On a quest to locate water and other volatile minerals in the Moon’s soil, the LCROSS experiment—Lunar Crater Observation and Sensing Satellite—hurtled a spent Centaur rocket into a dark crater at the lunar South Pole last year. The crater, known as Cabeus, is one of the permanently shadowed regions of the Moon, and researchers believe it is also one of the coldest.
When the empty shell of the rocket struck the bottom of the crater, a plume of debris, dust, and vapor became visible to the “shepherding” LCROSS spacecraft trailing behind it. Now, data from that shepherding craft has allowed researchers to describe the impact event in detail and provide an estimate of the total concentration of water ice in Cabeus crater.
About 155 kilograms (342 pounds) of water vapor and water ice were blown out of the darkness of the crater and into the LCROSS field of view, according to Anthony Colaprete from the NASA Ames Research Center in Moffett Field, California, and colleagues from across the United States who analyzed data from the near-infrared and ultraviolet/visible spectrometers onboard the shepherding spacecraft. They estimate that approximately 5.6% of the total mass inside Cabeus crater (plus or minus 2.9%) could be attributed to water ice alone.
These results and more from the LCROSS experiment are reported with six separate reports in the 22 October issue of the journal Science, which is published by AAAS.
“These permanently shadowed regions of the Moon have not received direct sunlight for billions of years,” said David Paige, a researcher from the University of California-Los Angeles (UCLA), who also interpreted the LCROSS data and authored one of the Science reports. “We suspected that they may be cold enough to trap water ice, but the big question prior to LCROSS was: how much?”
Aside from water, this collaboration of researchers also report the detection of other volatile compounds in the plume of debris during the few seconds it was visible to the spacecraft, including a number of light hydrocarbons, sulfur-bearing species, and carbon dioxide.
“The range of volatile compounds observed during the LCROSS impact is the same we see in icy comets in the outer solar system,” Paige said.
In one Science report, Peter Schultz from Brown University in Providence, Rhode Island, and colleagues in California describe how they monitored the many stages of the impact and its resulting plume of debris. These researchers say the rocket impact created a crater about 25 to 30 meters wide, and that somewhere between 4,000 kilograms (8,818 pounds) and 6,000 kilograms (13,228 pounds) of debris, dust, and vapor was blown out of the dark crater and into the sunlit LCROSS field of view.
And when the empty LCROSS rocket slammed into the pitch-black bottom of Cabeus crater, the Lunar Reconnaissance Orbiter (LRO) was also in orbit around the Moon—and this spacecraft captured many more important details of the impact.
“LRO and LCROSS were launched together on the same rocket, but they took very different paths once they got to the Moon,” said G. Randall Gladstone from the Southwest Research Institute in San Antonio, Texas. “LRO went into a low-altitude, two-hour orbit around the Moon, while LCROSS went into a 37-day orbit around the Earth. They intersected with the Moon’s Cabeus crater on 9 October 2009.”
Gladstone and colleagues utilized an ultraviolet spectrograph onboard the LRO to visualize the debris, dust, and vapor created by the LCROSS impact and identify numerous elements and compounds in the plume, including molecular hydrogen, carbon monoxide, calcium, mercury, and magnesium. These observations support the idea that the dark, freezing-cold, permanently shadowed regions of the Moon can trap volatile compounds—delivered from deep space or other areas of the Moon—and preserve them for eons.
In another Science report, Paul Hayne from UCLA and colleagues describe how they measured the thermal signature of the LCROSS impact with the Diviner Lunar Radiometer onboard the LRO. Their observations provide insight into how energy is dissipated and matter is slowly cooled during such planetary impacts.
These researchers suggest that, during the LCROSS impact, a region of 30 to 200 square meters of the Cabeus crater’s floor was heated from approximately 40 degrees Kelvin to at least 950 degrees Kelvin—and that the residual heat was enough to turn approximately 300 kilograms (661 pounds) of ice directly into vapor in just four minutes after the impact, consistent with the LCROSS findings.
But, how did scientists choose the LCROSS impact site in the first place? Well before the empty rocket crashed into Cabeus crater, researchers were analyzing the Moon’s surface, searching for regions where volatile minerals might become trapped.
At that time, Paige and colleagues from across the United States used instruments onboard the LRO to map surface temperatures near the Moon’s South Pole and identify expansive areas that were theoretically cold enough to trap such volatile minerals. The researchers developed a thermal model of the lunar surface that accurately balanced the Moon’s topography with solar and infrared radiation to calculate the average amount of heat that those regions harbored. Their findings suggested that the floors of large impact craters that receive no direct sunlight are the coldest regions of the Moon—and they identified Cabeus crater as one of the coldest candidates.
In another of the Science reports, Igor Mitrofanov from the Institute for Space Research of the Russian Academy of Science in Moscow, Russia, and colleagues from both Russian and the United States describe how they used the Lunar Exploration Neutron Detector, or LEND, onboard the LRO to analyze the distribution of hydrogen near the southern lunar pole. Their findings confirm that the Cabeus crater contained a high concentration of hydrogen—and that it was indeed an ideal impact site for LCROSS.
“To me, the take-home message is that, yes, as has long been speculated, the Moon’s permanently shadowed regions are great cold traps and hold lots of volatiles—not just water, but many other interesting materials,” Gladstone said. “It seems likely that some of the species found will have important implications for future exploration or resource utilization planning. Scientifically, since these permanently shadowed regions are thought to have existed for a billion years or more, they likely hold an impressive record of solar system history.”
Read the abstracts for the related articles in Science:
Listen to Robert Frederick’s Science Podcast interview with Anthony Colaprete.
Read the abstracts for the related articles in Science.