Atomic collisions in solids: Astronomical applications

Transcription

1 Nuclear Instruments and Methods in Physics Research B 193 (2002) Atomic collisions in solids: Astronomical applications R.A. Baragiola *, C.L. Atteberry 1, C.A. Dukes, M. Fama, B.D. Teolis Laboratory for Atomic and Surface Physics, Engineering Physics, University of Virginia, Thornton Hall, Charlottesville, VA , USA Received in revised form 30 September 2001 Abstract Airless bodies in space are subject to irradiation with energetic atomic particles, which generate atmospheres by sputtering and alter the surface composition. Astronomical observations with telescopes and space probes continuously provide new data that require new laboratory experiments for their interpretation. Many of these experiments also serve to expand the current frontier of atomic collisions in solids by discovering previously unknown phenomena. Some of the experimental techniques used in these experiments could find applications in other areas of atomic collisions in solids. We present results from our current experimental research program on sputtering and surface modification of ices and minerals and point out opportunities for research in this area. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction * Corresponding author. Tel.: ; fax: address: raul@virginia.edu (R.A. Baragiola). 1 Present address: Department of Physics, USAF Academy, CO 80840, USA. This is a progress report on recent work from our laboratory on atomic collisions with astronomical surfaces to which we have added key references to work done elsewhere. Statements of some unsolved problems in this area are intended to stimulate the interest of other researchers in atomic collisions in solids. We start by pointing out that the solar system is a natural laboratory for atomic collisions in solids. The harsh space environment is populated by energetic ions, electrons and photons that impact the surface of any body not protected by a relatively thick atmosphere, like most satellites, asteroids, comets, Mercury, Pluto and spacecraft. Radiation from the Sun that can alter materials consists of UV photons, especially Lyman-a (10.2 ev), the 1 kev/amu solar wind, and occasional solar flares. Fluxes decay with the square of the distance R to the Sun; near Earth (R ¼ 1 AU) they are, on average, Ly-a/cm 2 and ions (electrons)/cm 2. Magnetospheric ion fluxes around Jupiter (R ¼ 5:2 AU) and Saturn (R ¼ 9:54 AU) are more intense and mostly H þ, oxygen and sulfur ions; their energy distribution has a broad peak at kev and a thermal component. Indication of atomic collision processes comes from the optical reflectance (sunlight reflected from the surface), optical emission from ionospheres and, for the Moon, from laboratory analysis of actual surface rocks. Johnson s book [1] is an invaluable resource, describing the radiation environment, observations, and modeling of atomic collisions on planetary surfaces up till Many new X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S X(02)

2 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) discoveries appeared since then from improved telescopes and new space probes. Laboratory simulations can mimic the low pressures of the tenuous atmospheres but not astronomical time scales, thus extrapolations are needed. Lifetimes of a planetary satellite surface between significant micrometeorite impacts may be years. Ingredients to consider in the extrapolation are the relative fluxes of incoming radiation and atmospheric gases, of sublimation from surfaces, and the rates of chemical reactions, diffusion, segregation, phase transformations, etc. Unfortunately, very little is known about those processes and about the actual porosity, roughness and detailed composition of surfaces. Thus, experiments aim at the basic understanding of physical processes needed to model a large number of possible situations. Below we report different experiments designed to understand how atomic collisions change the surface region of icy satellites, and the rocky surfaces of the Moon, Mercury and asteroids. The experiments are done in ultrahigh vacuum, using mass spectrometry, optical spectroscopy (0.1 1 lm), and X-ray photoelectron spectroscopy (XPS). Details on the different experimental methods have been published [2 4]. Mineral surfaces are produced by fracturing while thin ice films are grown by vapor deposition onto a cooled microbalance. We measure sputtering yields with the microbalance and from the flux of sputtered species with a mass spectrometer (MS). This instrument is also used to measure the gas desorbed from irradiated films while they are heated using a linear temperature ramp. Irradiation fluxes are insufficient to significantly heat the samples. Our techniques complement studies using infrared spectroscopy [5,6] and incident electrons [7 9] and UV light [10 12]. Here we give representative references, usually to the most recent work, which can lead to all relevant references that