COLD SPOT: A thermal map of the moon's south pole shows the LCROSS impact site. Purple regions are coldest, with temperatures ranging from roughly –250 to –215 degrees Celsius. Image: The journal Science & UCLA/JPL/GSFC/NASA
A spent rocket stage that NASA sent hurtling into the moon last year in hopes of kicking up water from a polar crater delivered on that mission, revealing that at least a moderate portion of its target was indeed made of ice. But the Lunar Crater Observation and Sensing Satellite (LCROSS) revealed much more than that—hinting at a rich mixture of chemical species in the crater, including carbon monoxide, mercury and possibly silver.
Far from a "mission to bomb the moon," LCROSS involved two complementary pieces of hardware—a spent Centaur rocket as an impactor that produced a towering debris plume, and a sophisticated shepherding spacecraft that trailed it to sniff out water in the plume before crashing into the moon as well four minutes later. Data collected by the sensor-laden spacecraft, as well as measurements from a lunar orbiter passing overhead, are the basis for a suite of research papers about the mission appearing in the October 22 issue of Science.
When the Centaur struck the lunar crater Cabeus, near the moon's south pole, on October 9, 2009, astronomers around the world tuned in to try to glimpse the impact flash, thanks in part to a public relations push from NASA that drew attention to the event and its viewing potential. But most telescopes trained on the moon saw little. "It was sort of considered a flop," says Randall Gladstone, a planetary scientist at the Southwest Research Institute in San Antonio and lead author of one of the new studies. "People were expecting an awful lot from such a small impact velocity."
The Centaur crashed into the moon at about 2.5 kilometers per second, roughly one quarter the speed of the impactor that struck Comet Tempel 1 in 2005 as part of NASA's Deep Impact mission. But LCROSS, Gladstone says, "wasn't a flop scientifically—we found out all kinds of great stuff." Mission scientists announced preliminary results from the experiment in November 2009, including the spectral signature of water in the debris plume tossed up by the impact. But after months of analysis the picture has become clearer, and a few surprising characteristics of the target crater have emerged.
"What we have now is we actually have concentrations," says Peter Schultz, a planetary geologist at Brown University who co-authored two of the Science papers. "Before, we had to do some hand-waving to do our estimates." Measurements from the shepherding spacecraft detected the presence of water in the infrared spectra of the plume. The researchers estimate that the shepherding spacecraft saw as much as 155 kilograms of water in its field of view; a series of measurements imply concentrations of roughly 5.6 percent water in the lunar soil that was churned up.
Scientists have speculated that ice could persist for long periods of time without sublimating to vapor in places such as Cabeus, because the rim of the crater, coupled with the low angle of the sun, keeps part of the crater floor in perpetual darkness. With temperatures that can range well below –200 degrees Celsius, permanently shadowed regions on the moon are among the most frigid places in the solar system and form cold traps that preserve all manner of chemical species that arrive there.
Other instruments provided additional information about the contents of Cabeus, including an ultraviolet spectrometer on board the LCROSS shepherding spacecraft and a spectrometer on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that launched along with LCROSS in 2009 and that passed over the impact site about 90 seconds after the Centaur crashed into Cabeus.
Among the chemical species detected in the plume by the trailing spacecraft are carbon dioxide and hydrogen sulfide as well as sodium and silver, although not all detections were conclusive. "We have a lot of different things stored," Schultz says. "Things migrate to the poles and get trapped in this deep, deep freeze." Some of those materials, he adds, were most plausibly delivered by comets impacting the moon. Diffused into the tenuous lunar atmosphere or liberated by the occasional impact, heavy atoms such as silver could bounce around the moon until they reached a cold trap. "Whatever we find at the poles probably built up atom by atom, molecule by molecule, from impacts all over the globe," he says.
As the trailing LCROSS spacecraft approached the lunar surface, documenting the debris cloud en route to its own impact just a few minutes later, LRO had its own spectrometer trained on the airspace over Cabeus. The Centaur impact blasted up material high enough for LRO to see from orbit, dozens of kilometers above the surface.
The plume spectra registered by LRO point to a vapor cloud containing hundreds of kilograms of carbon monoxide, molecular hydrogen and calcium. Most surprisingly, Gladstone says, was a significant spectral signature of mercury, probably more than 100 kilograms of the stuff. "With the mercury, we tried to find something else that would fit [the spectra], but it just does a great job," Gladstone says.
Initially the team was in disbelief, he says, until one of the researchers recalled an obscure 1999 paper in Meteoritics & Planetary Science about the possibility that lunar cold traps might harbor significant amounts of mercury as well as ice. The paper, authored by Argonne National Laboratory researcher George W. Reed, was cheekily entitled "Don't Drink the Water". According to what LRO saw in the debris plume over Cabeus, Reed may have been correct. "He did this back-of-the-envelope calculation to make this wild prediction—and it's dead-on, as far as we can tell," Gladstone says.
Although the new papers provide some answers about the lunar poles, the impact, so to speak, of LCROSS is still being felt. "I'm still working in this stuff," Schultz says. "I think this has raised a lot of questions." Among them: How do the materials trapped in polar craters arrive on the moon? How do they migrate to high latitudes? And can their relative concentrations be measured, as was done for water ice?
For the time being, at least one longstanding issue has been resolved. "The legacy of Apollo is: The moon is bone dry," Schultz says. "The legacy of LCROSS is: Not everywhere."