Mercury's Polar Deposits

Ground-based radar observations of Mercury have revealed the presence of unusually bright radar-reflective regions at the planet's north and south poles (Slade et al. 1992, Science 258, p. 635, and Harmon and Slade 1992, Science 258, p. 640). The similarity of both the strength and polarization behavior of these radar echoes to the observed radar characteristics of the icy Gallilean satellites and the residual south polar cap of Mars prompted the observers to conclude that water ice or some other volatile material might be present at the poles of the otherwise hot planet (see also Butler et al. 1993, J. Geophys. Res. 98, p. 15,003). 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. Katherine Rawlins, Kevin Zahnle, Luke Dones, and I have used 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 rate to that at Mercury, we find that continual micrometeoritic bombardment of Mercury over the last 3.5 billion years could have resulted in the delivery of (3–60) × 10¹⁶ g 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 micrometeorite 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) × 10¹⁶ g of water to Mercury's polar regions (corresponding to ice deposits 0.05–60 m thick), Halley-type comets can supply (0.2–20) × 10¹⁶ g of water to the poles (0.07–7 m of ice), and asteroids can provide (0.4–20) × 10¹⁶ g of water to the poles (0.1–8 m of ice). Although all these 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. In Figure 1, the amount of water delivered to Mercury from individual impactors in one simulation is shown as a function of (a) mass of impactor, and (b) collision probability. Asteroids are in green, Jupiter-family comets are in red, and Halley-type comets are in blue. Each impactor is marked by an open circle; stars depict the objects that deliver the most water in each simulation. Note that most of the water is supplied by a small number of objects with high masses and average-to-high impact probabilities.

Note:  This material was cannibalized from Moses et al. (1999, Icarus 137, p. 197). Further information about Mercury's polar deposits can be found within these papers and references therein.

Last updated
April 4, 2008