we have no space to cite. 2. Atomic collisions with ices Sputtering causes surface erosion by ejecting molecules that contribute to the local atmosphere around the astronomical object. Knowledge of the velocity distribution of sputtered particles is limited but important because it determines the fraction of particles that can escape gravity; those that do not escape return to the surface (perhaps tens of km away) and contribute transiently or permanently to the atmosphere. Planetary surfaces are very porous due to micrometeorite bombardment. Porosity alters sputtering since, e.g. atoms sputtered from the walls of a pore may redeposit [13]. Due to varying microscopic and macroscopic topography a range of impact angles are important; in addition, ion fluxes depend strongly on latitude and longitude; thus sputtering effects and local atmospheres are highly inhomogeneous. Irradiation alters the chemical composition of many materials; exceptions are solid H 2 O, O 2 and N 2, which approximately maintain their stoichiometry during irradiation. Atmosphere generation by sputtering dominates over sublimation in the icy satellites of the giant planets, due to large fluxes of magnetospheric ions [14] and low temperatures. Additional sputtering occurs due to solar UV [10]. Examples include the recently detected atmospheres of oxygen at Europa [15] and Ganymede [16], and hydrogen at Ganymede [17]. Atmospheres created by sputtering likely occur over other icy satellites, the icy rings of Saturn, and comets. Although sputtering data exist, especially for MeV light ions [18], many questions appear when trying to model ion irradiation effects, as in our study of the production of atmospheres around the icy satellites of Jupiter and Saturn [14] Sputtering of water ice Water ice, the main condensed gas on the icy satellites (except for Io) sputters more efficiently by electronic excitations (electronic sputtering) than from the typical recoil sputtering prevalent in refractory materials. Fig. 1 summarizes measurements done by others and us (adapted from [19]), which are dominated by electronic sputtering [18,20] (recoil sputtering is apparent in the low energy plateaus ). Largely unknown are the sputtering yields of mixed ices, an important concern since optical remote sensing has revealed that

3 722 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) Fig. 1. Sputtering yield of water ice versus energy/amu for different singly charged ions [19]. other volatile components (e.g. CO 2,SO 2,O 2,O 3, H 2 O 2,SO 4 H 2 ) exist on the icy satellites [14] either segregated, trapped in inclusions, or dissolved in the ice. Optical reflectance samples a depth more than two orders of magnitude larger than that responsible for sputtering; and therefore is not usually useful to characterize the surface, where the composition depends on a still unclear combination of sputtering, diffusion, sublimation, recondensation and molecular synthesis. that O 2 formed in the ice cannot be trapped permanently; it diffuses out at Ganymede s reported temperatures [23]. Transient trapping of O 2 in ice can be made by co-depositing O 2 and water in a film. When warmed above 70 K, the absorption bands become those of liquid oxygen, and different from those observed on Ganymede [24]. Our explanation is that very cold regions exist on Ganymede, made of segregated, bright ice patches, which are not visible to the Galileo infrared radiometer. Details of the findings and discussion of the model were published recently [25]. Further studies are needed on other materials (like silicates) that can trap O 2. Related to this problem, we have measured the synthesis of O 2 molecules in water ice by 100 kev Ar þ to simulate irradiation of the Jovian satellites (Ar þ has not been observed but should behave similarly to the abundant S þ ions, an important sputtering source). Fig. 2 shows that the temperature dependence of O 2 emission is very strong compared to that of H 2 O (the data were taken after saturation of the fluence dependence [26]). Improving on earlier observations for MeV ions [26,27], we measured the relative efficiencies of our 2.2. Molecular synthesis in water ice Solid O 2 was detected on Ganymede from its absorption bands in the red [21], which prompted the question: how can O 2 exist at the reported high temperatures where the vapor pressure would exceed the atmospheric pressure by many orders of magnitude? There has been a controversy in the literature on the explanation of this puzzle. Johnson and Jesser have proposed that O 2 is formed inside the ice by radiolysis, and trapped in bubbles or inclusions [22]. Our experiments show instead Fig. 2. Total and partial sputtering yields of water for 100 kev Ar þ versus temperature. (- - -): yield calculated from the MS contributions, normalized to the yield measured by the microbalance at 17 K, taking into account that the relative contribution of O 2 and H 2 O is according to their mass ratio (36=20) using H 18 2 O.

