Mercury's Polar Caps and the Generation of an OH Exosphere

Abstract

We predict the OH column that will be present in the polar regions of the mercurian exosphere for physically realistic ice deposits at the poles, including both surface and buried ice. The probable rates of accretion by meteoritic, asteroidal, and cometary sources are computed and compared with loss rates. The rate of accretion of water at the poles from the nominal meteoritic infall is 2.5–8 × 10 8molecules cm −2sec −1. Including the uncertainty in meteoritic plus asteroidal infall rate, the accretion rate is 1–12 × 10 8cm −2sec −1. For T< 115 K, the limiting loss process for surface ice is vaporization by micrometeoroids. The loss rate due to meteoritic vaporization of freshly deposited ice, with subsequent dissociation to OH, is about 1–2 × 10 8cm −2sec −1. Thus, the net accretion rate from meteoritic impact is 0–4.5 m/4 × 10 9years. Since the steady-state influx of water from meteoroids equals or exceeds the loss rate from all processes, any additional water accreted from a comet or extinct comet nucleus would be retained. The most probable value of the depth of water at the pole from comet impacts is 3 m, with large uncertainties. The background zenith OH 3085-Å emission from vaporized meteoritic material in the absence of ice deposits is expected to be 12 R in the equatorial region and 0.3–0.7 R above the poles at aphelion. The background exceeds the emission from an outgassing source from buried ice deposits at <112–118 K. An OH exosphere resulting from buried ice deposits would be difficult to observe from ground-based or Earth-orbiting instruments, but fresh deposits would be easily observable. A UV spectrograph in orbit about Mercury that could determine both latitudinal variations and scale heights could be used to infer buried deposits at T> 112 K or surface ice deposits.