Strange Brew at LCROSS's Crash Site

LCROSS and its Centaur rocket prepare to crash into the Moon.


It would be fair to say that the crashy culmination of NASA's LCROSS mission on October 9th was a technical success but a public-relations fizzle.

On the plus side, the engineering team for LCROSS (short for Lunar Crater Observation and Sensing Satellite) delivered as promised, deftly driving a spent 2½-ton Centaur rocket into a target zone near the Moon's south pole only 2 miles (3½ km) across. Four minutes later, after flying through the debris cloud raised by the rocket's crash, an instrument-packed 600-kg "shepherding spacecraft" augered in not far away.

But the team's hope of finding abundant water buried in the permanently shadowed floor of Cabeus, the 61-mile-wide target crater, has yet to pan out. Water molecules have strong spectral signatures in the near-infrared, and even one part water ice in 200 parts lunar dust should have been easy to spot.

So far, the LCROSS team has been mum on what's been found by the shepherd craft's nine instruments, apart from a heavily processed composite image showing a faint puff where the Centaur crashed.


Extensive image processing of images taken by the LCROSS shepherding spacecraft 15 seconds after the Centaur rocket's demise reveals a dim debris plume (6 to 8 km across) in the shadowed part of Cabeus crater.

Tony Colaprete, LCROSS's chief scientist, says that the rocket's impact created a pit about 92 feet (28 meters) across, close to expectations. And the debris plume from the crash attained roughly the size and height expected, though he concedes that it was only about a tenth as massive as he'd hoped (nowhere near the 350 tons touted in some predictions).

We may never learn the reasons for the paltry particle production, though right now Brown University impact specialists Peter Schultz and Brendan Hermalyn are saying, "Told ya so!" Their modeling, based on small-scale hypervelocity collisions at NASA's Ames Research Center, suggest that a low yield should have been expected — both because the empty Centaur collapsed into itself as it hit and because the spray of debris went mostly "out" instead of "up."

It's also possible that the Centaur pancaked into the crater's floor. "It was definitely rotating or tumbling," notes observer Marc Buie, who tracked the rocket's final hours with the 2.4-m telescope at Magdalena Ridge Observatory in New Mexico.

All this speculation is intriguing — but "Where's the beef?" you might ask. Colaprete assures me that all the instruments in the shepherding spacecraft got great results, and that the delay in revealing the compositional analyses stems from having lots of spectral signatures to sort through and categorize. Colaprete says some of these findings will be made public in a couple of weeks. (Don't be surprised if he announces that one of the spectrometers did, indeed, detect water in the plume.)

For now, let me tantalize you with a preliminary result from the Lunar Reconnaissance Orbiter, which viewed the Centaur's demise from nearly overhead and just 48 miles (76 km) up. An instrument dubbed the Lyman-Alpha Mapping Project (LAMP) probed the ultraviolet spectrum of the impact plume after it had risen high enough to be projected against black space above the lunar limb.

"We definitely saw something," notes LAMP scientist Randy Gladstone (Southwest Research Institute). But that "something" wasn't water. Nor was it oxygen or hydrogen atoms, both of which have strong ultraviolet emissions. There's some hint of hydrogen molecules (H2) — and though water is one source of hydrogen, it could also have come from silicate minerals, solar-wind gas trapped in the lunar soil, or (most likely) residual fuel in the Centaur's tanks.

LAMP's strongest and most intriguing observation came at the ultraviolet wavelength of 184-185 nanometers. Gladstone says the only known elements able to create that line are iron, perhaps magnesium … and mercury. "Both mercury and iron still look like the best bets for explaining the plume emission we see with LAMP," Gladstone reiterates, though the spectral match is still tentative and more data-crunching is in progress.

Liquid mercury on the Moon? Really? Gladstone directed me to an obscure, decade-old research paper titled "Don't Drink the Water" written by George W. Reed Jr. (Argonne National Laboratory). Reed describes how mercury was found in lunar regolith returned by the crews of Apollos 12, 15, 16, and 17, and other work suggests it might be present in the Moon's wispy-thin exosphere.

No matter what its source, Reed concludes, some of this mercury must end up as deposits in the ultracold interiors of permanently shadowed lunar craters. Moreover, the Centaur slam may not have created the big splash everyone wanted, but it only needed to heat the target area to about 200°F to release any mercury trapped in the dark dirt. And thermal imaging from the Diviner instrument aboard LRO argues that the impact site got that hot and then some.

This is all starting to make sense. Back in mid-September, UCLA scientist David Paige announced, based on Diviner's thermal mapping, that the lunar polar regions are far colder than expected, down near 35 kelvins (-397°F). This means the shadowed floors within Cabeus and its neighbors are the most frigid places known in the entire solar system. More to the point, Paige notes, "The temperatures in these super-cold regions are definitely low enough to cold-trap water ice, as well as other more volatile compounds for extended periods."

So is lunar water safe to drink? Future astronaut crews had better bring along some serious water-purification gear if they intend to live off what they scavenge from the lunar poles.