4 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) MS for O 2 and H 2 O, and have avoided problems related to direct line of sight detection (noise induced by scattered projectiles and the dissimilar velocities of sputtered molecules). Some of the radiolytic O 2 is transiently stored in the solid, but eventually leaves by diffusion. We found that 0.52 O 2 are produced and trapped in the ice by each 200 kev H þ (G 2: per 100 ev of deposited electronic energy) [28]. The experiments showed also the synthesis of smaller quantities of HO 2 and H 2 O 2. Detailed studies of the temperature dependence of H 2 O 2 synthesis in ice by fast H þ were done recently by infrared spectroscopy [29] to understand evidence of this molecule on the surface of Europa, a satellite of Jupiter Ozone synthesis in ices containing oxygen Ozone has been detected on the surface of Ganymede [30], and the Saturnian satellites Rhea and Dione [31]; a possible origin is radiolysis of solid oxygen condensed from the atmosphere. We found that H þ irradiation of condensed O 2,CO 2 and H 2 O 2 leads to the formation of ozone [32] (Fig. 3). In contrast, we found no detectable O 3 synthesis in water ice using either optical Fig. 3. Absorbance of Hartley O 3 band produced by 100 kev H þ on different ices at 20 K (5 K for O 2 and the 1:1 H 2 O:O 2 mixture). spectroscopy or mass spectrometry during thermal desorption of irradiated films. We quantify O 3 production from the depth of the Hartley absorption band in ultraviolet reflectance spectra. The shape of the band can account for part of the ultraviolet absorption seen on Ganymede, Dione and Rhea the differences may be due to sulfur compounds formed by implantation of magnetospheric sulfur. Microscopic modeling of the fluence dependence of band depth (Fig. 3) shows that fast H þ synthesize O 3 orders of magnitude more efficiently than previously thought [33]. Additional studies of O 3 synthesis in solid O 2 are reported elsewhere in this volume [34]. 3. Atomic collisions in minerals We are interested in the question of the source of ordinary chondritic meteorites, the most abundant on Earth. Probable meteoritic parent bodies are S(IV)-type asteroids that, however, show spectral reflectance strikingly different from that of chondritic meteorites. This may result from prolonged irradiation by solar wind ions, visible and UV radiation, and micrometeoritic bombardment. We made the first in situ UHV quantitative study of chemical changes of olivine due to irradiation with 1 kev protons and 4 kev helium ions using XPS [35] and found that the primary chemical effect is reduction of iron to the metallic form. In a meteorite this iron will reoxidize instantaneously when entering the Earth s atmosphere. The effect of iron reduction on the spectral reflectance of asteroids remains to be demonstrated. Fig. 4 shows the change in composition of labradorite (a plagioclase feldspar found in Moon basalt) versus fluence of 4 kev He þ. Unlike olivine, the surface of labradorite is very stable, only the Na concentration is seen to decrease significantly while the C shows a peculiar behavior. These differences indicate the difficulty of extrapolating data from one mineral to another. Impact desorption of alkalis from surface minerals is thought to contribute to exospheres of Na and K at Mercury and the Moon, a topic of great current interest [13,36 40]. Recent studies show that Na is photodesorbed from evaporated oxides

