💥 The day Mercury became icy in a single impact

Mercury, the closest planet to the Sun, seems like the last place you would expect to find ice. Yet telescopic observations in the 1990s, confirmed by the MESSENGER probe, revealed immense reserves of frozen water in polar craters that never see sunlight. A situation that has long puzzled researchers.

A new study proposes a surprising explanation: a single colossal impact could have transported and trapped this water in record time. In the span of a single Mercurian day, which is nonetheless 176 Earth days, an impact the size of the one that formed the Hokusai crater could have spread water across the entire planet, and that water would have persisted in permanent shadow zones.


To test this hypothesis, a team led by Parvathy Prem of the Johns Hopkins Applied Physics Laboratory used computer simulations. They simulate an impactor about 10.6 miles (17 km) in diameter hitting Mercury at about 67,000 mph (30 km/s). The impact generates a temporary dense atmosphere rich in water vapor that envelops the planet in a little over an hour, according to the researchers.

This water vapor atmosphere acts as a shield. It protects water molecules from the Sun's intense ultraviolet rays, slowing their destruction. Result: a large amount of water survives long enough to migrate to polar craters where it settles as ice. Most of this deposition occurs within a single Mercurian day, i.e., 176 Earth days, the study published in Journal of Geophysical Research: Planets indicates.

This discovery could indicate that Mercury's ice formed rapidly rather than gradually over long periods. It would also explain its apparent purity, as a sudden influx would limit mixing with other materials. Scientists hope that the BepiColombo mission, which is set to enter orbit around Mercury in November, will bring new clues about the origin of these polar deposits.

In the meantime, this single-impact scenario offers an elegant answer to a longstanding puzzle. It shows how a cataclysmic event can, in a matter of hours, redistribute volatile elements across an entire planet, transforming a torrid world into a reservoir of ice.