Permanently shadowed craters on the Moon are cold enough to trap volatile compounds. Credit: NASA/GSFC/Arizona State University

NASA has unveiled a swathe of findings obtained by its Lunar Reconnaissance Orbiter (LRO), which has been orbiting the Moon since June.

Previous findings had identified the Moon as the coldest place in the Solar System, but the latest results push the temperature even lower, all the way to 26 kelvin (-249 °C) — well below the freezing points of nitrogen and oxygen on Earth, and only a dozen degrees above the freezing point of hydrogen.

"Nothing like this has ever been measured in the Solar System," says David Paige, a planetary scientist from the University of California, Los Angeles, and principal investigator for the spacecraft's temperature-sensing instrument. "One would probably have to travel far beyond the Kuiper Belt to find an object whose temperatures are this low."

The measurements were made in October, when the Moon's north pole was at mid-winter (see video of a flyover of the lunar north pole).

Because the Moon is tilted a mere 1.54° on its axis (compared with 23.5° for Earth), its seasons are not very pronounced. But the poles have small regions where the Sun doesn't rise for around six months. It was there that the LRO's instruments measured the new minimum.

The satellite also found that permanently shadowed 'cold traps' near the Moon's south pole get only 20 °C warmer than the coldest regions of the north pole, even in mid-summer. "We now have a good idea of what all the temperatures (both minimum and maximum) are at the cold traps," Paige says, "which will help us understand what volatile [compounds] they may contain."

The impact of NASA's Lunar Crater Observation and Sensing Satellite (LCROSS) in October (see 'Lunar impact tosses up water and stranger stuff') revealed that one of these cold traps contained water. But that might be just the beginning. "There could be all kinds of interesting compounds trapped there," Paige said earlier this week at a meeting of the American Geophysical Union in San Francisco, California.

Cosmic-ray risk

Other researchers at the meeting found that the danger posed to astronauts by exposure to cosmic rays was likely not serious enough to rule out long lunar missions.

Harlan Spence, of Boston University in Massachusetts, is principal investigator for an instrument on the LRO called CRaTER (Cosmic Ray Telescope for the Effects of Radiation), which uses a special 'tissue-equivalent plastic' that simulates the properties of human tissue to measure the effect of galactic cosmic rays on astronauts.

Galactic cosmic rays are electrically charged particles that enter the Solar System at nearly the speed of light. Spence's team found that they deliver a radiation dose of about 50 milli-Grays per year. That is about 10 times the dose received on Earth by X-ray tecnicians, nuclear power workers, and uranium miners. But astronauts would only be fully exposed to such radiation during the three-day transit from the Earth to the Moon, says Spence. "At the Moon, you have structures and regolith to work with, and that would provide substantial shielding to lower the dose by a lot," he adds.

Better yet, the measurements came at a time of unusually high galactic cosmic-ray flux, as sunspot activity is at its lowest since the dawn of the space age.

High sunspot activity poses a risk of solar flares, which can produce their own cosmic rays during intense storms of solar radiation. But super-low sunspot activity is also a risk because there is less solar magnetic field to deflect incoming cosmic rays away from the inner Solar System.

Less sanguine, however, was a finding that the Moon seems to be emitting radiation of its own, probably as a by-product of galactic cosmic rays hitting its surface at atom-smashing speeds.

Scientists had long anticipated the lunar surface to be radioactive for this reason (and, in fact, the LRO carries an instrument specifically geared to detect this type of radioactivity) but they had expected to find neutron radiation there. The surprise, Spence says, is that CRaTER is also detecting something new, and powerful. "Although not yet definitive," he says, "the data suggest strongly that they are protons."

Still, even though the new radiation source probably adds about 30-40% to the dose, it shouldn't prevent long-term visits to the Moon, Spence says, even when levels of cosmic-ray bombardment are as high as they are now. "It's something we're used to dealing with."

Lunar faults

A third team, led by Mark Robinson of Arizona State University in Phoenix, has been using the LRO's camera to take detailed images of the lunar surface. Some of the more dramatic images are of the Apollo landing sites, with the astronauts' footprints clearly visible. But they have also taken important images of the surrounding terrain.

For example, Robinson says, the images are giving more details on fault scarps that may have been produced by the crust shrinking as the Moon cools. "This tells us something about stresses in the crust," Robinson says. "As the planet shrinks, something has to give, and it is the crust that buckles, forming these scarps."

One set of photos, from near the Apollo 17 landing site, reveals a scarp being pushed up and over some tiny impact craters, never before seen. From the images, Robinson says, the scarp is clearly younger than had been anticipated, "potentially in the order of 10 million years". However, this estimate is not terribly precise, he says. "It's not one million," he says, "but not a billion [years either]."

The upshot, he adds, is that scientists are finding that the Moon is much more geologically complex, and far more varied, than the Apollo astronauts could see from the handful of landing sites.