J. Phys. Chem. 1983, 87, 4218-4220
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when a comet is captured by the solar system, ita evolution is a fast decay implying a fractionation, the most volatile components being lost first. Physicochemical data suggest that amorphous ice may initially play an important role, but it must disappear in the surface layers in favor of a transition to cubic ice, not much later than when the comet is inside the orbit of Jupiter. An icy grain halo, constantly sublimating but replenished by ice grains dragged away by vaporizing gases from the cometary nucleus, is likely to exist at moderate he-
liocentric distances but difficult to detect except in favorable cases.
Acknowledgment. I thank B. Levin for his preprint that reminded me of Laplace’s quotation. NSF Grant AST82-07435 and NASA Grant NSG-7301 (planetary atmospheres) are gratefully acknowledged. Registry No. Nitrogen, 7727-37-9;carbon monoxide, 630-08-0; methane, 74-82-8; formaldehyde, 50-00-0; ammonia, 7664-41-7; carbon dioxide, 124-38-9; hydrogen cyanide, 74-90-8; water, 7732-18-5.
Energetic Charged Particle Erosion of Ices in the Solar System R. E. Johnson, Department of Nuclear Engineering and Engineering Physics, University of Virginia, Charlottesvllle, Vlrglnle 22903
W. L. Brown, and L. J. Lanzerotti Bell Laboratoty, Murray Hill, New Jersey 07974 (Recehred: August 23, 1983)
Ice is known to be a pervasive constituent of the solar system and, probably, the interstellar medium. In most regions of space icy objects and ice-covered surfaces are exposed to energetic charged particle (electron and ion) bombardment. These charged particles change the surface layers via implantation, bond rearrangement, and erosion. Experiments and observations related to the erosion and modification of icy surfaces in space are briefly reviewed here.
Introduction Astronomical observations both from earth and from spacecraft continue to expand the experimental evidence for ice as pervasive constituents of the solar system.l Frozen volatiles are found in the polar caps of Earth and Mars, in the satellites of the giant planets,24 in the rings of S a t ~ r nand , ~ in the nuclei of comets.5 Indeed, reasonable inferences suggest that, except for the near proximity of individual stars, frozen volatiles may be the dominant surface constituents of interstellar grains and other solid, cold bodies throughout the galaxy. Spacecraft measurements over the past two decades have shown that the solar system is filled with energetic ions. These ions are primarily hydrogen and helium with a wide range of energies and fluxes. Ions with energies of approximately 1 keV/amu comprise the solar wind, continually flowing out from the sun.6 Eruptions on the sun produce sporadic outbursts of solar cosmic rays with energies from 10’s of keV to 10’s of MeV/amu and gallatic cosmic ray particles have energies of 10’s to 100’s of MeV.7 ~
(1) J. S. Lebofsky, Zcarus, 25, 205 (1975). (2) C. B. Pilcher, S. T. Ridgeway, and T. B. McCord, Science, 178, 1087 (1972); L. A. Lebofsky, Nature (London,) 269, 785 (1977). (3) A. L. Broadfoot et al., Science, 204,979 (1979); B. A. Smith, et al., Zbid., 212, 163 (1981). (4) H. Gehrels, “Satellites of Jupiter”, University of Arizona Press, Tuscon, 1982. (5) F. L. Whipple in ‘Comets, Asteroids and Meteorites”, D. H. Delsemme, Ed., University of Toledo Press, Toledo, 1977, p 25. F. L. Whipple, Astrophys. J.,11, 375 (1950). (6) M. A. Stroscio, L. Katz, G. K. Yates, B. Stellars, and F. A. Hanser, J. Geophys. Res., 81, 283 (1976); L. J. Lanzerotti and C. G. Maclennan, ibid., 78, 3935 (1973). 0022-3654/83/2087-4218$01.50/0
Planets with magnetic fields such as the Earth, Jupiter, and Saturn can trap ions with energies of a few keV to 100’s of MeV/amu in their radiation belts.* Recognition that the ices are invitably bombarded by at least some of these ions led to the initiation of experimentsgJOto measure the consequences of such bombardment in terms of the erosion and modification of ice layers and the formation of molecular fragments and more complex molecules. These experimental results and particle flux measurementa can be used to consider the effect of charged particle bombardment on ice grains,ll comets,12the rings of Sat(7) M. A. I. Van Hollebeke, F. B. McDonald, J. H. Trainor, and T. T. von Rosenvinge, J. Geophys. Res., 83,4723 (1978); P. Meyer, R. Ramaty, and W. R. Webber, Phys. Today, 27, 23 (1974); C. W. Allen, “Astrophysical Quantities”, 3rd ed, The Athlone Press, London, 1973, p 255. (8) S. M. Krimigis et al., Science, 206, 977 (1979);S. M. Krimigis, et al. Zbid., 215, 571 (1982). (9) W. L. Brown, L. J. Lanzerotti, J. M. Poate, and W. M. Augustyniak, Phys. Reu. Lett., 40, 1027 (1978); W. L. Brown, W. M. Augustyniak, E. Brody, L. J. Lanzerotti, A. L. Ramirez, R. Evatt, and R. E. Johnson, Nucl. Instrum. Methods, 170,321 (1980);W. L. Brown, W. M. Augustyniak. L. J. Lanzerotti, R. E. Johnson, and R. Evatt, Phys. Rev. Lett., 