Although the details are probably complex, the concept of lunar ice deposition is simple. Water ice is stable in the cold, dark areas near the lunar poles. Any water that is made or deposited on the Moon is not stable in sunlit areas, and so will migrate across the surface. If it gets into one of these polar cold traps, it is there forever – no known process exists to remove it. Thus, even though the addition of water is extremely slow, over very long periods of time, a substantial amount of water may accumulate there.
But what is the physical nature of these deposits? Our expectations derived from experience here on Earth are probably misleading. We deal daily with water in liquid form and ice on Earth is usually made by the freezing of liquid water. This results in crystalline ice, as water molecules in solid form assume an ordered, tightly bonded lattice structure. As everyone knows, this material is both hard and tough and greatly resists attempts to break or gather it using normal digging tools. Water also can crystallize directly from vapor form into a solid as frost, usually found as an extremely thin coating that is very soft and easily scraped and removed from the object on which it forms.
Water that freezes within soil forms a tough, indurated deposit that can be quite difficult to dig or excavate. In the polar regions of the Earth, this material is frozen solid year-round and is called permafrost. Permafrost is extremely hard and difficult to excavate. Buildings in arctic regions require heavy equipment to dig and move the permafrost, including the use of explosives to break up the rock-hard frozen soil. If lunar ice is like this stuff, it will be extremely difficult to dig up and mine.
In contrast, snow is soft and easy to excavate. Snow is created when precipitation droplets (rain) freeze before they land on the ground. Typically, airborne dust particles will nucleate these droplets. Small drops have time to crystallize into magnificent ice crystals which famously, are each unique and individual. Sometimes larger water drops freeze quickly in flight and form ice blobs which may land on the ground as hail. In any event, if this material accumulates on the ground, we have a porous, weakly bonded deposit that is easily scooped up, usually by cursing inhabitants wielding large flat shovels.
Neither of these two accumulation scenarios occur on the Moon. We don’t know whether lunar ice is deposited (e.g., by comets hitting the Moon) or made (e.g., by solar wind hydrogen reacting with mineral surfaces). But however it is deposited, the water exists as individual molecules in gaseous form. Although this water is found all over the Moon, it is not stable everywhere. The molecules hop around the surface randomly, not slowing down until they land at a cooler locality and don’t stop until they reach a cold trap. The Moon loses most of these water molecules by a variety of mechanisms, including escape, disassociation and combination with minerals. The lucky few that reach a polar cold trap are there forever.
So what form do lunar ice deposits take? They are not now and never have been in liquid form, so crystallization into dense, “ponds” of ice is not likely. This lack of history as a liquid also means that “permafrost” (at least as we understand that term from terrestrial experience) is not likely either. Both of these ice forms ultimately require a freeze-thaw cycle, even if the time frame for such a cycle is hundreds of years. The lunar cold traps are cold now and have been for billions of years. And for this length of time, they have been gathering water molecules, sometimes at very high rates of accumulation (as when a comet strikes the Moon nearby) but usually at very slow, steady rates of accumulation.
Lunar ice probably is very porous, or at least “solid” but weakly bound together. The tight bonding of crystalline ice is made during the transition from liquid to solid during freezing. This doesn’t happen on the Moon; the water is added to the surface through direct ballistic deposition as individual molecules. In addition to the accumulation of water in the form of extremely tenuous vapor, dust and soil particles may interact with the water, creating a deposit with variable strength and water content. Even this material is likely to be loosely bound, as this mixing occurs at low temperatures and the water does not have a chance to re-crystallize, the usual reason for the steel-like hardness of permafrost. In astrophysics, a fine-grained, loosely bound structure is referred to as “ fairy castle structure.”
Do we have any evidence that this guess may be correct? We have only a few indirect clues at present. The ejecta plume observed during the impact of the LCROSS upper stage was unusually narrow. The science team suggested that this was a result of impact into an unusually low density soil; the term they used to describe it was “fluffy.” In addition to the high CPR fill of anomalous craters seen in the Mini-RF radar images of the poles (which we interpret as ice), we also observe anomalously low CPR in the areas surrounding the anomalous craters. Extremely low CPR implies fine-grained, lower than average density deposits with few rocks. Yet because polar ice is geologically young (less than a couple of billion years), if there were rock-hard, crystalline ice in abundance, we might expect a higher than average radar CPR, caused by abundant angular blocks excavated by impacts. Such a signal is not observed.
Admittedly, the evidence for this story is very weak. To determine the true physical and chemical nature of lunar polar ice, we must examine and study it in detail from a suitably equipped surface rover. Such a mission has been repeatedly proposed and I note that it is one of the proposed mission studies in the National Academy’s Planetary Exploration Decadal Survey. For a resource that may change the rules of spaceflight, determining its properties should be a high priority for exploration.