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Nuclear Instruments North-Holland. and Methods in Physics Research B14 (1986) 373 373 377 Amsterdam Section I. SpuItering of ices ASTROPHYSICAL IMPLICATIONS L.J. LANZEROTTI and W.L. BROWN OF ICE SPUTTERING A T&T Bell L.uhorcrrone.~s,Murray Hdl. New Jersyv 07974. C’SA Volatiles Saturn) condensed arc important particle environments on surfaces and condensed as major constituent, constituents such as the solar wind and planetary icy obJects. producing physical and chemical alterations. ob~ccts, using laboratory-based of astronomical of the solar system and, likely. the interstellar and spacecraft-measured. medium. bodies (e.g.. comets or the satellites of In most regions of space the charged magnetospheres can interact directly with the ice-covered surfaces and the Many of the recent studies of the effects of charged particles on solar system data are summarized 1. Introduction The solar system is filled with photons from the sun and the stars, and with charged particles (plasma populations), primarily electrons and protons, in the solar wind, from solar flare-produced cosmic rays, from cosmic rays in the galaxy, and in the magnetospheres of many of the planets. If the surfaces of planets and planetary satellites are not shielded by atmospheres or magnetic fields, these particles can impact the surfaces, physically eroding and chemically changing them. Other, smaller objects in the solar system such as asteroids, minor planets, comets and grains are generally not shielded and are thus subject to the full irradiation of the particle environments in which they are found. Stimulated particularly by considerations that are important for understanding lunar materials obtained during the Apollo missions to the moon, a number of investigations have reported the effects of radiation on the surfaces of planetary objects or their satellites e.g. refs. [l-3]. In the same era, Kimura [4] proposed that cosmic rays could modify and limit the growth of grains in interstellar space and thus affect their properties. This work does not seem to have been followed up in any detail. except in very general considerations of the growth. destruction, and stability of grain mantles, e.g., ref. 151. Recently the first evidence has been reported of cosmic ray tracks in interplanetary grains, incident on Earth and collected at high altitudes [6]. The constituent materials of the outer solar system are vastly different than those of bodies orbiting within a few astronomical units of the sun (one astronomical unit (AU) is defined as the Sun-Earth distance). In addition to the giant gaseous planets Jupiter (- 5 AU) here. and Saturn (- 10 AU), with densities near one, the outer reaches of the solar system consist of planetary satellites made up largely of condensed volatiles, water in the case of several of Jupiter’s satellites and Saturn’s rings, and possibly water and ammonia mixtures in the case of some of the Saturnian satellites. The temperatures continue to drop for objects more distant from the sun and volatiles such as methane can be condensed on the rings and moons of Uranus, and probably on the surface of the planet Pluto and its satellite Charon. Characteristic temperatures in the solar system are shown in fig. 1 171. The solid line is the temperature profile expected from simple black-body radiation equilibrium on a rapidly rotating object with an albedo (reflectance coefficient) equal to zero 181. The solid point for each planet indicates the effective average temperature of the respective planet using measured albedos. On the right-hand scale are indicated the melting points for various molecular species found in the solar system and the effective average of temperatures (broken lines) of bodies composed of the indicated ices exposed to solar photon irradiation [9]. It is clear that although close to the sun the icy objects will lose mass rapidly through sublimation, in the outer solar system the condensed-gas frosts will be stable. In addition to the icy satellites of the planets, comets and ice grains are constituents of the outer regions of the solar system and, in particular, of the regions beyond the orbit of Pluto (aphelion - 50 AU). Halley’s comet is actually a quite nearby resident of the sun, with an aphelion distance of - 35 AU, just beyond Neptune. Virtually all of the nonperiodic comets which enter the solar system are believed to originate in a large “cloud” of icy bodies residing at distances up to - 5 x 1. SPUTTERING OF ICES L.J. 374 10 -0.1 I Lmrerotti I et al. / Astrophysical I 1 DISTANCE I 10 FROM SUN implications - of icesputtering H2 50 (AU) Fig. 1. Temperature in K as a function of distance from the sun in astronomical units (AU; 1 AU = average Sun-Earth distance). Solid points: effective average temperature of the indicated planets. Solid line: black body temperature on a rapidly rotating body with zero albedo. Dashed and dashed-dotted lines: effective average temperature (determined by equilibrium between solar photon irradiation and sublimation) of an object composed of the indicated ice. Melting points for several ices indicated on right-hand axis. Adapted from [7]. 