Data obtained from the exploration of the outer solar system has led to a 6 new area of physics: electronically induced sputtering of low-temperature, 7 condensed-gas solids, here referred to as ices. Icy bodies in the outer solar system 8 are bombarded by relatively intense fluxes of ions and electrons, as well as the 9 background solar UV flux, causing changes in their optical reflectance and ejection 10 (sputtering/desorption) of molecules from their surfaces. The low cohesive energies 11 of

more »

... ices lead to relatively large sputtering rates by both momentum transfer ('knock-12 on' collisions) and the electronic excitations produced by the incident particles. 13 Such sputtering produces an ambient gas about an icy body, often the source of the 14 local plasma. This chapter focuses on the ejection of material by ionizing radiation 15 from a surface that is predominantly a molecular condensed gas solid. The incident 16 radiation types considered are photons, electrons and ions with the emphasis on the 17 ejection processes. This radiation also produces the chemical effects described in 18 the chapters of sections II and III. The induced-chemistry can produce both more 19 refractory and more volatile products and so affect the molecular ejection rate. 20 The emphasis in this chapter is on the production of gas-phase species from icy 21 surfaces in space. We describe the physics and chemistry leading to the ejection of 22 atoms and molecules, give semi-empirical expressions based on these processes, 23 and describe some applications. 25 We are in an exciting period of exploration of the outer solar system by spacecraft, 26 orbiting telescopes, and remarkably improved ground-based observations. These 27 studies have revolutionized our understanding of this region, revealing worlds very 28 different from ours, some of which are bizarre. Because of the low surface 29 temperatures, typically <120 K, the 'rock' of outer-solar-system bodies is water 30 ice. That is, excluding the four giant planets, it is the structural and thermal 31 properties of ice that principally determine the geology of the surfaces of many 32 of the small objects from Jupiter's orbit and beyond. Other more volatile molecular 33 species, such as N 2 , O 2 , CO, CO 2 , NH 3 , and CH 4 , form atmospheres and polar ices, 34 or can cause the surface to be active. An interesting exception is Io, a moon of 35 Jupiter, which does not have water ice. Owing to its tidal interactions, Io is 36 volcanically active and has been desiccated, losing all of its water and other light 37 volatiles which are stripped from its transient atmosphere by the Jovian plasma 38 (Johnson 2004). Because of this, frozen SO 2 , a familiar, heavy volcanic gas on 39 Earth, covers Io's surface (e.g., Bagenal et al. 2004 ). 40 Since most small outer-solar-system bodies, with the exception of Titan, have 41 either no atmospheres or tenuous ones at best, their surfaces are exposed to the solar 42 ultraviolet radiation and local plasma. These irradiations can alter their surfaces 43 physically and chemically (Johnson 1990; Carlson et al. 2009) and produce 44 atmospheres (Johnson et al. 2009), such as the recent discovery of an O 2 -CO 2 45 atmosphere on Saturn's moon Rhea (Teolis et al. 2010). Since the Pioneer and 46 Voyager spacecraft found that both Jupiter and Saturn had surprisingly intense 47 plasmas (e.g., Fig. 17 .1) trapped in their giant magnetic fields, the effect of the 48 plasma bombardment on their icy moons was investigated. Therefore, following 49 the Pioneer encounters with Jupiter and in preparation for the Voyager encounters, 50 W. L. Brown, L. J. Lanzerotti, and colleagues carried out a series of experiments to 51 measure the ejection of molecules induced by energetic ion impact