The galaxy and the solar system: Edited by R. Smoluchowski, J. N. Bahcall, and M. S. Matthews. Pp. 483. University of Arizona Press. 1987. $29.95

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Radar images have revealed the possible presence of ice deposits in Mercury's polar regions. Although thermal models indicate that water ice can be stable in permanently shaded regions near Mercury's poles, the ultimate source of the water remains unclear. We use stochastic models and other theoretical methods to investigate the role of external sources in supplying Mercury with the requisite amount of water. By extrapolating the current terrestrial influx of interplanetary dust particles to that at Mercury, we find that continual micrometeoritic bombardment of Mercury over the last 3.5 byr could have resulted in the delivery of (3–60)×1016g of water ice to the permanently shaded regions at Mercury's poles (equivalent to an average ice thickness of 0.8–20 m). Erosion by micrometeoritic impact on exposed ice deposits could reduce the above value by about a half. For comparison, the current ice deposits on Mercury are believed to be somewhere between ∼2 and 20 m thick. Using a Monte Carlo model to simulate the impact history of Mercury, we find that asteroids and comets can also deliver an amount of water consistent with the observations. Impacts from Jupiter-family comets over the last 3.5 billion years can supply (0.1–200)×1016g of water to Mercury's polar regions (corresponding to ice deposits 0.05–60 m thick), Halley-type comets can supply (0.2–20)×1016g of water to the poles (0.07–7 m of ice), and asteroids can provide (0.4–20)×1016g of water to the poles (0.1–8 m of ice). Although all these external sources are nominally sufficient to explain the estimated amount of ice currently at Mercury's poles, impacts by a few large comets and/or asteroids seem to provide the best explanation for both the amount and cleanliness of the ice deposits on Mercury. Despite their low population estimates in the inner solar system, Jupiter-family comets are particularly promising candidates for delivering water to Mercury because they have a larger volatile content than asteroids and more favorable orbital and impact characteristics than Halley-type comets.