104AU from the sun (the Oort cloud of comets). These objects, some lOI to 1013 in number, are believed to surround the solar system. Individual members can occasionally be perturbed by the gravitational force of a passing star, causing the icy object to enter the inner solar system where its ices can be sublimed. The plasma environments in the solar system have a wide range of particle energies and intensities. For example, the solar wind, continually flowing out from the sun with a density of a few particles per cubic centimeter at the orbit of Earth, is composed of electrons and ions (primarily protons with a few percent helium) with energies of approximately 1 keV. Eruptions on the sun produce sporadic outbursts of solar cosmic rays with energies from tens of thousands to tens of millions of electron volts. The magnetosphere particle populations (fluences and compositions) vary widely from planet to planet and have spatial and temporal dependencies at each planet. The magnetosphere of Jupiter contains intense fluxes of keV-energy sulfur and oxygen ions, while Saturn has a population of nitrogen and oxygen ions. The Pioneer and Voyager spacecraft missions to Jupiter, Saturn, and the outer solar system have been crucial in providing the necessary data without which it would have been impossible to assess the effects of particle impacts on the icy bodies in the outer solar system. Summarized briefly below are a number of the studies that have reported on the effects of charged particles on icy bodies in the solar system. As there has been little work in relating specific laboratory experiments to extra-solar system astrophysical problems, the concentration here is on the solar system. Details of the work can be found in the listed references. References primarily to laboratory experiments and data not specifically addressing astophysical problems are largely omitted. Several recent reviews of various aspects of this work include those of Brown et al. [lo], Lanzerotti et al. [ll], and Cheng et al. [12]. 2. Applications A number of problems related to solar system astrophysics require consideration of the effects of charged particles on condensed volatiles. A molecule or atom eroded from a surface in space by a charged particle can suffer a variety of fates. The sketches in fig. 2 illustrate some of these, with the resultant effect depending upon the physical characteristics (size, density, gravitational attraction) of the body (ice grain or planetary satellite, for example) and the mechanism(s) of the sputtering process [lo]. An eroded particle can escape directly or can strike the surface again (fig. 2a), depending upon the gravitational attraction of the body and the sputtered particle energy. If the body has a sufficient atmosphere (as some planetary satellites do), the outgoing sputtered particle will be stopped by atmospheric collisions, dif- L.J. Lanrerotti et al. / Astrophysrcal implications of ice sputtering 375 cbl (a) Fig. 2. Schematic illustration of possible fates of material eroded from a surface, depending upon gravitational attraction and external environment of the object. (a) An eroded atom or molecule can escape or enter into a ballistic trajectory, depending on its ejection energy and the gravitational attraction of the object. In the case of an interplanetary or interstellar grain, the incident particle may pass completely through and the gravitational attraction on the ejected species will be negligible. (b) An atmosphere around a planetary satellite can produce scattering and re-impact of the ejected species on the surface. (c) Ionization by solar or stellar photons (hv) of an ejected species in the presence of a externally-imposed magnetic field can cause a loss of the species. In practice. various combinations of these three conditions can exist in most environments. tary or interplanetary magnetic field). Some relevant physical parameters of some representative solar system objects are listed in table 1. Interplanetary grains. The solar wind and solar energetic particles can dominate sublimation, and thus be the principal determinant of the lifetime of icy (H,O) fuse in the atmosphere, and ultimately be recondensed on the surface (fig. 2b). A molecule or atom eroded from a body can be ionized by solar or stellar photons or by an external astrophysical plasma (fig. 2c), with the ionized species probably escaping, especially if there is an externally imposed magnetic field (such as a plane- Table 1 Escape energies and velocities Object Jovian satellites 10 Europa Ganymede Callisto Saturnian satellites Mimas Encelades Tethys Dione Rhea Titan A-ring object Moon Earth from the surfaces Radius Density (km) (g/cm’ of various ) solar system bodies. &m/z’ ) 1820 1500 2640 2500 3.5 3.5 2.0 1.6 178 147 147 114 195 250 525 560 765 2570 - O.M)l 1738 6378 1.2 1 1.1 1.4 1.3 1.9 6.6 7 15 22 28 136 - 2.6x 1O-5 164 980 -1 3.34 5.5 Escape energy (evfamu) 0.035 0.023 0.040 0.029 0.00013 0.0002 0.0008 0.0013 0.0023 0.036 - 2.6X10_‘5 0.029 0.64 Escape velocity (m/s) 2580 2080 2790 2390 150 190 400 500 660 2640 - 7.2~10-~ 2400 11200 I. SPUTTERING OF ICES 376 L.J. Lanzerotti et al. /Astrophysical implications of ice sputtering grains, for distances beyond - 2 AU [13-151. For the erosion of large surface areas in interplanetary space, the solar wind dominates the erosion of H,O ice surfaces - 5 AU and the erosion of