Mercury can hold water ice because some deep polar crater floors remain permanently shadowed and extremely cold. Sunlight heats much of the planet, but it never reaches every crater. Spacecraft observations support the presence of water ice in these cold traps, sometimes beneath a dark covering.
A world of extremes
Mercury’s surface can become intensely hot in sunlight, but temperature depends strongly on location. Near the poles, the Sun stays low on the horizon. Ridges can block its light from crater floors, leaving patches in darkness for geological timescales. In these “cold traps,” water molecules do not receive enough energy to escape easily. NASA’s MESSENGER mission found several independent lines of evidence consistent with polar water ice. [1]
Where could the water have come from?
Comets and water-rich asteroids may have delivered ice during impacts. Some water could also have escaped from Mercury’s interior. Once a water molecule reached a permanently shaded crater, it could freeze and remain there. The evidence does not mean the whole planet is icy: deposits are localized, and some may be buried beneath darker material.
What observations tell us
Radar-bright patches near Mercury’s poles were identified before spacecraft mapped the region in detail. MESSENGER then measured hydrogen-rich deposits and observed crater surfaces whose temperatures allow ice to persist. Researchers combine these clues because no single measurement alone provides a simple photograph of a thick ice sheet. The results make Mercury a useful example of how sunlight, topography and temperature can create tiny cold refuges.
Does Mercury have ice everywhere?
No. Strong evidence points to ice in some persistently shadowed polar craters, not across the sunlit surface.
Sources and evidence
- NASA MESSENGER — Water Ice on Mercury ↗
MESSENGER imaging and interpretation of water ice in permanently shaded polar craters.
- NASA NSSDC — Ice on Mercury ↗
Evidence and context for radar-bright polar deposits and cold traps.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against the linked institutional and research sources.