Astronomy
The Moon's Frozen Shadows
Quick fact
Craters of eternal darkness are permanently shadowed regions near the lunar poles where temperatures remain low enough to trap water ice and other volatiles for billions of years.
Why this is interesting
Imagine a place where the sun hasn't shone for billions of years—what secrets might be locked in that endless night?
Read the full explanation
Understanding The Moon's Frozen Shadows
The Moon has a very small axial tilt—only about 1.5 degrees—so its poles are almost perpendicular to the Sun's rays. Deep craters near the poles have floors that never receive direct sunlight. These permanently shadowed regions act as cold traps: any volatile molecule, like water, that enters them freezes and stays frozen because the temperature is always extremely low, typically below -200°C. Over billions of years, water delivered by comets, asteroids, or formed by the solar wind interacting with lunar soil accumulates in these cold traps, preserving ancient ice that records the history of the solar system.
A deeper explanation
Permanently shadowed regions (PSRs) exist because the Moon's spin axis is nearly perpendicular to the ecliptic plane. In deep polar craters, the crater rim blocks sunlight year-round. Without direct solar heating, these areas radiate heat to space and reach equilibrium temperatures as low as 25–40 K. Volatile substances—water, carbon dioxide, methane—have very low vapor pressures at these temperatures, so once they land on the surface, they cannot sublimate away. This process, called cold trapping, can preserve ice for geological timescales. The ice likely originates from multiple sources: cometary impacts, asteroidal debris, and solar wind protons reacting with oxygen in the regolith to form hydroxyl and water. Over time, these volatiles migrate across the surface, hopping from warmer regions until they become trapped in PSRs. Evidence for ice comes from neutron spectroscopy, radar reflections, and impact experiments like LCROSS. The ice layers may be mixed with regolith and could be ancient, offering a pristine record of volatile delivery to the inner solar system. Understanding these deposits is crucial for both planetary science and future in-situ resource utilization.