Vacuum Surface Environments and Processes I

Spring 2003

Exosphere at Surface

Objects:

Moon, Mercury, Asteroids, Phobos, Deimos; Inner Solar System Rocky bodies

Surface-bounded exospheres

Molecules are gravitationally trapped, but collisionless: mean free path > scale height

Molecules travel on ballistic trajectories

Atmosphere more in contact with surface and space than with itself

Outer Solar System Icy bodies

exist in very different surface environments

Consequences

No micrometeorite shielding;

smoothing and abrasion on Moon, Mercury

Unmodulated solar heating and reradiation.

Mercury 10x Moon; 180/0 Hot pole, 90/270 warm pole (2/1 Difference)

Unfiltered solar UV,X and Cosmic radiation degrades minerals, removes gases.

Trace atmospheres on the Moon and Mercury

O, Na, He, K, H on Mercury (in order of decreasing abundance)

O, Na, K result from vaporization of meteorites, sputtering of surface minerals, diffusion from subsurface (on Mercury, dayside)

H₂, He delivered by solar wind as ions, converted to neutrals at surface (ion recombination)

Ar on Moon results from radioactive decay of K in crust

Spatial/temporal variations in abundance are clues

Water, CO₂ from comets, meteorites and possibly volcanism

Several efficient loss mechanisms

Exposed to solar wind, UV, X-ray, and cosmic ray bombardment

Gases subject to thermal (Jeans) escape, photodissociation, photoionization, sputtering; timescales are short

Consequence for atmospheres: efficient removal, little retention

Present atmospheric constituents continually re-supplied

Volatiles must be condensed to survive in large quantities

General darkening, but compostion still shows through

Phobos darker than Moon, but receives only 1/2.23 solar radiation

Lunar Upland/Maria differ in albedo 2/1

Mercury albedo similar to Moon, maybe even higher, but very little contrast (Compositionally different?)

Darkening of Rays--devitrified glass beads gardened by impact

Direct implanting of solar wind components; Earth’s geotail effect

Phobos; 1/100 g; 1/2.25 Solar; 1/90 nighttime; no cold traps.

Behavior of Volatiles on Vacuum Surfaces

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Stability controlled by vapor pressure of solid phase in coldest location (WMB)

sources; endogenic, meteorites, comets, solar wind (Hydrogen)

Water by far the most stable of common volatiles in Inner Solar System

Migration process; random walk in presence of high loss mechanisms

Migration of condensable volatiles (WMB, Butler (1997)

Ballistic hops until lost to photodissociation, high energy collision or cold trapping

Molecules accommodate thermally to surface, emerge with appropriate temperature and emission angle.

Monte Carlo model

Permanently shaded areas in polar regions; scattered light, dust layers

0.5 % surface area

Loss from cold traps

Temperatures within cold traps should be less than minimum nightside temperature, governed by balance of energy input from indirect scattering and intrinsic heatflow.

Sublimation rate a function only of temperature: partial pressure of a vapor over a condensed phase is equal to vapor pressure, f(T), when in equilibrium

Episodic vs. smoothed sources;

obliquity variations

Bibliography for Moon/Mercury Ice

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Butler, B. J. 1997. The migration of volatiles on the surfaces of Mercury and the Moon. J. Geophys. Res.102, 19283-19291.

Butler, B. J., D. O. Muhleman, and M. A. Slade 1993. Mercury: Full disk radar images and the detection and stability of ice at the north pole, J. Geophys. Res. 98, 15003-2603.

Hapke, B. 1990. Coherent backscatter and the radar characteristics of outer planet satellites.Icarus 88, 407-417

Harmon, J. K., M. A. Slade, R. A. Veles, A. Crespo, M. J. Dreyer, and J. M. Johnson 1994. Radar mapping of Mercury’s polar anomalies. Nature 369, 213-215.

Hodges, R. R. 1980. Lunar cold traps and their influence on argon – 40. Proc. Lunar Planet. Sci. Conf. 11, 2463-2477.

Ingersoll, A. P , T. Svitek, and B. C. Murray 1992. Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars. Icarus 100, 40-47.

Paige, D. A., S. E. Wood, and A. R. Vasavada 1992. The thermal stability of water ice at the poles of Mercury.Science 258, 643-646.

Rignot, E. 1995. Backscatter model for the unusual radar properties of the Greenland Ice Sheet.J. Geophys. Res. 100,9389-9400

Slade, M. A., B. J. Buttler, and D. O. Muhleman 1992. Mercury radar imaging: evidence for polar ice. Science 258, 635-640.

Watson, K., B. C. Murray, and H. Brown 1961. The behavior of volatiles on the lunar surface. J. Geophys. Res. 66, 3033-3045.

- Murray and Vasavada -

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