Astronomy
Deposition of Water Ice in Permanently Shadowed Lunar Craters
Quick fact
Data from lunar orbiters suggest that permanently shadowed polar craters may store billions of tons of water ice, some of which might have been deposited over billions of years.
Why this is interesting
Imagine a place on the Moon that has never seen sunlight. Yet, hidden in its shadows, water ice may be silently accumulating. How does that happen?
Read the full explanation
Understanding Deposition of Water Ice in Permanently Shadowed Lunar Craters
On the Moon, certain craters near the poles have floors that never receive direct sunlight because the Sun never rises high enough above the horizon. These are called permanently shadowed regions. Because they are permanently dark, they are also extremely cold, with temperatures dipping below -230°C. In these frigid zones, any water molecules that wander in from space—from comet impacts, asteroid debris, or even interactions of the solar wind with lunar soil—will slow down, lose energy, and stick to the surfaces like frost forming on a cold window. Over time, layer upon layer of this icy frost can accumulate, creating deposits of water ice.
A deeper explanation
The key mechanism is cold trapping. Water molecules in the lunar exosphere hop along the surface due to thermal motion, each hop sending them into a ballistic trajectory. When one enters a permanently shadowed crater, it encounters surfaces so cold that on impact it cannot gain enough energy to escape. The molecule's thermal energy is insufficient to overcome the weak binding forces that hold it to the cold soil. Thus, it remains frozen. Over billions of years, this gradual accretion builds up substantial ice layers. This process not only preserves a record of past water delivery events but also makes these ice deposits a potentially valuable resource for future human exploration, supplying water and rocket propellant without needing to haul them from Earth.