45, 1632 (1980). (10) L. E. Seiberling, C. K. Meins, B. H. Cooper, J. E. Griffith, M. H. Mendenhall, and T. A. Tombrello, Nucl. Instrum. Methods, 198, 17 (1982). C. L. Melcher. D. J. Le Poire, B. H. CooDer. and T. A. Tombrello, Geophys. Res. Lett., 9, 1151 (1982). (11) L. J. Lanzerotti, W. L. Brown, J. M. Poate, and W. M. Augustyniak, Nature (London),272, 431 (1978); H. Patashnick and G. Rupprecht, Appl. J., 197, L79 (1975); Icarus, 30, 402 (1977); T. Mukai and G. Schwehm, Astron. Astrophys., 95,373 (1981);S. Wyckoff in ‘Comets”, L. L. Wilkening, Ed., University of Arizona Press, Tucson, 1982. (12) R. E. Johnson, L. J. Lanzerotti, W. L. Brown, W. M. Augustyniak, and C. M u d , Astron. Astrophys., in press; M. Moore and B. Donn, Astrophys. J.,in press; J. M. Greenberg, Astrophys. Space Sci., 39, 9 (1975).
0 1983 American Chemical Society
Energetic Charged Particle Erosion of Ices
urn,13 and the surfaces of the satellites of Jupiter and Saturn.14J5 Much of this material has been summarized recently in two articles.le Here we give a brief overview and literature references as a means of guiding readers interested in examining this field more deeply. Discussion Incident charged-particle radiation is known to modify the chemical composition of most condensed-gas species.17 The modification produced when the dominant energy loss of the incident particles is to the electronic system of the solid is similar to those modifications produced when energetic photons are incident in the solid. The major difference in all cases is determined by differences in density of deposited electronic energy. On electronic relaxation in such insulating materials a significant fraction of the electronic energy deposited goes into atomic motion,18J9 which results in the further breaking of chemical bonds and, in the surface region, the ejection of atoms and molecules from the material (desorption, sputtering). In addition, slow ions whose dominant energy loss is to direct collisions with the nuclei of the solid produce similar effects. As the icy objects in the solar system are exposed to particle radiations it is of interest to be able to quantitatively describe the effects produced by long-term charged-particle irradiation. Recent experiments have shown that light fast ions (protons and helium) with energies comparable to those in the cosmic ray and energetic solar-particle spectra erode and modify ice layers rather efficiently. For example, MeV to keV protons and helium ions eject of the order of 1-30 water molecules per incident p a r t i ~ l e .Electrons ~ at keV energies are expected to produce effects comparable to protons of equal velocities.18 As the yields depend non0 and linearly on the energy deposited, heavier ions (e.g., ' S+ in the magnetospheric plasmas of Jupiter and Saturn)8Jo produce much larger yields (100's to 1000's of molecules per i ~ n ) . ~ These J ~ erosion yields are very nearly independent of the ice temperature below about 100 K and increase monotonically with temperatures above thisSg Mass spectra have shown that the dominant ejected species for erosion of DzO is mass 20 (DzO) at low temperatures and 4 (Dz) become a large fraction of and masses 32 (0,) the yield at the higher temperatures.21 The rather large erosion effects found suggest that the surfaces of icy objects exposed to charged-particle radiations for long periods of time in space are indeed signifi(13)R. W. Carlson, Nature (London),283,481 (1980);A. F. Cheng and L. J. Lanzerotti, J. Geophys. Res., 83,2597 (1978). (14)D.L. Matson, T. V. Johnson, and F. P. Fanale, Appl. J.,192,L43 (1974);A. F. Cheng, Astrophys. J.,242,812 (1980);P. K. Haff, C. C. Watson, and Y. L. Yung, J. Geophys. Res., 86,6933 (1981);L.J. Lanzerotti, W. L. Brown, W. M. Augustyniak, R. E. Johnson and T. P. Armstrong, Astrophys. J.,259,920 (1982). (15)L. J. Lanzerotti, W. L. Brown, J. M. Poate, and W. M. Augustyniak, Geophys. Res. Lett., 5,155(1978);R. E. Johnson, L. J. Lanzerotti, W. L. Brown, and T. P. Armstrong, Science, 212, 1027 (1981). (16)R. E. Johnson, L. J. Lanzerotti, and W. L. Brown, Nucl. Instrum. Methods, 198, 147 (1982);W. L. Brown, L. J. Lanzerotti, and R. E. Johnson, Science, 218,525 (1982). (17)E. J. Hart and R. L. Platzman, 'Mechanisms in Radiation Biology", H. Errera and A. Forssberg, Ed., Academic Press, New York, 1961. (18)R. E. Johnson and W. L. Brown, Nucl. Instrum. Methods, 198, 103 (1982);W. L. Brown, L. J. Lanzerotti, W. M. Augustynick, and R. E. Johnson, "Diet-I Proceedings", Plenum, New York, in press. (19)R. E.Johnson and M. Inokbuti, Nucl. Inst. Meth., in press. (20)F. Bagenal and J. D. Sullivan, J. Geophys. Res., 86,8447(1981); J. W. Belcher, C. K. Goertz, and H. S Bridge, ibid., 7,17 (1980). H.S. Bridge et al., Science, 215,563 (1982). (21)W. L. Brown, W. M. Augustyniak, E. Simmons, K. J. Marcantonio, L. J. Lanzerotti, R. E. Johnson, J. W. Boring, C. T. Reimann, G. Foti, and V. Pirronello, Nucl. Instrum. Methods, 198,1 (1982).
