Abstract

Magnetic anomalies near the lunar poles correlate strongly with water-ice deposits, but the physical mechanism behind this relationship has remained qualitative. Here we develop the first quantitative framework showing how crustal magnetic fields create mini-magnetospheres capable of protecting polar cold traps from solar-wind sputtering, though only hundreds of nanotesla in strength. Using SI-consistent pressure balance, grazing-incidence solar-wind geometry, and standoff-distance scaling, we show that polar arrival angles (≈85° from vertical) amplify magnetic deflection efficiency by more than an order of magnitude. Even 100–200 nT anomalies can generate upstream bow shocks ~30–40 km from the source and produce extended magnetic shadows that reduce ion flux by factors of 5–10.

We couple this bow-shock geometry to sputtering and delivery budgets to compute ice accumulation over 3.5 Gyr. The model predicts 5–10× thicker ice in magnetically protected regions, asymmetric “comet-tail’’ spatial patterns within craters, and a natural explanation for the long-standing puzzle of the south-to-north polar ice asymmetry. All predictions are directly testable using existing Diviner, LEND, and Kaguya datasets, and forthcoming missions such as VIPER and Lunar Vertex.

This work converts the correlation identified by Hood et al. (2022) into a causal, predictive mechanism and provides a framework for mapping ice abundance anywhere on the Moon using magnetic topology and solar-wind geometry alone.

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1. Introduction

1.1 The Lunar Polar Ice Paradox

Water ice was definitively confirmed at the lunar poles in 2018 (Li et al., 2018), residing in permanently shadowed regions (PSRs) where temperatures remain below 110 K. These deposits represent both a scientific puzzle and a practical resource for future exploration. The puzzle is simple to state and surprisingly hard to resolve: how does water ice survive for billions of years on an airless body constantly bombarded by the solar wind?

The Moon has no global magnetic field and only a ghost-thin exosphere—neither provides meaningful protection from the ∼400 km/s stream of charged particles flowing outward from the Sun. Solar wind sputtering should erode exposed ice at rates of ~0.1–1 monolayers per 10⁵ years (Crider & Vondrak 2000; Farrell et al. 2019). Integrated over the 3–4 billion year age of the polar cold traps, this predicts complete removal of surficial ice unless one of three mitigating conditions holds:

1. Continuous replenishment
   Cometary and asteroidal impacts deliver water to the lunar surface, but most of it is vaporized, and only a small fraction migrates into PSRs. Delivery helps, but does not by itself offset steady sputtering losses.
2. Subsurface sequestration
   Regolith mixing (micrometeorite gardening) can bury ice over time, but surface layers remain exposed and should still be eroded faster than they are replenished.
3. Solar wind shielding
   Some mechanism must reduce the flux of incoming ions enough to slow sputtering by at least an order of magnitude.

The third explanation—shielding—became compelling only recently. Hood et al. (2022) mapped crustal magnetic anomalies at the lunar poles using Kaguya LMAG data and found a striking correlation: PSRs containing strong crustal magnetic fields tend to host more ice, while thermally similar PSRs lacking anomalies tend to be ice-poor. The implication is that even weak, localized magnetic fields can alter the solar wind flow enough to change the long-term volatile budget.

What was missing was the physics: how does a 100–200 nT magnetic patch produce a protected zone large enough to matter? Why does grazing incidence at the poles amplify the effect? And what ice distributions should this mechanism produce?

This work provides the quantitative explanation.