The source of the significant water ice deposits hidden in Mercury’s polar regions has been a topic of debate among researchers. A new study, published in the Journal of Geophysical Research: Planets, suggests that these deposits were accumulated in only one Mercurian day (176 Earth days) by a large impactor, such as a comet or asteroid. While previous studies have suggested a similar scenario, this is the first study to fully model the impact. Furthermore, these new models suggest that the impactor may have been larger and slower than previously suggested.
Mercury’s mysterious water ice
Being the closest planet to the sun, Mercury sees daytime temperatures of up to 430°C (806°F). On top of that, Mercury doesn’t have a true atmosphere. Instead, it has an ultra-thin, tenuous layer of gas, called an exosphere, in which gases are constantly blown into space and then replenished by the solar wind. While these aspects of Mercury should make water retention extremely difficult, both Earth-based and orbital observations have found reflective areas that indicate the presence of water ice hidden in permanently shadowed regions (PSRs) near Mercury’s north and south poles.
Scientists have suggested several potential sources of the ice found in PSRs. Some hypotheses include steady delivery by micrometeoroids, solar wind, or a single large, volatile-rich impact. Some studies have found that the ice appears to be relatively pure and “young” (at only a few 100 million years). These findings suggest a rapid, episodic delivery rather than slow accumulation, according to more recent studies.
It’s thought that the impactor that created the 97 km diameter Hokusai crater on the surface of Mercury may be the origin of the water ice now found in the PSRs. The PSRs are thought to act as cold traps, which have temperatures cold enough to harbor the ice even in Mercury’s harsh environment.
Simulating water release scenarios
To determine the details of how a Hokusai-sized impact (from a 17 km diameter, 30 km/s comet or asteroid) might result in the transport and distribution of water to PSRs on Mercury, the team involved in the new study conducted models incorporating updated maps of PSRs and realistic surface temperature models.
They compared two scenarios: one in which water is released into a thin exosphere, and another where water is released into a dense, impact-generated atmosphere. The first scenario was simulated to update older estimates of the efficiency of water transport in Mercury’s exosphere, while the second actually simulated the impact with a range of impact parameters for the Hokusai-forming impactor.
The simulations showed that a Hokusai-scale impact could deliver about 2.3 × 10¹³ kg of water ice to Mercury’s polar cold traps, which matched the lower end of current estimates for the amount of polar ice. Less than an hour after the impact, the impact-generated water vapor would have expanded to entirely surround the planet, creating a temporary, water-rich atmosphere. Much of this atmosphere would be quickly broken down by interactions with photons in a process called photolysis.
The rest of the water manages to migrate to the poles and into the PSRs in the simulation. In a large enough impact, the simulations showed that a process called atmospheric self-shielding greatly increases the fraction of water reaching the cold traps, while decreasing the amount lost in photolysis, compared to the first baseline simulations. This also resulted in a more even distribution between the poles.
“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.
“Due to the efficacy of atmospheric self-shielding from photolysis, much more water—22.4% of the mass modeled (i.e., ∼31% of non-escaping vapor)—is cold-trapped in the aftermath of the Hokusai-like impact, compared to 3.4% of non-escaping vapor in the baseline simulation. The slower rate of photolysis also allows more water from the northern hemisphere impact to reach the south polar cold traps than in the baseline, optically thin scenario,” the study authors explain.
A larger, slower impactor for thicker ice deposits
One aspect of these simulations seemed off to the researchers, namely that the pools of ice were too thin to match observed data. Data from observations suggested that the ice was meters thick, while the simulations showed that the ice was only tens of centimeters thick, suggesting that certain parameters may need tweaking.
The team writes, “While the total mass of water found to be delivered to the poles in this work is consistent with previous estimates, we also find that the resulting deposits may be too thin (37 cm at most, compared to the several meters required to be radar-bright). This suggests that if a single impact did indeed deliver the bulk of Mercury’s polar water, a slower impactor larger than that modeled here may be required.”
The researchers note that there are limitations in their study, as they only modeled water and not other volatiles from the impact. Processes that occur on time-scales longer than one Mercurian day, like impact gardening or space weathering that could affect ice after deposition were also not included.
The team says that further modeling of different impact parameters, such as size, speed, angle is needed, and that observations from upcoming missions like BepiColombo could provide more data on ice thickness and distribution.
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Publication details
Parvathy Prem et al, Modeling the Delivery of Mercury’s Polar Ice by a Volatile‐Rich Impact, Journal of Geophysical Research: Planets (2026). DOI: 10.1029/2025je009399
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Mercury’s water ice may have been deposited by a larger, slower impactor than previously thought—in only one day (2026, May 26)
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