For decades, scientists have wondered if frost persists inside the dark and cold craters of the Moon's poles. The recent discovery of unusually bright areas near the Moon's south pole suggests this very well may be the case. But as a potential source of water for aspiring lunar colonists, the quantity of this surface frost may come as a disappointment.
Credit: NASA's Goddard Space Flight Center/Scientific Visualisation Studio
We tend to think of the Moon as a cold, dead place, but with no atmosphere to protect it, the lunar surface warms to a scorching 100C during the day, making it difficult for frost to remain at the surface. That said, scientists have suspected that frost may exist inside of dark craters found at the poles, but scant evidence exists to support this claim. Last year, NASA research suggested that patterns of suspected surface ice correlated well with the tilting of the Moon. Scientists have also found traces of water molecules in the thin layer of gases above the lunar surface. But as for hard evidence of frozen water on the Moon, that remains elusive.
In an effort to solidify the case for ice on the Moon, a research team led by Elizabeth Fisher from the University of Hawaii at Māno used instruments on board NASA's Lunar Reconnaissance Orbiter (LRO) to take surface temperature measurements, and to illuminate the lunar surface with laser pulses. These observations revealed unusually bright regions at the southern pole indicative of frost, where the temperature is consistently low enough to sustain water in a solid state. The details of this discovery now appear in the science journal Icarus.
"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," noted Fisher in a press release.
The frost — if that's what it is — was found in permanently dark areas, namely the floors of deep craters that don't receive direct sunlight. In these craters, temperatures remain below minus 163C, potentially allowing water ice to persist for millions or billions of years.
These observations suggest that surface water ice deposits are thin, and mixed in with the surface layer of soil, dust and small rocks known as regolith. Such deposits appear to be scattered around the lunar surface in a series of small patches. The researchers estimate that the total volume of frozen water strewn across the southern polar region is only enough to fill an Olympic-sized swimming pool — bad news for future lunar colonists, who will need water to survive. If we're going to make the Moon our home, we're going to have to find another source of this precious liquid.
This study bolsters the case that frost exists on the Moon, at least in the southern polar regions. The same signatures were not observed at at the Moon's north pole. That said, previous data collected by India's Chandrayaan-1 spacecraft suggests that icy deposits exist at the northern pole as well. Future investigations will have to resolve these contradictory findings.
"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.
If frozen water truly exists on the Moon, a big question now is to understand how it got there. One possibility is that it arrived via icy comets or asteroids, which would make the water as old as the Solar System itself. Alternately, the water could have been produced by chemical reactions driven by the solar wind, making it much more recent in terms of age. Both scenarios may be true. Another tantalising possibility — one that should make future lunar colonists excited — is that ancient ice deposits may be buried deep below ground. But that has yet to be proven.