1,4- 1Space Sciences Laboratory, University of California, Berkeley, CA, USA
- 2NASA Goddard Space Flight Center, Greenbelt, MD, USA
- 3American University, Washington, DC, USA
- 4Department of Physics, Umeå University, Umeå, Sweden
Even though it is the closest planet to the Sun, Mercury’s polar craters harbor large volatile deposits [1]. Observations of radar brightness [2], neutron absorption [3], and optical reflection [4] have led to the conclusion that the deposits are largely made up of water ice. Ultimately, a total water ice mass of 1013 – 1015 kg has been estimated [3].
Due to the low number of craters on the icy surfaces, it has been suggested that the ice has been deposited in the past few 100 million years [4], for example by a single cometary impact [5]. However, recent analysis of the ages of ice-filled craters indicates a significant correlation of crater age and water ice amount [6]. Such a behavior would rather point to a contribution from a steadily ongoing process. Previous work has discussed dust impacts and solar wind delivery as potential sources [7, 8]. Jones et al. [8] developed a model to quantify the accumulation of water in Mercury’s permanently shadowed regions (PSRs) following solar-wind H+ implantation. They estimate a small contribution on the order of 1013 kg to today’s deposits over the past billion years, assuming identical delivery rates as today.
However, observations of other stars have allowed us to constrain the behavior of the younger Sun and it has been established that it was much more active in the past. In particular, the mass loss rate via solar wind emission was orders of magnitude higher [9], which potentially provides a much-increased source of solar-wind implanted water. At the same time, the Sun’s Lyman alpha emission was much brighter [10], which increases the H2O photodissociation rate and limits the efficiency of migration into Mercury’s PSRs. Several studies have also suggested that Mercury’s dynamo history has evolved significantly in the past, likely requiring a much stronger early dynamo to explain today’s remanent surface magnetization [11].
In this study, we aim to quantify the potential solar wind contribution to Mercury’s H2O deposits by accounting for these different aspects. We perform hybrid simulations using the Amitis code [12] for a more intense young solar wind, considering a range of different dynamo strengths. Based on the results of solar wind H+ implantation, we calculate rates of water delivery into Mercury’s PSRs by accounting for the increased Lyman alpha strength in the past (up to a factor of 3).
When calculating the estimated total water amount accumulated over the past 3.7 billion years, we find that the result significantly depends on the assumptions regarding Mercury’s dynamo history. A gradual decrease of dynamo strength to today’s value typically leads to similar estimates as the original calculation from Jones et al. (2020) of a minor contribution of 1013 kg. However, if there were extensive time periods with a dynamo comparable to today’s strength or no dynamo at all, the calculated water deposits increase up to several 1014 kg, which would represent an important contribution to today’s ice deposits.
Our study thus further supports that solar wind implantation is a possible source of water ice in Mercury’s PSRs. The planet’s volatile deposits might also be closely connected to the history of the planet’s dynamo. Further characterization of Mercury’s PSRs with BepiColombo, as well as more detailed modeling and additional characterization of Mercury’s interior and its dynamo history will allow us to better understand the accumulation of water ice through solar-wind implantation.
References
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[5] C.M. Ernst, et al., Journal of Geophysical Research: Planets 123 (2018), 2628.
[6] S. Bertoli, et al., Planetary and Space Science 264 (2025), 106150.
[7] K. Frantseva, et al., Icarus 383 (2022), 114980.
[8] B.M. Jones, et al., The Astrophysical Journal Letters 891 (2020), L43.
[9] V.S. Airapetian, et al., The Astrophysical Journal 916 (2021), 96.
[10] I. Ribas, Proceedings of the International Astronomical Union 5 (2009), 3.
[11] I.S. Narrett, et al., EPSC-DPS2025-1118.
[12] S. Fatemi, et al., Journal of Physics: Conference Series 837.1 (2017).
How to cite: Szabo, P. S., Poppe, A. R., Airapetian, V. S., and Fatemi, S.: Estimating Contributions to Mercury’s Water Ice Deposits from the Young Solar Wind, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-708, https://doi.org/10.5194/epsc2026-708, 2026.
