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 8 molecules cm −2 sec −1 . Including the uncertainty in meteoritic plus asteroidal infall rate, the accretion rate is 1–12 × 10 8 cm −2 sec −1 . For T 8 cm −2 sec −1 . Thus, the net accretion rate from meteoritic impact is 0–4.5 m/4 × 10 9 years. 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-A 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 T > 112 K or surface ice deposits.

[1]  W. Huebner,et al.  Solar photo rates for planetary atmospheres and atmospheric pollutants , 1984 .

[2]  E. Opik The lunar atmosphere , 1962 .

[3]  W. Ip,et al.  Steady-state injection of short-period comets from the trans-Neptunian cometary belt , 1991 .

[4]  M. Slade,et al.  Radar Mapping of Mercury: Full-Disk Images and Polar Anomalies , 1992, Science.

[5]  B. Donn,et al.  Structure and origin of cometary nuclei , 1981 .

[6]  T. H. Morgan,et al.  Potassium in the atmosphere of Mercury , 1986 .

[7]  A. Potter,et al.  Impact-driven supply of sodium and potassium to the atmosphere of Mercury , 1987 .

[8]  Thomas J. Ahrens,et al.  Cometary and meteorite swarm impact on planetary surfaces , 1982 .

[9]  M. S. Matthews,et al.  Hazards Due to Comets and Asteroids , 1992 .

[10]  Martin A. Slade,et al.  Mercury: Full-disk radar images and the detection and stability of ice at the North Pole , 1993 .

[11]  C. Chyba Terrestrial mantle siderophiles and the lunar impact record , 1991 .

[12]  J. Benkhoff,et al.  Influence of the Vapor Flux on Temperature, Density, and Abundance , 1995 .

[13]  Harold F. Levison,et al.  The Long-Term Dynamical Behavior of Short-Period Comets , 1993 .

[14]  D. Brownlee,et al.  A Direct Measurement of the Terrestrial Mass Accretion Rate of Cosmic Dust , 1993, Science.

[15]  T. H. Morgan,et al.  A non-stoichiometric model of the composition of the atmospheres of Mercury and the Moon , 1997 .

[16]  R. Killen,et al.  Spatial distribution of sodium vapor in the atmosphere of Mercury , 1990 .

[17]  D. Muhleman,et al.  Radar Images of Mars , 1991, Science.

[18]  T. Ahrens,et al.  Meteorite impact ejecta: dependence of mass and energy lost on planetary escape velocity. , 1977, Science.

[19]  M. Neugebauer The Quasi-Stationary and Transient States of the Solar Wind , 1991, Science.

[20]  Andrew P. Ingersoll,et al.  Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars , 1992 .

[21]  John W. Salisbury,et al.  Thermal Infrared Spectra of the Moon , 1995 .

[22]  R. Velez,et al.  Radar mapping of Mercury's polar anomalies , 1994, Nature.

[23]  Louis J. Lanzerotti,et al.  Ice in the polar regions of the Moon , 1981 .

[24]  H. J. Melosh,et al.  Impact erosion of the primordial atmosphere of Mars , 1989, Nature.

[25]  G. Schubert,et al.  Magnetism and thermal evolution of the terrestrial planets , 1983 .

[26]  Donald M. Hunten,et al.  Sulfur at Mercury, Elemental at the Poles and Sulfides in the Regolith , 1995 .

[27]  D. Muhleman,et al.  Mercury Radar Imaging: Evidence for Polar Ice , 1992, Science.

[28]  A. Potter,et al.  Discovery of Sodium in the Atmosphere of Mercury , 1985, Science.

[29]  W. Ip,et al.  Exchange of condensed matter among the outer and terrestrial protoplanets and the effect on surface impact and atmospheric accretion , 1988 .

[30]  R. Behrisch,et al.  Sputtering by Particle Bombardment III , 1981 .

[31]  J. Zimbelman Planetary impact probabilities for long-period comets , 1984 .

[32]  Michael F. A'Hearn,et al.  The fluorescence of cometary OH , 1988 .

[33]  A. Vasavada,et al.  The Thermal Stability of Water Ice at the Poles of Mercury , 1992, Science.

[34]  F. Hörz Impact cratering and regolith dynamics , 1977 .

[35]  T. H. Morgan,et al.  Limits to the lunar atmosphere , 1991 .

[36]  A. Lyle Broadfoot,et al.  The airglow spectrum, 3100-10,000 A , 1968 .

[37]  F. L. Whipple,et al.  Do comets play a role in galactic chemistry and gamma-ray bursts , 1975 .

[38]  S. Suess,et al.  Mercury: Magnetospheric processes and the atmospheric supply and loss rates , 1981 .

[39]  Bruce C. Murray,et al.  The behavior of volatiles on the lunar surface , 1961 .

[40]  Mark J. Cintala,et al.  Impact‐induced thermal effects in the lunar and Mercurian regoliths , 1992 .

[41]  E. Shoemaker,et al.  The Flux of Periodic Comets Near Earth , 1993 .

[42]  A. Potter Chemical sputtering could produce sodium vapor and ice on Mercury , 1995 .

[43]  J. W. Chamberlain Theory of planetary atmospheres , 1978 .

[44]  Robert A Kolvoord,et al.  Collision lifetimes and impact statistics of near-Earth asteroids , 1993 .

[45]  A. Kouchi Vapour pressure of amorphous H2O ice and its astrophysical implications , 1987, Nature.

[46]  D. Kessler Average relative velocity of sporadic meteoroids in interplanetary space , 1969 .

[47]  A. Sprague,et al.  Mercury: Evidence for Anorthosite and Basalt from Mid-infrared (7.3-13.5 μm) Spectroscopy , 1994 .

[48]  B. Hapke,et al.  Photometry and polarimetry of Mercury , 1988 .

[49]  Peter H. Schultz,et al.  Cometary collisions on the Moon and Mercury , 1980, Nature.