Using data from NASA’s Lunar Reconnaissance Orbiter (LRO), scientists, including University of Hawaiʻi at Mānoaresearcher Paul Lucey, 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 laser light is reflected off the moon’s surface from one of the LRO instruments.
“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. Fisher carried out the data analysis while doing research with Lucey at the UH Mānoa Hawaiʻi Institute of Geophysics and Planetology after earning her undergraduate degree. She is now a graduate student at Brown University.
The icy deposits appear to be patchy and thin, and it’s possible that they are mixed in with the surface layer of soil, dust and small rocks called the regolith. The researchers say they are not seeing expanses of ice similar to a frozen pond or skating rink. Instead, they are seeing signs of surface frost.
“We estimate that the ice detected would fill about one Olympic-sized swimming pool,” said Lucey.
The frost was found in permanently dark areas—located on the floors of deep craters that don’t receive direct sunlight—where temperatures remain below minus 260 degrees Fahrenheit (minus 163 degrees Celsius). Under these conditions, water ice can persist for millions or billions of years.
More than a half-century ago, scientists suggested that permanently dark areas could store water ice, but confirming that hypothesis turned out to be challenging.
“These findings demonstrate once again the value of studying the moon from orbit long-term,” said John Keller, the LRO project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “All of this work begins with comprehensive data sets made up of years’ worth of continuous measurements.”
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, Texas, and a co-author on the study.