Mercury may have gained all of its unexpected water in a single day

Around 100 million years ago, Mercury’s surface underwent a sudden transformation that led to the formation of thick ice deposits at its poles. Despite the planet’s proximity to the Sun and extreme daytime temperatures exceeding 430°C (806°F), permanently shadowed craters at the poles harbor several meters of ice. New simulations led by Parvathy Prem at the Johns Hopkins Applied Physics Laboratory suggest that a single large impact event created these ice deposits over the course of one Mercurian day, equivalent to 176 Earth days. The research proposes that a slower, larger impactor struck Mercury, forming the prominent Hokusai crater visible today. This collision vaporized much of the impactor, generating a transient, water-rich atmosphere around the planet. Although this atmosphere was extremely tenuous and short-lived, the simulations indicate that over 20% of the water vapor could have migrated to the permanently shadowed polar regions, where it condensed and remained trapped as ice. This scenario aligns more closely with data from NASA’s Messenger spacecraft, which orbited Mercury from 2011 to 2015 and detected substantial ice deposits in these shadowed craters. Previous hypotheses suggested a smaller, faster impactor delivered the water, but the new model’s larger, slower collision better explains the volume of ice observed. The findings provide a detailed visualization of how Mercury’s ice could have formed rapidly, challenging earlier assumptions that the deposits accumulated gradually over time. This event marks a significant moment in Mercury’s geological history, offering insights into volatile delivery and retention on airless bodies in the inner solar system. Understanding Mercury’s ice origins has broader implications for planetary science, including the study of water distribution and preservation on other rocky planets and moons. It also sheds light on the processes that can create and maintain volatile reservoirs in extreme environments, which is crucial for future exploration and the search for life beyond Earth.