Mercury's crater ice may have come from a single comet or asteroid

New research suggests that the ice found in Mercury's permanently shadowed craters likely originated from a single comet or asteroid impact. Despite being the closest planet to the Sun, Mercury harbours frozen water in deep polar craters where sunlight never reaches. Scientists are working to understand how this ice was delivered.

Mercury, the solar system's innermost planet, has long puzzled scientists with one surprising feature: deposits of water ice hiding in the permanently shadowed floors of craters near its poles. Now, new research suggests this ice may not have accumulated gradually over billions of years from countless impacts — instead, it could have been delivered in a single dramatic event by one comet or asteroid.

Despite scorching daytime temperatures that can exceed 400°C, Mercury has almost no atmosphere to distribute heat, meaning the floors of certain polar craters never see sunlight and remain permanently frozen. These cold traps have preserved ice for potentially billions of years, shielded from the Sun's intense radiation.

A Single Cosmic Delivery

The new hypothesis challenges earlier assumptions that Mercury's ice built up slowly through repeated impacts from water-bearing objects over geological time. Researchers now argue that the isotopic and spatial distribution of the ice is more consistent with a single large delivery event — a comet or asteroid carrying substantial quantities of water that struck the planet at some point in the distant past.

Mercury itself carries no liquid water on its scorching surface, and its thin exosphere — far too tenuous to be called a true atmosphere — contains no water vapour under normal conditions. This makes the polar ice deposits all the more remarkable as a preserved record of ancient cosmic collisions.

What This Means for Planetary Science

Understanding the origin of Mercury's ice has broader implications for planetary science, including questions about how water was distributed across the inner solar system in its early history. If a single impact could deliver enough ice to account for Mercury's polar deposits, similar processes may have played a role in delivering water to other rocky planets — including early Earth.