Water ice exists in permanently shadowed craters at Mercury’s poles, despite the planet’s proximity to the Sun and daytime temperatures exceeding 430°C. For years, scientists debated whether this ice arrived gradually through micrometeoroids and solar wind or in a single, large event. A 2026 study published in the Journal of Geophysical Research: Planets now suggests the latter: a massive impact, likely forming the 97-kilometer Hokusai crater, could have delivered the majority of Mercury’s polar ice in just one Mercurian day—176 Earth days.

The researchers modeled two scenarios: one where water vapor entered Mercury’s thin exosphere, and another where the impact created a dense, temporary atmosphere. In the latter case, atmospheric self-shielding helped protect water molecules from being broken apart by sunlight, allowing more to reach the poles. Simulations showed such an impact could deliver up to 2.3 × 10^11 kilograms of water—matching lower estimates of current ice mass.

However, a key discrepancy remains: the model produced ice layers up to 37 centimeters thick, while radar observations suggest deposits several meters deep. This implies the actual impactor may have been larger and slower than simulated. The study also didn’t account for other volatiles, and researchers await data from the BepiColombo mission to refine ice thickness and distribution.