In the coming years, NASA and other space agencies hope to explore the southern polar region of the Moon. Recent surveys of this region have revealed an environment rich in volatiles – elements that vaporize rapidly due to changes in conditions. In particular, missions like NASA’s Lunar Reconnaissance Orbiter (LRO) and the Lunar CRater Observation and Sensing Satellite (LCROSS) have detected abundant water ice in the permanently-shadowed craters around the South Pole-Aitken Basin.
Where this water came from has remained the subject of much debate, with theories ranging from it being deposited by volcanic activity or solar wind to being delivered by comets. After examining LCROSS data on the Cabeus crater near the Moon’s south pole, a multinational team of researchers from the U.S. and France determined that the water ice and volatiles in the crater were likely delivered by the impactor (a comet) that created it.
Before and after the GRAIL twins impacts on the Moon December 17, 2012. The LROC Narrow Angle Camera (NAC) directors were able to resolve the impact sites on February 28, 2013, revealing both to be about 5 meters in diameter. Upper panels show the area before the impact; lower panels after the impact. Arrows point to crater locations. LROC NAC observations M186085512R, M186078336L, M1116736474R and M1116736474L. Credit: NASA/GSFC/Arizona State University.
On December 17, 2012, the GRAIL mission came to an end, and the two washing machine-sized spacecraft performed a flying finale with a planned formation-flying double impact into the southern face of 2.5-kilometer- (1.5-mile-) tall mountain on a crater rim near the Moon’s north pole. The Lunar Reconnaissance Orbiter has now imaged the impact sites, which show evidence of the crashes.
But surprisingly, these impacts were not what was expected, says the LRO and GRAIL teams. The ejecta around both craters is dark. Usually, ejecta from craters is lighter in color – with a higher reflectance – than the regolith on surface.
“I expected the ejecta to be bright,” said LROC PI Mark Robinson at a press conference from the Lunar and Planetary Science Conference today, “because everybody knows impact rays on the Moon are bright. We are speculating it could be from hydrocarbons from the spacecraft.”
GRAIL A site seen before and after the impact event. Crater center is located at 75.609°N, 333.407°E/ Credit: NASA/GSFC/Arizona State University.
Typically ejecta from craters is brighter, since subsurface regolith tends to have a higher reflectance. The lunar regolith on the surface tends to be darker because of its exposure to the vacuum of space, cosmic radiation, solar wind bombardment, and micrometeorite impacts. Slowly over time, these processes tend to darken the surface soil.
Robinson said the hydrocarbons could have come from fuel left in the fuel lines (JPL estimated a quarter to half a kilogram of fuel may have remained in the spacecraft – so, not very much) or from the spacecraft itself, which is made out of carbon material.
GRAIL B site seen before and after impact event. Crater center is located at 75.651°N, 333.168°E. Credit: NASA/GSFC/Arizona State University.
Additionally, the impact craters’ shapes were not as expected. The impacts formed craters about 5 m (15 ft) in diameter, and there is little ejecta to the south – the direction from which the spacecraft were traveling. “The spacecraft came in at a 1 or 2 degree impact angle,” said Robinson, “so this not a normal impact, as all the ejecta went upstream in the direction of travel.”
“I was expecting to see skid marks, myself,” said GRAIL principal investigator Maria Zuber. She added that she was committed to using every bit of fuel to mapping the gravity field at as low an altitude as possible. “I was determined that we would not end the mission with unused fuel because that would have meant we could mapped it even lower.
LRO Wide Angle Camera (WAC) image of the GRAIL impact area on the south side of the unnamed massif. Credit: NASA/GSFC/ASU.
Robinson said he was skeptical that they could find the impact craters, since the team has yet to find the impact sites of the Apollo ascent stages, which should be much bigger than the GRAIL impacts.
“Finding the impact crater was like finding a needle in haystack,” Robinson said, “as the images are looking at an area that is about 8 km wide and 30 to 40 km tall, and we were looking for something that is a couple of pixels wide.”
Robinson said he spent hours looking for it with no luck, only to see it later when he was on a conference call and was just looking at it out of the corner of his eye.
“It was really fun to find the craters,” he said. LRO did take images in early January, but better images were taken on February 28, 2013.
While LRO’s camera was not able to image the actual impact since it occurred on the night-side of the Moon, the LAMP instrument (Lyman Alpha Mapping Project) on LRO was able to detect the plumes of the impacts.
Kurt Retherford, PI of LAMP said the UV spectrograph was pointed towards the limb of the Moon — and actually looking in the direction of the constellation Orion at the time of the GRAIL impact — to observe the gases coming out of the plumes. They did detect the two impact plumes which clearly showed an excess of emissions from hydrogen atoms. “We were excited to see this detection of atomic hydrogen coming from the impact sites,” Retherford said. “This is our first detection of native hydrogen atoms from the lunar environment.”
This video shows LRO as it flies over the north pole of the Moon, where it has a very good view of the GRAIL impact. The second part is the view from LRO through LAMP’s slit, showing the impact and the resulting plume. The orbits, impact locations, terrain, LAMP field of view, and starfield are accurately rendered.
Retherford said further studies from this will help in determining the processes of how the implantation of solar wind protons on the lunar surface could create the water and hydroxyl that has been recently detected on the lunar surface by other spacecraft and in studies of lunar rocks returned by the Apollo missions.
Using the dim light of distant stars reflecting off of the surface of the Moon, scientists using a spectrometer aboard NASA’s Lunar Reconnaissance Orbiter have found traces of the Moon’s tenuous atmosphere. But don’t expect to take off your protective spacesuit. The Moon’s atmosphere is made of helium.
“The question now becomes, does the helium originate from inside the Moon, for example, due to radioactive decay in rocks, or from an exterior source, such as the solar wind.” says Dr. Alan Stern, LAMP principal investigator and associate vice president of the Space Science and Engineering Division at Southwest Research Institute, Boulder, Colo.
Scientists designed the Lyman Alpha Mapping Project (LAMP) spectrometer aboard LRO to map the lunar surface but the confirmation that helium surrounds Earth’s largest natural satellite was a bonus, Stern told Universe Today.
“LAMP was designed to simply do what we had not done in 40 years; to look closely at the surface of the Moon,” Stern said. “This really is a breakthrough, a capability discovery.”
LAMP’s findings support work done by the Lunar Atmosphere Composition Experiment, or LACE, that was left behind by Apollo 17 astronauts in 1972. LAMP is designed to examine far ultraviolet emissions in the tenuous atmosphere above the Moon’s surface.
Some elements found on the Moon, such as carbon or sodium, can be studied from Earth. Helium is not one of these, Stern says. Helium only shows very weakly in the far ultraviolet part of the spectrum. The signature is too weak to be seen from the 250,000 miles separating the Moon from Earth. Earth’s ozone layer also absorbs ultraviolet radiation making detection from ground-based detectors impossible.
And with LAMP moving over the lunar surface, we can see more than we’d see with a simple lander, Stern said.
During its mission, LACE detected argon but so far only helium has been confirmed from LAMP’s spectrograph. Although, the noble gas argon is much fainter than helium to the spectrograph, LAMP will seek this and other gases as well.
John Williams is a science writer and owner of TerraZoom, a Colorado-based web development shop specializing in web mapping and online image zooms. He also writes the award-winning blog, StarryCritters, an interactive site devoted to looking at images from NASA’s Great Observatories and other sources in a different way. A former contributing editor for Final Frontier, his work has appeared in the Planetary Society Blog, Air & Space Smithsonian, Astronomy, Earth, MX Developer’s Journal, The Kansas City Star and many other newspapers and magazines.
