GEOPHYSICAL RESEARCH LETTERS, VOL. 18, NO. 11, PAGES 2169-2172, NOVEMBER 1991 LUNAR SURFACE' SPUTTERING AND SECONDARY ION MASS SPECTROMETRY R.E. Johnson and R. Baragiola Department of Nuclear Engineering and Engineering Physics University of Virginia Abstract. We combine laboratory and Apollo year on the moon) every surface species has been observations to describe the sputtering of the struck and subsequent ions eject chemically- lunar surface and the composition of the ejecta altered species. At high fluences sputtering with special reference to O. The atmospheric proceeds stoichiometrically (Johnson 1990). inventory appears to be dominated by micro- Three sputtering mechanismsoccur. Electronic meteorite vaporization of lunar grains. sputtering (by ions, electrons, or EUV-photons) Sputtering effects are observable in the local results from excitation of electrons to repulsive plasma due to ion ejection, in the extended states and, for refractory-materials, applies to atmosphere through energetic neutral ejection, adsorbed species (H20, Na). Sputtering by and on grain surfaces through the chemical momentum-transfer is the dominant process for fractionation of the redeposited sputter-ejecta. solar wind ions. [Morgan & Shemansky (1990) Ionization of the micrometeorite-vapor also suggest the 0 yields are due to electronic contributes to the local plasma. sputtering'the cross sections,'10 -21 cm 2 are too small.] Finally, chemical sputtering arises Introduction after long-term bombardment or when bombarding with chemically reactive species (H, O, C). Apollo data indicated that solar-wind ions The total yield for oxides by low energy (0.1- created amorphous outer layers on lunar grains 10 keV), light ions (H,He) are similar to metals '0.02•m, changed these grains isotopically and and peak near the energy-per-nucleon of the solar chemicallyand the energetic-ionscreated tracks wind' Ni peaksat 0.016 around 1.2 keV for H+; and spallation. These effects were used to for He + , Y = 0.2 atoms/ion at 2 keV, close to estimate regolith turnover rates. Because solar that for SiO 2 seen in Fig.(1). Since the solar wind-ions reach the surface, sputtering also wind has 5% He ++, the contribution is of the occurs. We examine this here. Since micro- same order as for protons. These yields increase meteorite bombardment modifies lunar grains with increasing angle of incidence having a - (impact-glasses and agglutinates, Taylor 1982) maximum due to surface roughness at 55-85 ø. At and produces a vapor we compare it to sputtering. normal incidence the ejecta distribution peaks Sputtering by space plasmas and detection of toward the surface normal, a behavior also ejected ions can determine the composition of drastically affected by roughness. A decrease in surfaces by analogy with the laboratory technique the effective yield occurs for a porous-regolith SIMS(Secondary Ion Mass Spectrometry), as due to redeposition Fig. 1. This can be suggested for Saturn's moons (Johnson & Sittler estimated (Hapke 1986' Johnson 1989) from 1990) and for the lunar surface (Stern 1991' information on the ejected species and angles, Elphic et al., this issue). After describing the and the sticking probabilities (Hapke & Cassidy sputtering process we consider its application to 1978' Kasi & Rabalais 1990). The sticking of the lunar surface and its importance relative to the ejecta results in a net transport into the micrometeorite produced vapor, Fig. 1. regolith. Finally, the energy distribution of - collisionally sputtered atoms peaks at Ui/2. Emission of H20 (chemical-sputtering) has been Laboratory Samples' Sputtering and SIMS observed in long-term H-bombardment of glasses and oxides' i.e. the formation of a volatile The sputtering yield of a species (number of which is easily sputtered or desorbs thermally. ejected per incident ion) is roughly proportional In a hydrogen glow-discharge [Ishibe & Oyama to its surface concentration and inversely 1979] this yield decreases with dose from 0.01 proportional to its binding energy, U i. In this H20/H to -0.003, typical of physical sputtering, process neutrals are more readily ejected than as the layer is reduced (depleted in O). Both ions and chemical-binding controls the e]ecta physical and chemical sputtering lead to composition (e.g. Johnson 1990). The yield preferential e•ection of oxygen[Thomas & Hofmann dependsonly weakly on mass leading to slight 1985] leaving microscopic metallic regions preferential ejection of the lighter species (Walters et al 1989), a process possibly (Kerridge & Kaplan 1978). For a molecular-solid occurringon the Moon (Taylor & Epstein 1973). the yield is fluence (exposure-time) dependent. Since sputtering is a non-equilibrium process, In the laboratory, this dependence is partially molecular species form' for Na2S neutral S, S2, due to projectile-induced surface structures. Na, Na2S, NaS, Na 2 were found after a large Only a fraction of the ion•s energy goes into fluence, in order of decreasing signal (Chrisey sputtering, most of it producin• chemical et al 1988). Since S also comes from cracking of alteration. After a fluence of 101•ions/cm2(<l molecules in the mass spectrometer, S 2 was estimated to be the dominant sulfur product. For Na20 in a silicate 02 should be the dominant Copyright 1991 by the American Geophysic• Union. speciesfor long term exposure, with the surface slowly replenished in O by beam-enhanced Paper number 91GL02095 diffusion from the bulk. Surface-charging can 0094-8534/91/91GL-02095503.00 increase the sputtering yield [Bach et al 1974] 2169