The lunar rocks brought back to Earth by the Apollo astronauts were found to have very little water, and were much drier than rocks on Earth. An explanation for this was that the moon formed billions of years ago in the solar system's turbulent youth, when a Mars-sized planet crashed into Earth. The impact stripped away our planet's outer layer, sending it into orbit. The pieces later coalesced under their own gravity to form our moon. Heat from all this mayhem vaporized most of the water in the lunar material, so the water was lost to space.
However, there was still a chance that water might be found in special places on the moon. 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 perhaps delivered there by comet impacts or chemical reactions with hydrogen carried by the solar wind.
In October 9, 2009, NASA's LCROSS (Lunar Crater Remote Observation and Sensing Satellite) was intentionally crashed into the Cabeus crater near the lunar south pole. The idea was to kick up debris from the bottom of the crater so its composition could be analyzed. LCROSS hit at over 9,000 kilometers (5,600 miles) per hour, sending up a plume of material over 19 kilometers (12 miles) high.
"Seeing mostly pure water ice grains in the plume means water ice was somehow delivered or chemical processes are causing ice to accumulate in large quantities," said Anthony Colaprete, LCROSS project scientist and principal investigator at NASA's Ames Research Center, Moffett Field, Calif. "Furthermore, the diversity and abundance of certain materials called volatiles in the plume, suggest a variety of sources, like comets and asteroids, and an active water cycle within the lunar shadows."
LCROSS was a companion mission to NASA's Lunar Reconnaissance Orbiter (LRO) mission. The two missions were designed to work together, and support from LRO was critical to the success of LCROSS. During impact, LRO, which is normally looking at the lunar surface, was tilted toward the horizon so it could observe the plume. Shortly after LCROSS hit the moon, LRO flew past debris and gas from the impact while its instruments collected data.
"LRO assisted LCROSS in two primary ways - selecting the impact site and confirming the LCROSS observations," said Gordon Chin of NASA's Goddard Space Flight Center, Greenbelt, Md., LRO associate project scientist.
"Since observatories on Earth were also planning to view the LCROSS impact, there were a lot of constraints on the location - the impact plume had to rise out of the crater and into sunlight, and it had to be visible from Earth," said Chin.
"Originally, the LCROSS team was going with a site farther north than the Cabeus crater, because it was better for Earth visibility," said Chin. "However, LEND revealed that the area did not have a high hydrogen concentration, but Cabeus did. Also, Diviner showed that Cabeus was one of the coldest sites, and LOLA indicated it was in permanent shadow. So, we were able to influence the decision to aim for Cabeus farther south -- while it was a little less visible from Earth, Cabeus was ultimately better for what we were trying to find."
The Diviner instrument aboard the Lunar Reconnaissance Orbiter was built and is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif. Temperature maps from LRO's Diviner instrument were also crucial to identify where the coldest places were.
David Paige, principal investigator of the Diviner instrument from the University of California, Los Angeles, used temperature measurements of the lunar south pole obtained by Diviner to model the stability of water ice both at and near the surface.
"The temperatures inside these permanently shadowed craters are even colder than we had expected. Our model results indicate that in these extreme cold conditions, surface deposits of water ice would almost certainly be stable," said Paige, "but perhaps more significantly, these areas are surrounded by much larger permafrost regions where ice could be stable just beneath the surface."
"We conclude that large areas of the lunar south pole are cold enough to trap not only water ice, but other volatile compounds (substances with low boiling points) such as sulfur dioxide, carbon dioxide, formaldehyde, ammonia, methanol, mercury and sodium," Paige added.
A UCLA graduate student and Diviner team member, Paul Hayne, was monitoring the data in real-time as it was sent back from Diviner.
"During the flyby 90 seconds after impact, all seven of Diviner's infrared channels measured an enhanced thermal signal from the crater. The more sensitive of its two solar channels also measured the thermal signal, along with reflected sunlight from the impact plume. Two hours later, the three longest wavelength channels picked up the signal, and after four hours only one channel detected anything above the background temperature."
Scientists were able to learn two things from these measurements: first, they were able to constrain the mass of material that was ejected outwards into space from the impact crater; second, they were able to infer the initial temperature and make estimates about the effects of ice in the soil on the observed cooling behavior.
Another LRO instrument, the Lyman-Alpha Mapping Project (LAMP), used data on the gas cloud to confirm the presence of the gases molecular hydrogen, carbon monoxide and atomic mercury, along with smaller amounts of calcium and magnesium, also in gas form.
"We had hints from Apollo soils and models that the volatiles we see in the impact plume have been long collecting near the moon's polar regions," said Randy Gladstone, LAMP acting principal investigator, of Southwest Research Institute in San Antonio. "Now we have confirmation."
"The detection of mercury in the soil was the biggest surprise, especially that it's in about the same abundance as the water detected by LCROSS," said Kurt Retherford, LAMP team member, also of Southwest Research Institute.
"The observations by the suite of LRO and LCROSS instruments demonstrate the moon has a complex environment that experiences intriguing chemical processes," said Richard Vondrak, LRO project scientist at NASA Goddard. "This knowledge can open doors to new areas of research and exploration."
LCROSS launched with LRO aboard an Atlas V rocket from Cape Canaveral, Fla., on June 18, 2009.
The research was funded by NASA's Exploration Systems Missions Directorate at NASA Headquarters in Washington. LRO was built and is managed by NASA's Goddard Space Flight Center in Greenbelt, Md. LCROSS is managed by NASA's Ames Research Center, Moffett Field, Calif. LAMP was developed by the Southwest Research Institute in San Antonio, Texas; LOLA was built by NASA Goddard; LROC was provided by Arizona State University, Tempe; LEND was provided by Institute for Space Research, Moscow; The Diviner instrument was built and is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif. UCLA is the home institution of Diviner's principal investigator.
For more information on Diviner, visit:
http://diviner.ucla.edu.
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