NEAR-SURFACE ICE ON MERCURY AND THE MOON - A TOPOGRAPHIC THERMAL-MODEL

A thermal model that can be easily adapted to craters of arbitrary sha pe is developed and applied to high-latitude impact craters on Mercury and the Moon, Chao Meng Fu crater at -87.5 degrees L on Mercury, an u nnamed bowl-shaped crater at 86.7 degrees L, on Mercury, and Peary cra ter at 88.6 degrees L on the Moon. For an assumed input topography and grid of surface elements, the model computes for each element the irr adiation from direct insolation and reflected and emitted radiation fr om other elements, taking into account shadowing by the walls of the c rater, partial obscuration of the solar disk near the poles and the di urnal, orbital, and seasonal cycles. Temperatures are computed over th e surface grid as functions of depth and time from the surface to a sp ecified depth and over the pertinent astronomical cycles, including th e effects of direct and indirect surface irradiation, infrared radiati on, heat conduction, and interior heating. Vapor fluxes and ice recess ion times are computed as functions of ice depth over the surface grid . Temperatures profiles, vapor fluxes, and ice recession times were co mputed for flat surfaces not associated with craters near the poles of Mercury and the Moon. It was found that water ice could have existed throughout geologic time within the maximum radar detection depth of r ecent observation of Mercury (J. K. Harmon and M. A. Slade, 1992, Scie nce 258, 640-643) poleward of similar to 87-88 degrees L. on Mercury a nd poleward of similar to 73 degrees L, on the moon. For Chao Meng Fu crater it was found that similar to 40% of the crater floor is permane ntly shadowed from direct solar insolation, while the remainder of the crater floor is periodically illuminated by a partially obscured Sun. Temperatures at the upper levels of the south wall can slightly excee d 550 K. Surface temperatures in the permanently shadowed region of th e crater floor are under similar to 130 K, which could have allowed wa ter ice to exist throughout geologic time within the radar detection d epth of recent observation of Mercury. For the small bowl-shaped crate r on Mercury, it was found that most of the crater is permanently shad owed from direct solar radiation, except for a narrow semicircular ban d bordering the north rim. However, temperatures in the permanently sh adowed region periodically reach a maximum near similar to 315 K due t o efficient heating of the small crater by thermal emission and reflec tion from the small sunlit region, which periodically reaches temperat ures exceeding 630 K. Water ice could not have existed throughout geol ogic time anywhere in this crater within the radar detection depth. Fo r Peary crater on the Moon, the entire crater poor is permanently shad owed from direct solar insolation with maximum temperatures under 120 K. The upper level of the north wall periodically reaches a maximum te mperature near 310 K. The low temperatures on the crater floor would h ave allowed water ice to exist near the surface throughout geologic ti me, provided that the Moon's obliquity was always as low as it is at p resent. (C) 1994 Academic Press, Inc.