Scientists using data from NASA's lunar orbiter have identified bright areas in craters near the Moon's south pole that are cold enough to have frost present on the surface.
The new evidence comes from an analysis that combined surface temperatures with information about how much light is 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 the journal Icarus.
The icy deposits appear to be patchy and thin, and it is possible that they are mixed in with the surface layer of soil, dust and small rocks called the regolith.
The researchers said they are not seeing expanses of ice similar to a frozen pond or skating rink. Instead, they are seeing signs of surface frost.
Cold traps are permanently dark areas - located either on the floor of a deep crater or along a section of crater wall that does not receive direct sunlight, where temperatures remain below minus 163 degrees Celsius.
Under these conditions, water ice can persist for millions or billions of years.
Understanding the nature of these deposits has been one of the driving goals of Lunar Reconnaissance Orbiter, or LRO, which has been orbiting the Moon since 2009.
Fisher, who carried out the analysis while at the University of Hawaii at Manoa, 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.
In these comparisons, the coldest areas near the south pole also were very bright, indicating the presence of ice or other highly reflective materials.
The researchers looked at the peak surface temperatures, because water ice won't last if the temperature creeps above the crucial threshold.
The study strengthens 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.
"What has always been intriguing about the Moon is that we expect to find ice wherever the temperatures are cold enough for ice, but that's not quite what we see," said Matt Siegler, a researcher with the Planetary Science Institute in Dallas, and a co-author on the study.