NASA LRO: Metamorphosis of moon's water ice explained

Panoramic lunar view taken by the Lunar Reconnaissance Orbiter Camera of the north rim of Cabeus crater. 

The distance from left to right is about 75 kilometers (46 miles). 

Image courtesy of NASA/GSFC/Arizona State Univ.

Using data gathered by NASA's Lunar Reconnaissance Orbiter (LRO) mission, scientists believe they have solved a mystery from one of the solar system's coldest regions—a permanently shadowed crater on the moon.

They have explained how energetic particles penetrating lunar soil can create molecular hydrogen from water ice.

The finding provides insight into how radiation can change the chemistry of water ice throughout the solar system.

Space scientists from the University of New Hampshire and NASA's Goddard Space Flight Center have published their results online in the Journal of Geophysical Research (JGR): Planets.

Lead author of the paper is research scientist Andrew Jordan of the University of New Hampshire's Institute for the Study of Earth, Oceans, and Space (EOS).

Discovering molecular hydrogen on the moon was a surprise result from NASA's Lunar Crater Observation Sensing Satellite (LCROSS) mission, which crash-landed the LCROSS satellite's spent Centaur rocket at 5,600 miles per hour into the Cabeus crater in the permanently shadowed region of the moon. 

These regions have never been exposed to sunlight and have remained at temperatures near absolute zero for billions of years, thus preserving the pristine nature of the lunar soil, or regolith.

Instruments on board LCROSS trained on the resulting immense debris plume detected water vapor and water ice, the mission's hoped-for quarry, while LRO, already in orbit around the moon, saw molecular hydrogen—a surprise.

"LRO's Lyman Alpha Mapping Project (LAMP), detected the signature of molecular hydrogen, which was unexpected and unexplained," says Jordan.

Jordan's JGR paper, "The formation of molecular hydrogen from water ice in the lunar regolith by energetic charged particles," quantifies an explanation of how molecular hydrogen, which is comprised of two hydrogen atoms and denoted chemically as H2, may be created below the moon's surface.

"After the finding, there were a couple of ideas for how molecular hydrogen could be formed but none of them seemed to work for the conditions in the crater or with the rocket impact." Jordan says.

"Our analysis shows that the galactic cosmic rays, which are charged particles energetic enough to penetrate below the lunar surface, can dissociate the water, H2O, into H2 through various potential pathways."

That analysis was based on data gathered by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the LRO spacecraft.

Jordan is a member of the CRaTER scientific team, which is headed up by principal investigator Nathan Schwadron of EOS.

Schwadron, a co-author on the JGR paper, was the first to suggest energetic particles as the possible mechanism for creating molecular hydrogen.

CRaTER characterises the global lunar radiation environment by measuring radiation dose rates from galactic cosmic rays and solar energetic particles.

Says Jordan, "We used the CRaTER measurements to get a handle on how much molecular hydrogen has been formed from the water ice via charged particles."

Jordan's computer model incorporated the CRaTER data and showed that these energetic particles can form between 10 and 100 percent of the H2 measured by LAMP.

The study notes that narrowing down that percent range requires particle accelerator experiments on water ice to more accurately gauge the number of chemical reactions that result per unit of energy deposited by cosmic rays and solar energetic particles.

NASA GRAIL Mission: LRO's LAMP Captures Lunar Impact

These models show the time evolution for hydrogen (left) and mercury (right) as plumes of gas rapidly expand into the vacuum of space following the planned impact of the GRAIL twins onto the lunar surface. 

Data from the Lyman-Alpha Mapping Project (LAMP) aboard NASA’s Lunar Reconnaissance Orbiter accurately constrain such models used to understand the impact event. 

Credit: JHUAPL/SwRI/NASA

When NASA's twin GRAIL spacecraft made their final descent for impact onto the Moon's surface last December, the Lunar Reconnaissance Orbiter's sophisticated payload was in position to observe the effects.