on ice (Brown 52 et al. 1978; Lanzerotti et al. 1978) . Their discovery that the sputtering of low 53 temperature ice by fast, light ions is principally determined by the electronic 54 excitations produced in the ice, rather than by knock-on collisions of the incident 55 ions with water molecules, initiated a new area of physics, the study of electronic 56 sputtering of low-temperature condensed-gas solids referred to here as ices. 57 The study of sputtering in refractory materials has a long history, as it is a tool for 58 producing a vapor from a low-vapor-pressure solid (Sigmund 1993). Therefore the 59 application of the standard sputtering process to ices is first discussed, followed by a 60 description of electronic sputtering of ices and the chemical changes that effect the 61 ejection of molecules. Finally, the relevance of sputtering to recent observations of 62 icy bodies in the outer solar system is described. 552 R.E. Johnson et al. 63 17.2 Sputtering and Desorption 64 A fast ion or electron penetrating a solid gradually loses energy to the constituent 65 atoms. The average rate of energy transfer is represented by a quantity dE/dx, 66 the energy loss per unit path length of the charged particle in the solid, called the 67 'stopping power' of the material. Since an incident ion loses energy both by 68 nuclear-elastic (knock-on) collisions and by electronic excitations and ionizations 69 of the molecules in the material, the stopping power is often written: (dE/dx)~(dE/ 70 dx) n + (dE/dx) e . These quantities are given in Fig. 17.2a for O + incident on water 71 ice, a system of interest in both the Jovian and Saturnian magnetospheres. Energetic 72 electrons exhibit only a (dE/dx) e component; for cosmic ray ions and shocks in the 73 interstellar medium such quantities are discussed in Bringa and Johnson (2003). 74 Sputtering, the ejection of atoms or molecules from the solid into the vacuum, is 75 initiated when the energy deposited by the incident particles sets atoms and 76 molecules in motion in the surface region. The study of ion-induced sputtering by 77 knock-on collision, (dE/dx) n , has a long history in part due to its usefulness in 78 17 Sputtering of Ices 553 79 late 1970s it was discovered that weakly bound insulators such as the ices were also 80 sputtered by the electronic energy deposited, (dE/dx) e (e.g., Lanzerotti et al. 1978; 81 Brown et al. 1978 81 Brown et al. , 1982. Whereas the electronic excitations and ionizations 82 produced are rapidly quenched in a metal, those produced in these weakly bound 83 solid insulators can be converted to kinetic energy through a variety of pathways 84 (Brown and Johnson 1986; Johnson 1996). 85 A measure of the sputtering rate for a solid is the yield, Y(E i ,y i ): the number of 86 molecules ejected per incident ion or electron with energy E i and angle to the 87 surface normal, y i . Y is shown in Fig. 17 .2b for y i ¼ 0 for H + , O n+ and S n+ on ice. 88 These ions, especially H + and O n+ , are common in the outer planet magnetospheres. 89 Comparing Fig. 17 .2a, b it is seen that both knock-on collisions and electronic 90 excitations sputter water ice. Although the energy deposited must act in concert, 91 Y is usually written as a sum of two components: Y~Y n + Y e . 92 In metals and other refractory materials knock-on collisions are typically the 93 dominant cause of sputtering, so that Y~Y n . On the other hand, the icy surfaces in 94 the Jovian magnetosphere are irradiated by very energetic ions. For such ions the 95 electronic component, Y e , is seen in Fig. 17 .2b to be dominant. Below we describe 96 the physics of electronic and knock-on sputtering. The latter is sometimes called 97 collision cascade or nuclear sputtering. out of their lattice sites form defects (interstitials and voids). Under long term 104 irradiation this process can also amorphize the penetrated region of a crystalline 105 solid (e.g., Strazzulla et al. 1992; Fama et al. 2010). 