ICARUS 125, 195–211 (1997) ARTICLE NO. IS965601 Mercury’s Polar Caps and the Generation of an OH Exosphere R. M. KILLEN, J. BENKHOFF, AND T. H. MORGAN Department of Space Science and Instrumentation, Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78228–0510 E-mail: rosemary@whipple.space.swri.edu Received March 14, 1996; revised June 28, 1996 face of the Moon by interaction of solar wind protons and oxygen-bearing minerals in the soil. Previous to the We predict the OH column that will be present in the polar regions of the mercurian exosphere for physically realistic ice Mariner 10 flyby, this process, known as chemical sput- deposits at the poles, including both surface and buried ice. tering (Roth 1983), was suggested by Thomas (1974) as a The probable rates of accretion by meteoritic, asteroidal, and source of water on Mercury. Nevertheless, the discovery cometary sources are computed and compared with loss rates. of radar-bright poles on Mercury came as a surprise. More The rate of accretion of water at the poles from the nominal recently, Potter (1995) suggested that water produced by meteoritic infall is 2.5–8  10 8 molecules cm 2 sec 1 . Including proton sputtering of silicates could be a source of ice in the uncertainty in meteoritic plus asteroidal infall rate, the the mercurian polar region. accretion rate is 1–12  10 8 cm 2 sec 1 . For T  115 K, the Although exposed water ice was shown to be stable limiting loss process for surface ice is vaporization by microme- against thermal evaporation at temperatures lower than teoroids. The loss rate due to meteoritic vaporization of freshly deposited ice, with subsequent dissociation to OH, is about 112 K, the modeled temperatures for flat surfaces at lati- 1–2  10 8 cm 2 sec 1 . Thus, the net accretion rate from meteor- tudes above 86° are 140–180 K (Paige et al. 1992); however, itic impact is 0–4.5 m/4  10 9 years. Since the steady-state thermal modeling (Butler et al. 1993) showed that tempera- influx of water from meteoroids equals or exceeds the loss rate tures could remain as cold as 60 K in the interiors of high- from all processes, any additional water accreted from a comet latitude deep craters, and therefore water would be stable or extinct comet nucleus would be retained. The most probable against thermal evaporation in permanently shadowed po- value of the depth of water at the pole from comet impacts is lar craters. Subsequently, high-resolution radar maps (Har- 3 m, with large uncertainties. The background zenith OH mon et al. 1994) showed that the regions of high radar 3085-A ˚ emission from vaporized meteoritic material in the ab- reflectivity are confined to the interiors of craters in both sence of ice deposits is expected to be 12 R in the equatorial the north and south polar regions, lending support to the region and 0.3–0.7 R above the poles at aphelion. The back- theory that condensation and cold trapping of a volatile ground exceeds the emission from an outgassing source from buried ice deposits at 112–118 K. An OH exosphere resulting substance are responsible for the radar anomalies. All of from buried ice deposits would be difficult to observe from the source craters that have been classified are relatively ground-based or Earth-orbiting instruments, but fresh deposits pristine, and thus do not have degraded rims. The southern would be easily observable. A UV spectrograph in orbit about polar feature is associated with the crater Chao Meng Fu, Mercury that could determine both latitudinal variations and whereas the northern polar features are associated with scale heights could be used to infer buried deposits at T  the crater Desperez and a number of smaller unnamed 112 K or surface ice deposits.  1997 Academic Press craters. Most of the larger features in the north are on the unmapped hemisphere. Other radar-bright features found at lower latitudes have been associated with a fresh Tycho 1. INTRODUCTION class impact crater, a possible shield volcano, and a linear feature possibly associated with smooth plains, respectively Radar mapping of mercurian polar regions revealed (Harmon and Slade 1995). bright polar anomalies that were attributed to the presence Rawlins et al. (1995) considered exogenic sources for of several meters of water ice (Harmon and Slade 1992, water at Mercury, and concluded that one-third of the Slade et al. 1992). The possible presence of water ice at predicted amount of water (several meters) could be ac- the lunar pole has been the subject of numerous papers creted by meteoritic bombardment, assuming that the me- over the past three decades, beginning with the pioneering teoroids contain 10% water; however, they did not consider work of Watson et al. (1961), who concluded that lunar polar regions were possible cold traps for water ice. Opik loss processes at the poles. We consider accretion rates by meteoroids, asteroids, and comets versus prompt and slow (1962) speculated that water could be created on the sur- 195 0019-1035/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.