As plumes of gas rose from the impacts, the Lyman Alpha Mapping Project (LAMP) aboard LRO detected the presence of mercury and hydrogen and measured their time evolution as the gas rapidly expanded into the vacuum of space at near-escape velocities.

NASA intentionally crashed the GRAIL twins onto the Moon on Dec. 17, 2012, following successful prime and extended science missions.

Both spacecraft hit a mountain near the lunar north pole, which was shrouded in shadow at the time.

Developed by Southwest Research Institute (SwRI), LAMP uses a novel method to peer into the darkness of the Moon's permanently shadowed regions, making it ideal for observations of the Moon's night-side and its tenuous atmospheric constituents.

Dr. Kurt Retherford

"While our results are still very new, our thinking is that the hydrogen detected from the GRAIL site might be related to an enhancement at the poles caused by hydrogen species migrating toward the colder polar regions," says Dr. Kurt Retherford, LAMP principal investigator and a principal scientist at SwRI.

"Combining GRAIL results with LCROSS results could tell us more about hydrogen and water near the poles," says Dr. Thomas Greathouse, a LAMP team member and SwRI senior research scientist.

“We have begun to understand that the amount of water ice near the polar regions is higher than was previously thought, but we don't fully understand how it gets there."

LAMP usually observes the night-side lunar surface using light from nearby space (and stars), which bathes all bodies in space in a soft glow.

This Lyman-alpha glow is invisible to human eyes but visible to LAMP as it reflects off the Moon.

However, the new detection of Lyman-alpha emissions from native lunar atomic hydrogen gas released by the impact is a first for LAMP, and for any previous instrument.

There's Metal In Moon Water

Bring a filter if you plan on drinking water from the moon.

Water ice recently discovered in dust at the bottom of a crater near the moon's south pole is accompanied by metallic elements like mercury, magnesium, calcium, and even a bit of silver.

Now you can add sodium to the mix, according to Dr. Rosemary Killen of NASA's Goddard Space Flight Center in Greenbelt, Md.

Recent discoveries of significant deposits of water on the moon were surprising because our moon has had a tough life.

Intense asteroid bombardments in its youth, coupled with its weak gravity and the Sun's powerful radiation, have left the moon with almost no atmosphere. This rendered the lunar surface barren and dry, compared to Earth.

However, due to the moon's orientation to the Sun, scientists theorized that deep craters at the lunar poles would be in permanent shadow and thus extremely cold, and able to trap volatile material like water as ice if such material were somehow transported there, perhaps by comet impacts or chemical reactions with hydrogen, a major component of the solar wind.

The October 9, 2009 impact of NASA's Lunar CRater Observation and Sensing Satellite (LCROSS) spacecraft into the permanently shadowed region of the Cabeus crater confirmed that a surprisingly large amount of water ice exists in this region, along with small amounts of many other elements, including metallic ones.

LCROSS almost missed the target

The Lunar Reconnaissance Orbiter recorded temperatures at the Cabeus crash site, shown here about 90 seconds after impact. 

The impact generated temperatures over 1340 degrees Fahrenheit (727 degrees Celsius), which appear as a tiny glowing dot near the center of the color swath. Credit: NASA/UCLA.

The existence of ice on the moon was revealed with a bang last year, when kamikaze spacecraft crashed into a crater at the lunar south pole, kicking up enough water for researchers to finally detect.

Today scientists in six separate studies announced new findings from the Oct. 9, 2009 LCROSS moon crash mission. They found, among other discoveries, substantial amounts of water ice at ground zero for the impact — water that could one day be key to the humanity's future in space.

But as successful as the mission proved in the end, the complicated affair was fraught with uncertainty and came perilously close to failure. Now scientists reveal the story of how they made their discovery and what challenges they faced along the way. [10 Coolest New Moon Discoveries]

LCROSS Confirms Lunar Prospector Findings: Water on the Moon

Feldman and his team didn't measure water directly, but their data showed evidence for inordinately large amounts of hydrogen in some craters.