106 17.3.1 Yield 107 Based on the picture described above, Y n (E i ,y i ), for an ion with energy incident E i 108 incident at an angle to the surface normal, y i , is determined by the number of recoils 109 set in motion near the surface. The recoil production is, in turn, roughly propor-110 tional to the energy deposited in the near surface region: (dE/dx) n . Since the ability 111 of an atom or molecule to be ejected is affected by its binding to the solid, Y n also 112 is, roughly, inversely dependent on the cohesive energy of the solid, U. For long 113 term sputtering, this is the average binding energy of the atoms to the surface, which 114 for the low energy ejecta is of the order of the sublimation energy. Therefore, the 115 yield is often written as Y n $ c½l dE=dx ð Þ n =U p (17.1) 116 where l is the mean spacing of the molecules (l~n À1/3 , n the molecular number 117 density). Therefore, [l(dE/dx) n ] is the average energy deposited per molecular layer 118 in the surface region and the term in brackets in Eq. 17.1 is dimensionless. The 119 fraction of produced recoils moving toward the surface is roughly accounted for by 120 c in Eq. 17.1. Since the recoils are initially forward directed, c~0.1-0.2 (Johnson 121 1990) in a number of material, and the exponent p is discussed below. 122 Based on Eq. 17.1, knock-on sputtering is much more efficient in ices than in 123 refractory materials, as the value of U is an order of magnitude smaller (Johnson 124 and Schou 1993). Although experiments clearly bear this out, using values of c 125 based on simulations, the U extracted from measurements can differ considerably 126 from the sublimation energy even for simple materials (Behrisch and Eckstein 127 2007). This is due to a number of effects, such as radiation induced damage. But 128 for the volatile materials of interest, when a large volume of material is ejected, the 129 sublimation energy of the individual molecule becomes irrelevant. In addition, 130 during sputtering, the volatility of a molecular ice, hence the effective U, can be 131 affected by the chemistry induced, as discussed shortly. 132 Since (dE/dx) n roughly scales as the square of the incident ion atomic number, it is 133 small for light ions and, therefore, produces a sparse recoil density. To first order the 134 recoil cascade is described as 'linear'; that is, it is assumed to be produced by a series 135 of independent binary collisions with each collision between a recoil atom and an atom 136 not previously struck. In this picture the probability of producing a recoil that leaves 137 the surface is roughly proportional to the energy deposited, hence, p ¼ 1 in Eq. 17.1 138 above some energy threshold, E t (Behrisch and Eckstein 2007; Johnson 1990). 139 A threshold value of 1.48 eV for incident eV Xe ions on ice was been recently 140 measured (Killelea et al. 2012). 17 Sputtering of Ices 555 Bradley JP (1994) Chemically anomalous, preaccretionally irradiated gains in interplanetary dust 725 from comets. Science 265:925-929 726 Bringa EM, Johnson RE (2003) Ion interaction with solids: astrophysical application. In: 727 Pirronello V, Krelowaky J (eds) Solid state astrochemistry. Kluwer, Netherlands, pp 357-393 728 Bringa EM, Johnson RE (2002) Coulomb explosion and thermal spikes. Phys Rev Lett 88:165501-1-4 729 Bringa EM, Johnson RE (2001) Angular dependence of the sputtering yield from cylindrical track. 