When you look up at the moon tonight, imagine there’s frost there. Not everywhere, just a few deep craters at the moon’s south pole, but an evocative image just the same. Scientists using data from NASA’s Lunar Reconnaissance Orbiter (LRO) have identified bright areas in craters near the moon’s south pole cold enough to have frost present on the surface. They base their conclusion on new evidence that combines surface temperatures with how much light gets reflected off the moon’s surface.
“We found that the coldest places near the moon’s south pole are also the brightest places—brighter than we would expect from soil alone—and that might indicate the presence of surface frost,” said Elizabeth Fisher, the lead author of the study, published in Icarus.
While orbiting spacecraft detected ice at the moon’s south pole several years ago, this is a little different. The icy deposits appear to be patchy and thin, and they may even be mixed with the surface soil layer of dust and small rock, so it’s not like ice on a pond or solid ice beneath the surface. Picture instead frost rimming or covering small fragments of lunar soil creating patches that appear brighter just like a frosty lawn is brighter and whiter than a green one.
The frost was found in what are called cold traps close to the moon’s south pole. These are areas located either on the floor of a deep crater or along a section of a crater wall that never sees sunlight and where temperatures remain below –260° F (–162° C). Under these serious chill conditions, water ice can persist for millions or billions of years.
Fisher and her colleagues found evidence of lunar frost by comparing temperature readings from LRO’s Diviner instrument with brightness measurements from the spacecraft’s Lunar Orbiter Laser Altimeter, or LOLA. They learned that the coldest areas near the south pole were also very bright, indicating the presence of ice or other highly reflective materials. The peak temperature in the polar craters was crucial because water ice will vaporize away if the temperature creeps above a certain threshold. Together, the two studies strengthen the case that there is frost in cold traps near the moon’s south pole. So far, however, researchers have not seen the same signs near the moon’s north pole.
Water ice and other deposits also have been identified in cold traps near the north pole on Mercury. Though it’s the closest planet to the sun, Mercury appears to have up to 400 times more ice than the moon.
We still don’t know how old the moon’s ice is. If the water was delivered by comet and icy asteroid impacts, it could be very ancient, dating back to the earliest days of the solar system. But the water may also have been produced by hydrogen in the sun’s solar wind interacting with the lunar rock to produce water, in which case, it would be much younger. Scientists in the study point out that both processes may be true — there could be eons-old ice deposits buried below ground and newer water at the surface.
The moon continues to be a cool place, if you’ll excuse the pun. Someday the ice there may provide needed water for lunar colonies as well as help us understand the origins of Earth’s water.