Lunar 'permafrost': Evidence for widespread water ice on the moon

Their findings appear Oct. 22 in two papers published in the journal Science. The research was funded by NASA.

Diviner, an infrared spectrometer aboard NASA's Lunar Reconnaissance Orbiter (LRO), has made the first infrared measurements of temperatures in the permanently shadowed craters at the moon's poles.

In October 2009, Diviner also made the first infrared observations of a controlled impact on the moon, when NASA's Lunar Crater Observation and Sensing Satellite (LCROSS), the companion spacecraft to the LRO, slammed into one of the coldest of these craters in an experiment to confirm the presence or absence of water ice.

UCLA professor of planetary science David Paige, Diviner's principal investigator and lead author of one of the Science papers, 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 stability of water ice is an indication that is has existed in a particular location over an extended period of time.

"The temperatures inside these permanently shadowed craters are even colder than we had expected," Paige said. "Our model results indicate that in these extreme cold conditions, surface deposits of water ice would almost certainly be stable; but perhaps more significantly, these areas are surrounded by much larger permafrost regions where ice could be stable just beneath the surface."

This lunar 'permafrost' is analogous to the high-latitude terrain found on the Earth and on Mars, where subfreezing temperatures persist below the surface throughout the year, Paige said.

"These permafrost regions may receive direct sunlight at certain times of the year, but they maintain annual maximum subsurface temperatures that are sufficiently cold to prevent significant amounts of ice from vaporizing," he said.

Given that these permafrost regions are not in permanent shadow, surface lighting and thermal conditions in these locations would be far more hospitable for humans, which makes them of prime interest for future manned missions to the moon, Paige said. Subsurface water ice deposits are also likely to be more stable than surface deposits of water ice because they are protected from bombardment by ultraviolet radiation and energetic cosmic particles.

"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 sulphur dioxide, carbon dioxide, formaldehyde, ammonia, methanol, mercury and sodium," Paige said.

A representative cross-section of these substances was detected by the LCROSS near-infrared spectrometers when its upper-stage rocket impacted the Cabeus crater, ejecting a host of material that was previously buried beneath the crater's surface.

The impact site was situated within a permanently shadowed part of Cabeus with an average annual temperature of 37 Kelvin (-393 degrees Fahrenheit), making it one of the coldest spots near the lunar south pole. Temperature data from Diviner played a key role in the selection of Cabeus as the target for LCROSS. When it came time for impact, Diviner scientists and engineers made sure the instrument had a front-row seat: Diviner targeted the impact site for eight orbits, spaced roughly two hours apart, the closest of which was timed to pass by 90 seconds after impact. It observed an enhanced thermal signal on this and two subsequent orbits.

Paul Hayne, a graduate student in the UCLA Department of Earth and Space Sciences and lead author of the second paper in Science, monitored the data in real time as it was sent back from Diviner.

"During the fly-by 90 seconds after impact, all seven of Diviner's infrared channels measured an enhanced thermal signal from the crater," Hayne said. "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: They were able to calculate a range for the mass of material that was ejected outwards into space from the impact crater, and they were able to infer the initial temperature and make estimates about the effects of ice in the soil on the observed cooling behavior.

"Diviner's solar channel measured scattered sunlight from the impact plume over an area of 54 square miles," Hayne said. "Using this measurement, we were able to calculate the mass of the cloud at between 2,600 and 12,800 pounds, which is consistent with measurements by the LCROSS Shepherding Spacecraft. This is important because the cloud mass is used to estimate the abundance of water observed by the LCROSS spectrometers.

"In addition, we determined that in order to agree with the data from each of Diviner's channels, the impact must have heated a region of 320 to 2,150 feet to at least 950 Kelvin (1,250 Fahrenheit). This concentrated region was surrounded by a larger, lower temperature component that would have included the surrounding blanket of material excavated by the impact."

Given that ice within pore spaces in the soil influences cooling because it uses up heat energy in the process of sublimating and conducts heat more efficiently than lunar soil itself does, scientists were able to use Diviner's measurements of cooling at the impact site to calculate a range for the proportion of volatile compounds present.

"The fact that heated material was still visible to Diviner after four hours indicates LCROSS did not hit a skating rink; the ice must have been mixed within the soil," Hayne said, "we estimate that for an area of 320 to 2,150 feet, the steaming crater could produce more than enough water vapor to account for what was observed by LCROSS over a four-minute period."

"Although Cabeus crater is typical of the coldest areas on the moon today, we have determined that billions of years ago, smaller craters with steeper walls would have made more favorable cold-traps," Paige said. "It is therefore possible that the craters which have accumulated the most ice are not the coldest ones."

The results presented in both papers represent strong evidence in support of the theory that volatile compounds have been delivered to the moon by impacts by icy bodies from the outer solar system and then 'cold-trapped' at the lunar poles.