Mercury, the solar system's innermost planet, may have acquired much of its polar water ice from a single massive asteroid impact, according to a study published recently. The research suggests that the collision delivered the ice within the span of one Mercurian day — roughly 59 Earth days.
The study focuses on the dynamics of a slow-moving asteroid, which would have allowed much of its icy material to survive the impact and settle in Mercury's permanently shadowed polar craters. These craters, shielded from the Sun's intense heat, have long been known to harbor water ice, but its origin has remained a puzzle.
Key to the hypothesis is the impact's timing and velocity. The researchers modeled scenarios where a slow impactor — traveling at lower speeds than typical asteroid strikes — deposited its water without vaporizing it. This process could explain both the quantity and distribution of ice observed at Mercury's poles by NASA's MESSENGER mission.
The findings challenge the prevailing view that Mercury's water came from multiple smaller impacts or cometary bombardment over billions of years. Instead, the study points to a single event as a dominant source, simplifying the story of how the planet closest to the Sun ended up with frozen water.
While the model is compelling, it relies on assumptions about asteroid composition and the precise conditions of the impact that remain unverified. Some planetary scientists argue that multiple sources are still needed to account for the full inventory of Mercury's polar volatiles, calling for further missions, such as the European-Japanese BepiColombo probe, to test the theory.