Other phenomena, such as the solar wind and outgassing, could account for relatively high levels of hydrogen, "but there was a sufficient amount in some of these craters that it would be hard to understand if it came only from the solar wind or through other processes," Feldman said.

"So in our paper we didn't call it 'evidence for hydrogen,' but 'evidence for water'."

Not everyone agreed, and some controversy surrounded the paper. But, when NASA went looking for water on the Moon with the LCROSS mission, it headed for the Cabeus crater, about 62 miles from the Moon's south pole, that Feldman's Lunar Prospector team had identified as having the maximum hydrogen signature among all the high-latitude craters surveyed.

Not everyone agreed, and some controversy surrounded the paper. But, when NASA went looking for water on the Moon with the LCROSS mission, it headed for the Cabeus crater, about 62 miles from the Moon's south pole, that Feldman's Lunar Prospector team had identified as having the maximum hydrogen signature among all the high-latitude craters surveyed.

"This is a big, permanently shaded crater," Feldman said. "In fact, you can't even see it from the Earth because it has a rim that hides it. It takes a satellite to see it."

"When we converted the hydrogen signal to the amount of water ice in the regolith, we found that it was only about 1.5 percent by weight," Feldman added. "That's the reason the radar researchers really couldn't see it. There aren't large enough deposits of high-grade water ice to create the signal needed to identify ice with radar."

So it turns out that Feldman and the Lunar Prospector team showed the first experimental evidence for water on the Moon, which has now been conclusively confirmed by the LCROSS mission.

"There's a lot of interest in water on the Moon right now," Feldman said. "And there is more to be learned. The whole story is not in yet."

LCROSS: Images of Moon's Cabeus Crater

Most mountains on the Earth are formed as plates collide and the crust buckles. Not so for the Moon, where mountains are formed as a result of impacts.

Images taken looking across the landscape rather than straight down really bring out topography and help us visualize the lunar landscape.

However such images can only be taken as the spacecraft rolls to the side, in this case about 70°, so the opportunities are limited.

Foreground is about 15 km wide, view is northeast across the north rim of Cabeus crater. Credit: NASA/GSFC/Arizona State University.

Cabeus crater is relatively old, 100-km in diameter, and contains significant areas of permanent shadow. Such regions are of great interest because they may harbor significant deposits of ices (water, methane, etc).

Cabeus crater is most famous as the site of the LCROSS Centaur impact (9 October 2009) that was intended to excavate and eject any volatiles that may be in the regolith (what we call the lunar soil). Though analyses of data collected during the impact are still ongoing, preliminary results suggest that yes, significant amounts of water ice may be trapped in these shadowed regions (at least at this one spot).

Two and a half days after the LCROSS impact the LRO spacecraft slewed 70° back towards Cabeus crater to allow LROC to acquire an overview image of a portion of the northern rim. The large mountain (or massif) in the right background (full panorama below) is a portion of the ancient rim of the South Pole Aitken basin, it rises some 6000 meters (19,685 feet) above the surrounding plains, and more than 9200 meters (30,184 feet) above the floor of Cabeus crater -- taller than any mountain on the Earth.

On the Moon mountains are formed in only minutes as huge amounts of energy are released when asteroids and comets slam into the surface at velocities greater than 16-km per second (more than ten times faster than a speeding bullet). In contrast, mountains on the Earth typically form over millions of years during slow motion collisions of tectonic plates.

Panoramic view looking across the North rim of Cabeus crater from the SW. The distance from left to right is about 75-km and from foreground to background in the center is about 50-km. The LCROSS impact was just off the bottom center of the panorama Credit: NASA/GSFC/Arizona State University.

Future astronauts will see the same view as they descend to the surface for a polar landing. Explore the rim of Cabeus on your own as you plan your landing spot!