Unlike Earth, whose axial tilt with respect to the Sun drives the change of seasons, the spin axes tilts of Moon and Mercury are very small. This unique configuration causes craters and other topographic depressions located near the poles to cast highly stable shadows that could persist for billions of years. In the absence of atmospheric heat transfer, these so-called "permanently shadowed regions" may cold-trap and preserve water ice for geologic time-scales.
Telescopic and remote sensing observations conducted in the past decades have found thick, glacier-like, ice deposits near the poles of Mercury. At least some of these deposits have been shown to be fresh, possibly delivered by a comet or an asteroid that impacted the planet a few tens or hundreds of millions of years ago. Somewhat unexpectedly, parallel observations conducted on the poles of the Moon around the same time found little evidence for thick ice deposits. This difference is even more puzzling given the similar thermal environments of these two planetary bodies.
In this work, we infer the existence of thick ice deposits in simple craters near the south pole of the Moon. Simple craters are usually formed by smaller, less energetic impactors, and tend to be more circular and symmetric than larger complex craters. We have analyzed the morphology of approximately 15,000 craters on Mercury and the Moon, and found their depth/diameter ratios decrease near the poles by about 10% compared to lower latitudes. Additionally, their colder pole facing slopes were on average slightly shallower than their warmer equator facing slopes, also indicative of ice accumulation. On the Moon, previous studies have found evidence for surface ice near the south pole. By correlating these measurements with the depth/diameter ratio of craters, we further inferred the surface ice deposits may be exhumed or connected to the subsurface deposits through diffusion.
Considering the mean crater diameter in our catalog is about 4 km, the 10% decrease in craters' depth/diameter ratio corresponds to an average approximate infill of 10 meters. We use these results to re-estimate the total mass of the ice trapped in the lunar poles. Previous estimates computed the total area fraction occupied by lunar cold-traps on the Moon to be roughly 10,000 squared km. If all cold-traps on the Moon hide a 10 meter thick pure ice deposit, the total mass of water ice on the Moon could be estimated to be about 100 million metric tons. The possibility that thick ice-rich deposits exist on the Moon may not only help resolve the outstanding question regarding its low ice abundance relative to Mercury, but may also have practical applications in preparation for a future permanent lunar settlement.