730 Nucl Instrum Methods B180:99-104 731 Bringa EM, Johnson RE (2000) Electronic sputtering of solid O 2 . Surf Sci 451:108-115 732 Bringa EM, Johnson RE (1999) Molecular dynamics study of non-equilibrium energy transport 733 from a C\cylindrical track: part II. Models for the sputtering yield. Nucl Instrum Methods 734 152:267-290 735 Bringa EM, Johnson RE, Papaleo RM (2002) Crater formation by single ions in the electronic 736 stopping regime: comparison of molecular dynamics simulation with experiments on organic 737 films. Phys Rev B65:094113-1-8 738 Bringa EM, Johnson RE, Jakas M (1999) Molecular-dynamics simulation of electronic sputtering. 739 Phys Rev B 60:15107-15116 740 Brown WL, Johnson RE (1986) Sputtering of ices: a review. Nucl Instrum Methods B13:295-303 741 Brown WL, Augustyniak WM, Marcantonio KJ, Simmons EN, Boring JW, Johnson RE, Reimann 742 CT (1984) Electronic sputtering of low temperature molecular solids. Nucl Instrum Methods 743 B1:307-314 744 Brown WL, Augustyniak WM, Simmons E, Marcantonio KJ, Lanzerotti LJ, Johnson RE, Boring 745 JW, Reimann CT, Foti G, Pirronello V (1982) Erosion and molecular formation in condensed 746 gas films by electronic energy loss of fast ions. Nucl Instrum Methods 198:1-8 747 Brown WL, Augustyniak WM, Brody E, Cooper B, Lanzerotti LJ, Ramirez A, Evatt E, Johnson 748 RE (1980a) Energy dependence of the erosion of H 2 O ice films by H and He ions. Nucl Instrum 749 Methods 170:321-325 750 Brown WL, Augustyniak WM, Lanzerotti LJ, Johnson RE, Evatt R (1980b) Linear and nonlinear 751 processes in the erosion of H 2 O Ice by fast light ions. Phys Rev Lett 45:1632-1635 752 Brown WL, Lanzerotti LJ, Poate JM, Augustyniak WM (1978) "Sputtering" of ice by MeV light 753 ions. Phys Rev Lett 49:1027-1030 754 Burger MH, Wagner R, Jaumann R, Cassidy TA (2010) Effects of the external environment on icy 755 satellites. Space Sci Rev 153:349-374 756 Calcagno L, Oostra DJ, Pedrys R, Haring A, de Vries AE (1986) Erosion of methane induced by 757 energetic electron bomdradment. Nucl Instrum Methods B17:22-24 758 Carlson RW, Calvin WM, Dalton JB, Hansen GB, Hudson RL, Johnson RE, McCord TB, Moore 759 MH (2009) Europa's surface composition. In: Pappalardo R et al (eds) Chapter in Europa. 760 University of Arizona Press, Tucson, pp 283-327 761 Cassidy TA, Johnson RE (2005) Monte Carlo model of sputtering and other ejection processes 762 within a regolith. Icarus 176:499-507 763 Cassidy T, Coll P, Raulin F, Carlson RW, Johnson RE, Loeffler MJ, Hand KH, Baragiola RA 764 (2010) Radiolysis and photolysis of icy satellite surfaces: experiments and theory. Space Sci 765 Rev 115:299-315. doi:10.1007/s11214-009-9625-3, Chapter in Exchange Processes in the 766 Outer Solar System 767 Cassidy TA, Johnson RE, Tucker OJ (2009) Trace constituents of Europa's atmosphere. Icarus 768 201:182-190 769 Chrisey DB, Brown WL, Boring JW (1990) Electronic excitation of condensed CO: sputtering and 770 chemical change. Surf Sci 225:130-142 771 Chrisey DB, Brown WL (1989) Electronic excitation of condensed CO: sputtering and chemical 772 change. Surf Sci 225:130-142 773 Chrisey DB, Boring JW, Johnson RE, Phipps JA (1988a) Molecular ejection from low temperature 774 sulfur by keV ions. Surf Sci 195:594-618 775 Chrisey DB, Johnson RE, Boring JW, Phipps JA (1988b) Ejection of sodium from sodium sulfide 776 by the sputtering of the surface of Io. Icarus 75:233-244 981 Pirronello V, Strazzulla G, Foti G, Brown WL, Simmons E (1984) Formaldehyde formation in 982 cometary nuclei. Astron Astrophys 134:204-206 983 Pirronello V, Brown WL, Lanzerotti LJ, Marcantonio KJ, Simmons E (1982) Formaldehyde 984 formation in a H 2 O/CO 2 ice mixture under irradiation by fast ions. Astrophys J 262:636-640 985 Ponciano CR, Farenzena LS, Collado VM, da Silveira EF, Wien K (2005) Secondary ion emission 986 from CO 2 -H 2 O ice irradiated by energetic heavy ions Part II: analysis-search for organic ions. 987 Int J Mass Spectrom 244:41-49