5 724 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) below a certain threshold wavelength by an electron transfer mechanism [41]. We test whether such processes occur also on more realistic planetary surfaces and for incident ions by irradiating sub-monolayers of Na on olivine with 4 kev He þ ions, typical of the solar wind. The decrease of the Na coverage with irradiation fluence follows the expected exponential decay, from which we derive a desorption cross-section r ¼ 1: cm 2, enormous compared to cm 2 for desorption of Na from SiO 2 by 40 ev electrons [41]. The exponential decay means that the sputtering yield Y ¼ rn S depends on the surface concentration N S. Experiments with labradorite (Fig. 4), which contains Na and K in the bulk show the depletion of Na but not of K; the sputtering yield of Na is quite lower than that of Na adsorbed on olivine. Open questions are the time dependence of replenishing Na from the bulk, under wide temperature ranges appropriate to the Moon and Mercury. 4. Electron emission and surface charging Fig. 4. Surface composition of labradorite versus fluence of 4 kev He þ ( ions/cm 2 corresponds to 4400 years at the asteroid belt). Oxygen (not shown) adds up to 100% composition. Most ices are electrical insulators and, therefore, charge when exposed to charged particles and ionizing photons [42]. This charging can affect the dynamical behavior of small grains in regions of significant electromagnetic fields, like space plasmas. The amount of charge accumulated on a grain depends on it properties, the balance between fluxes of incoming and ejected charges, their energy distribution, and the electrical potential of the grain. Detailed modeling reveals that the potential of ice grains in Saturn s E ring varies from negative to positive as a function of distance from the planet [43]. The accuracy of such models suffers from the scarcity of data on electron emission from insulators by ions and electrons at energies below 100 ev. Another situation occurs when the particle flux and/or electromagnetic fields is not uniform (for example part of the surface being in the shadow). The induced electric fields resulting from inhomogeneous or differential charging will affect electron emission and may cause dielectric breakdown. These conditions are also relevant for insulating surfaces in spacecraft, where the resulting breakdown can produce malfunction by spurious electrical noise, and also permanent damage [42,44]. The opportunity for studies is revealed by the scarcity of quantitative data on surface charging with ion beams. Questions include what is the maximum charge density that a surface or bulk can take [45], the nature of the charges, and the dependence of charging on insulator properties. 5. Remote planetary surface analysis Most knowledge of planetary surfaces comes from the analysis of the reflectance spectra, optical emission of atmospheric species, and ionic abundances measured by space probes. The local ion bombardment provided by solar wind and magnetospheric ions produces ion emission from the

6 R.A. Baragiola et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) surface, which can be analyzed by a MS on a spacecraft, similar to the secondary-ion mass spectrometry (SIMS) technique [13,46]. Space based SIMS with the Cassini ion-mass spectrometer [47] on Saturn s icy satellites, where ion escape is not hindered by atmospheric collisions, should allow detection of minority species not visible in optical reflectance spectra. Another technique that might be useful to analyze surface composition is the possible luminescence of the night side of icy satellites caused by irradiation with magnetospheric ions [48]. Ioninduced luminescence results from the decay of radiative levels populated by electronic excitations. In