NASA has released images and findings from the Lunar Reconnaissance Orbiter (LRO), which witnessed the impact of NASA's twin GRAIL (Gravity Recovery And Interior Laboratory) spacecraft as they struck the Moon near the North Pole in a controlled impact on Dec.17, 2012. The unmanned orbiter sent back before and after images of the impact sites and used its Lyman-Alpha Mapping Project (LAMP) instrument to study the plume of dust and gas thrown up by the double impact, producing new insights into the processes going on in the interior of the Moon.
The controlled impact of the GRAIL spacecraft came at the end of a very successful mission, where the twin probes orbited in tandem low over the lunar surface as they produced the most detailed gravity map of any body in the Solar System. However, by the end of the mission, the GRAIL spacecraft were low on fuel and their low orbits meant that they would eventually crash on the Moon. In order to save historical landing sites from contamination, NASA decided to aim the craft at a mountain in the lunar polar region.
"The two spacecraft were relatively small – cubes about the size of a washing machine with a mass of about 200 kilograms (440 lb) each at the time of impact," said Mark Robinson, LROC principal investigator at Arizona State University's School of Earth and Space Sciences. "When they were launched, the individual spacecraft mass was slightly more than 300 kg (661 lb), but each consumed just over 100 kg (200 lb) of fuel during the mission. The spacecraft were traveling about 6,070 km/h (3,771 mph) when they hit the surface. Both craters are relatively small, perhaps four to six meters (about 13 to 20 feet) in diameter and both have faint, dark, ejecta patterns, which is unusual. Fresh impact craters on the Moon are typically bright, but these may be dark due to spacecraft material being mixed with the ejecta."
"Both impact sites lie on the southern slope of an unnamed massif (mountain) that lies south of the crater Mouchez and northeast of the crater Philolaus," adds Robinson. "The massif stands as much as 2,500 meters (about 8,202 ft) above the surrounding plains. The impact sites are at an elevation of about 700 meters (around 2,296 feet) and 1,000 meters (3,281 ft), respectively, about 500 to 800 meters (approximately 1,640 to 2,625 ft) below the summit. The two impact craters are about 2,200 meters (roughly 7,218 ft) apart. GRAIL B (renamed Flow) impacted about 30 seconds after GRAIL A (Ebb) at a site to the west and north of GRAIL A."
The tricky bit, however, was making sure that the LRO was on station at the time of impact. Unfortunately, the LRO team was only told about where the GRAIL probes would hit three weeks beforehand. The LRO was already scheduled to make a maneuver to correct its orbit. Worse, it was also promised to observe the impact of ESA’s Herschel spacecraft, which was also making a controlled crash. This almost put paid to LRO watching GRAIL’s impact, but at the last minute ESA scratched the impact, which simplified LRO’s maneuvers.
“We postponed the station keeping until April 29, and the Flight Dynamics team turned on a dime, making the station-keeping maneuver into a phasing maneuver (to catch up with the Grail spacecraft) so we could observe the impact," said LRO Project Scientist John Keller of NASA's Goddard Space Flight Center.
Because the impact site was in darkness at the time GRAIL A and B came down, the LRO wasn’t about to see them actually hit. Instead, it used its LAMP, an ultraviolet imaging spectrograph, to watch the plume thrown up by the strike. This wasn’t just a way of making sure that the spacecraft had augured in, it was also a way of studying the Moon’s interior because the plume contained mercury and atomic hydrogen.
Hydrogen means, among other things, water, and mercury is very volatile. Scientists tend to think that mercury on the Moon will be found in dark craters that haven’t seen the Sun in a couple of billion years, but its presence in a sunlit area where the GRAIL probes hit raised some questions.
"The issue for the GRAIL impact was not so much that mercury was found – you would expect it to be present as an element from the Moon's formation, just like it is found on Earth," said Keller. "Rather, it is still reasonably concentrated near the surface instead of being driven off in an area where, for a very long time, the surface has been completely exposed to the space environment, including heat from the Sun, impacts from microscopic meteorites, and radiation."
"These new results help us continue to understand the nature of volatiles near the lunar poles," said Kurt Retherford, LAMP principal investigator at Southwest Research Institute, San Antonio, Texas. "In the last four years we have begun to understand that the amount of water ice near the polar regions is higher than previously thought. In addition to direct measurements of water from the LCROSS (Lunar Crater Observation and Sensing Satellite) impact plume there were several other volatile species detected in the Cabeus crater cold-trapping region, including mercury atoms and hydrogen molecules detected with the LAMP instrument.
"While our results are still very new, our thinking is that the mercury detected by LAMP from the GRAIL site might be related to an enhancement at the poles caused by mercury atoms generally hopping across the surface and eventually migrating toward the colder polar regions. The detection of hydrogen atoms from the GRAIL impact plumes compared with H2 molecules in the LCROSS impact plume might tell us more about hydrogen and/or water near the poles, but this is a work in progress."
"This gives insight into how volatile material is transported around the Moon," added Keller. "It gives us a data point that helps constrain models of volatile transport, especially for models that describe how volatile material can get transported from warm to cold areas on the Moon."
The LRO also used its Lunar Orbiter Laser Altimeter (LOLA) instrument to produce a 3D map of the impact area that confirmed how close the impact points were to expectations. This not only acted as a way evaluating the predictions, but gave further insights into the gravity mapping of the Moon.
"Combining the LRO LOLA topography map with GRAIL's gravity map yields some very interesting results," said Keller. "You expect that areas with mountains will have a little stronger gravity, while features like craters will have a little less. However, when you subtract out the topography, you get another map that reveals gravity differences that are not tied to the surface. It gives insight into structures deeper in the Moon's interior."
The NASA video below shows the GRAIL impacts and the aperture of LRO’s LAMPS instrument.