Mercury’s polar ice deposits may have formed in just one Mercurian day after a giant comet or asteroid slammed into the planet, according to a new study published in the Journal of Geophysical Research: Planets. Researchers say the collision likely created a temporary water-rich atmosphere that spread vapor across the planet before part of it became trapped near the poles.
Scientists have long been puzzled by the presence of ice on Mercury, the closest planet to the Sun. Surface temperatures can exceed 430°C, and the planet has only a very thin exosphere, conditions that make the survival of water appear highly unlikely.
Yet radar observations and spacecraft data have repeatedly revealed bright reflective regions near Mercury’s poles. The new study suggests the impact that formed the 97-kilometer-wide Hokusai crater may also explain how water reached permanently shadowed craters, where temperatures remain cold enough for ice to persist over extremely long periods.
Mercury May Have Formed in Vapor
To test the idea, researchers simulated the aftermath of a collision involving a roughly 17-kilometer-wide comet or asteroid traveling at 30 kilometers per second. The models included updated maps of Mercury’s permanently shadowed regions and improved surface temperature estimates.
The team compared two scenarios. In the first, water released by the impact dispersed directly into Mercury’s thin exosphere. In the second, the collision generated a dense temporary atmosphere filled with water vapor. That second scenario produced dramatically different results.
The study found that less than an hour after impact, the vapor cloud had expanded around the entire planet.with a water-rich atmosphere. Sunlight quickly destroyed part of the water through photolysis, but a significant fraction survived and eventually migrated toward cold polar craters.
Researchers also identified a process known as atmospheric self-shielding. In this situation, dense water vapor blocks part of the incoming solar radiation, protecting other water molecules from being broken apart. The paper stated that:
“The large amount of water released in a Hokusai-scale impact means that this self-shielding effect has a strong influence; by the end of one solar day, ∼96% of the water vapor released in the collisionless, optically thin simulation was photodestroyed, compared to ∼46% in the impact-generated atmosphere simulation.”
Simulations Produced Billions Of Kilograms Of Ice
The simulations showed that a Hokusai-scale impact could deliver about 2.3 × 10¹³ kilograms of water ice to Mercury’s polar regions. Researchers say that amount matches the lower end of current estimates for the planet’s ice reserves.
The simulations also produced a more balanced distribution of ice between Mercury’s northern and southern poles. Because vapor lasted longer in the denser atmosphere scenario, material released in the northern hemisphere was still able to reach southern cold traps.
Atmospheric self-shielding also sharply increased the amount of water preserved after the collision. In the baseline simulation with a thinner atmosphere, only 3.4% of non-escaping vapor became trapped in cold regions. In the denser atmosphere model, that number rose to 22.4%.
The findings support the idea that Mercury’s ice may have arrived during a relatively short and violent event rather than through slow accumulation over billions of years. Most of the process unfolded within a single Mercurian solar day, equal to 176 Earth days.
The Ice Deposits May Still Be Too Thin
Even though the simulations produced large amounts of ice, one issue remained. The deposits formed in the models were thinner than the ice layers scientists believe exist on Mercury today.
The study found that the simulated deposits reached a maximum thickness of around 37 centimeters. Radar observations suggest some real deposits may measure several meters thick.
Because of that difference, researchers think the original impactor may have been larger and slower than the one tested in the simulations. A slower-moving object could potentially preserve more water before solar radiation destroyed it.
The team also highlighted several limitations in the study. The models focused only on water and did not include other volatile materials released during the impact. Longer-term processes such as space weathering and impact gardening were not included either.
Future observations from the BepiColombo mission, which is currently traveling the planet, could help scientists better understand the thickness and distribution of the planet’s hidden ice.