addition, thermoluminescence may result from the decay of electron traps also excited by fast particles. With a UHV setup similar to that used in our studies of ion-induced luminescence of diamond [49] and solid argon [50], we sought luminescence of water ice surfaces during irradiation with 100 kev H þ. We found no detectable luminescence (<10 4 photons/h þ ) except for a very weak signal from sputtered excited OH [32]. A spectrometer orbiting around an icy satellite, however, may find luminescence on the night side due to impurities, which would provide further clues on surface composition. Luminescence measurements of mixed and impure ices are needed to test this idea. Acknowledgements Our research is supported by NASA s Office of Space Science, NSF-Astronomy, and NASA s Cassini program under JPL contract BDTgratefully acknowledges IGERTfellowship support under National Science Foundation Grant # References [1] R.E. Johnson, Energetic Charged-Particle Interactions with Atmospheres and Surfaces, Springer-Verlag, Berlin, [2] N.J. Sack, R.A. Baragiola, Phys. Rev. B 48 (1993) [3] M.S. Westley, R.A. Baragiola, R.E. Johnson, G.A. Baratta, Planet. Space Sci. 43 (1995) [4] M. Shi, D.E. Grosjean, J. Schou, R.A. Baragiola, Nucl. Instr. and Meth. B 96 (1995) 524. [5] G. Strazzulla, G.A. Baratta, M.E. Palumbo, Spectrochem. Acta A 57 (2001) 825. [6] M.H. Moore, R.L. Hudson, P.A. Gerakines, Spectrochem. Acta A 57 (2001) 843. [7] M.T. Sieger, W.C. Simpson, T.M. Orlando, Nature 394 (1998) 554. [8] L. Sanche, Scanning Microsc. 9 (1995) 619. [9] S. Lacombe et al., Phys. Rev. Lett. 79 (1997) [10] M.S. Westley, R.A. Baragiola, R.E. Johnson, G. Baratta, Nature 373 (1995) 405. [11] M.P. Bernstein et al., Astron. J. 454 (1995) 327. [12] P.A. Gerakines, W.A. Schutte, P. Ehrenfreund, Astron. Astrophys. 312 (1996) 289. [13] R.E. Johnson, R.A. Baragiola, Geophys. Res. Lett. 18 (1991) [14] M. Shi et al., J. Geophys. Res. 100 (1995) 26,387. [15] D.T. Hall et al., Nature 373 (1995) 677. [16] D.T. Hall, P.D. Feldman, M.A. McGrath, D.F. Strobel, Astron. J. 499 (1998) 475. [17] C.A. Barth et al., Geophys. Res. Lett. 24 (1997) [18] R.E. Johnson, in: B. Schmitt, C. De Bergh, M. Festou (Eds.), Solar System Ices, Kluwer, Dordrecht, 1998, p [19] R.A. Baragiola et al., Scanning Microsc., in press. [20] R.E. Johnson, in: B. Schmitt et al. (Eds.), Solar System Ices, Kluwer Academic, Dordrecht, [21] J.R. Spencer, W.M. Calvin, M.J. Person, J. Geophys. Res. 100 (1995) 19,049. [22] R.E. Johnson, W.A. Jesser, Astron. J. Lett. 480 (1997) L79. [23] R.A. Vidal, D. Bahr, R.A. Baragiola, M. Peters, Science 276 (1997) [24] R.A. Baragiola, D.A. Bahr, J. Geophys. Res. 103 (1998) [25] R.A. Baragiola, C.L. Atteberry, D.A. Bahr, M. Peters, J. Geophys. Res. E 104 (1999) 14,183. [26] C.T. Reimann et al., Surf. Sci. 147 (1984) 227. [27] W.L. Brown et al., Phys. Rev. Lett. 45 (1980) [28] D.A. Bahr, M.A. Fama, R.A. Vidal, R.A. Baragiola, J. Geophys. Res. E 106 (2001) 33, 285. [29] M.H. Moore, R.L. Hudson, Icarus 145 (2000) 282. [30] K.S. Noll et al., Science 273 (1996) 341. [31] K.S. Noll et al., Nature 388 (1997) 45. [32] C.L. Atteberry, M.Sc. Thesis, University of Virginia, [33] R.A. Baragiola, C.L. Atteberry, D.A. Bahr, M.M. Jakas, Nucl. Instr. and Meth. B 157 (1999) 233. [34] M. Fama, D.A. Bahr, B.D. Teolis, R.A. Baragiola, Nucl. Instr. and Meth. B 193 (2002) 775. [35] C.A. Dukes, R.A. Baragiola, L. McFadden, J. Geophys. Res. 104 (1999) [36] A.L. Sprague et al., Icarus 129 (1997) 506. [37] A.E. Potter, Geophys. Res. Lett. 22 (1998) [38] M. Mendillo, J. Baumgardner, J. Wilson, Icarus 137 (1999) 13.

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