected other volatile compounds in the debris plume including a number of light hydrocarbons, sulphur-bearing compounds and carbon dioxide.
By the time it was the turn of LCROSS to slam into the Cabeus crater, LRO was in position to capture details of that impact.
G Randall Gladstone and colleagues from the Southwest Research Institute in Texas used the ultraviolet spectrograph onboard the LRO to identify numerous elements and compounds in the plume, including molecular hydrogen, carbon monoxide, calcium, mercury and magnesium.
This supports the idea that dark, regions of the Moon can trap and preserve volatile compounds from other parts of the Moon or deep space.
"The detection of mercury in the soil was the biggest surprise, especially that it's in about the same abundance as the water detected by LCROSS," says Kurt Retherford, also of Southwest Research Institute. "Its toxicity could present a challenge for human exploration."
Calfornian Institute of Technology planetary scientist Paul Hayne and colleagues estimate the LRO impact heated up to 200 square metres of the Cabeus crater's floor from approximately 40 K (-233°C) to at least 950 K (670°C), sublimating some 300 kilograms water ice in just four minutes.