CO, ice surfaces beyond beyond - 20 AU [14]. The particles could also act as a means of fusing aggregates of grain constituents [16]. Comets and icy bodies. The icy conglomerate model of a comet nucleus [17] is consistent with the body of observations to date, although water ice in the solid phase may only recently have been identified [18]. There may be a number of icy bodies the size of a comet (nucleus a few to several tens of km in diameter) that reside in stable elliptical orbits with large ellipticities in the outer solar system. Even Halley’s, in a highly elliptical orbit with aphelion - 35 AU, spends - 75% of its - 76 year orbital period beyond the orbit of Uranus (- 20 AU). The amount of matter lost from solar wind erosion is small, though non-negligible, - 10m5 g/cm’ of H,O or - 2 X lop4 g/cm* of CO, per orbit for an object in a near-circular orbit at - 50 AU [14]. Of possibly greater importance is the modification of the surface layers of the comet due to particle impact on the ice mixtures [14,19-241. Since the particle fluxes are unknown beyond the orbit of Pluto, it is not possible to reliably estimate the irradiation history of comets in the Oort cloud. It is also very difficult to reliably determine the effects of interstellar ions on the mantles of interstellar grains, matter which may be important in comet formation [25]. It may well be that in some instances the processing of such mantles by particles should be considered together with the important UV irradiation processes [25]. Earth’s Moon. Only in the polar regions of the moon is the temperature low enough for frozen volatiles to exist. The redistribution of any frozen volatiles from the polar regions [26] by sputtering by the solar wind and by Earth’s magnetosphere particles effectively eliminates the possibility of surface water ice on that body [27]. Jupiter: lo. The intensely volcanic satellite 10 [28] has been a subject of much study since the discovery of the sodium emissions accompanying IO in its orbit [29]. Soon after, Matson et al. [30] proposed a sputtering hypothesis to remove sodium from the surface. Much subsequent work has been devoted to studies of the energetic particle erosion of SO, frost layers on colder portions of the satellite [31,32] in order to understand the complicated plasma torus discovered by the Voyager spacecraft [33]. The considerations for formation of the torus and of various atmosphere conditions around 10 have become very detailed, and are reviewed in Cheng et al. [12]. Jupiter: Europa, Ganymede, and Cal&to. These three Galilean satellites of Jupiter are all water-ice covered and, indeed, water is a major ice. There have been a number of considerations of the effects of particle bombardment of these bodies [34-391, using particle fluxes measured in the magnetosphere of the planet by the Voyager spacecraft. Recent measurements of the velocity distributions of the ejected species imply that, because of the graviational attraction of these Galilean satellites, less loss will occur than was previously thought, with more redistribution and production of thin “atmospheres” [39]. Saturn’s satellites and rings. Considerations similar to those of Jupiter’s satellites have been applied to Saturn’s, again using particle fluxes measured in the magnetosphere by Voyagers 1 and 2. Escape of eroded species is more likely because of the reduced gravitational attraction of these satellites (table 1, ref. [40]), with the eroded water products capable of forming a heavy ion (oxygen) torus in the inner magnetosphere [16,41]. It has been proposed that ammonia ice, suggested to be important in the internal dynamics of, for example, Enceladus [42,43], is not observed by Voyager remote sensing instruments because of the preferential erosion of NH, compared to H,O [44]. The magnetosphere particles decrease in intensity at the edge of the major A ring, so that there do not appear to be significant effects by the particles on the principal rings of Saturn. However, galactic cosmic rays do strike these ring particles, producing nuclear interactions and high energy protons and electrons from the decay of produced neutrons [45]. The ions in the magnetosphere can interact with the tenuous E ring (in the region of the satellites Enceladus and Tethys). The interactions, together possibly with micrometeroid impacts (whose fluxes are very uncertain), predict a very short (on a cosmic scale) lifetime of - lo*-lo4 years for this ring if it is not replenished [12,46]. Uranus and Pluto. It has been proposed, from laboratory-based results, that particle impacts on CH, ice in the rings of Uranus would produce a loss of hydrogen and a polymerization of the residue, making them dark, as observed [47]. These considerations have been extended to discussions of methane ice on the Uranian satellites [12,48] and on Pluto [49]. Solar wind and magnetosphere particles could also produce polymerization of organic ices and materials on other satellites in the solar system, such as Iapetus (around Saturn), which has an unusual large, dark surface feature. However, no quantitative discussions have yet been published. A proposal has been made that solar UV could produce the Iapetus feature by polymerization [50]. 3. 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