The Journal of Physlcal Chemistry, Vol. 87, No. 21, 1983 4219
cantly modified. For example, ice grains in the solar system of a size particularly stable against sublimation (20 pm) are eroded efficiently by s o h wind charged particles.'l cloud containing mixtures of HzO and Comets in the 001% COz (and/or CO) ices5 will have their surfaces layers modified considerably by cosmic ray bombardment. Larger molecules will be created and the irradiated mixture will be more volatile on the first approach of the comet to the sun.lZ The small icy satellites of Saturn must be eroded by the electrons and ions in the Saturnian magnetospheric plasmasz2 Tethys and Dione, for instance, have lost a couple of meters of material due to charged-particle erosion based on present estimates of the charged-particle Whereas such erosion may not have an observable effect on the surface (as compared to meteoric bombardmentM), it is probably the source of the heavier particles (O+)in the plasma itself.25 That is, the eroded (sputtered) material is ionized and contributes to the charged-particle density in the plasma and subsequent surface erosion. Similarly, the erosion of SOz frost on 1014and water ice on Europa, Ganymede, and Callisto15 are probably responsible for the observed "clouds" of neutral atoms and molecules orbiting with Io and the heavy particles (O+,S+, S2+)in the Jovian magnetospheric plasma.l4vZ6 Finally on the Gallilean satellites a large fraction of the eroded material cannot escape the gravitational field. (On these satellites also, unlike the icy Saturnian satellites, the surface temperatures are thought to be sufficient so that sublimation cannot be i g n ~ r e d . ~The ~ ~material ~ ~ ~ ~re-~ ) distributed across the surface by particle bombardment is probably responsible for the appearance of a frost in the polar regions of G a n ~ m e d eand ~ ~ the , ~ ~observation on Callisto of frost on the rims of craters shielded from the sun. The competition between erosion by and implantation of fast sulfur ions can account for the observation of SO bands in a water ice surface on the trailing side of Europa and suggests that considerable surface erosion occurs due to particle bombardment of this surface.16~27~29 Lastly, the sputtered and sublimated molecules produce very tenuous atmo~pheres'~7~~ on the Galilean satellites. These atmospheres will only reach significant these icy ice decomposes into Oz and HZ.l6 For the temperatures of importance on these icy satellites, decomposition by photons of sublimated water molecules in the gaseous state31will not be important, except possibly on C a l l i s t ~ . 'On ~ ~ the ~ ~ other hand, the more efficient decomposition of ice (and or gaseous water molecules) by charged-particle bombardment will lead to the production of a tenuous 0, (22)A. F. Cheng, L. J. Lanzerotti, and V. Pirronello, J.Geophys. Res., in press. (23)L. J. Lanzerotti, C. G. Maclennan, W. L. Brown, R. E. Johnson, L. A. Barton, C. T. Reimann,J. W. Garrett, and J. W. Boring, J.Geophys. Res., submitted for publication. (24)R. Wolff and D. Mendis, J. Geophys. Res., submitted for publication. (25)L. Frank, B. Burek, K. Ackerson, J. Wolfe, and J. M. Mihalov, J. Geophys. Res., 85,5995 (1980). (26)A. J. Dessler, Ed., "Physics of the Jovian Magnetosphere", University of Arizona Press, Tuscon, 1982;D. E. Shemansky and B. R. Sandel, J. Geophys. Res., 87,219(1928);R. A. Brown, D. E. Shemansky, and R. E. Johnson, Astrophys. J.,264,309 (1983). (27)E. Sieveka and R. E. Johnson, Icarus, 51,528 (1982). (28)C. B. Pilcher, Icarus, 37, 559 (1979);N. G. Purves and C. B. Pilcher, ibid., 43,51 (1980). (29)A. R. Lane, R. M. Nelson, and D. L. Matson, Nature (London), 292,38 (1981). (30)R. E.Johnson, J. W. Boring, C. T. Reimann, L. A. Barton, E. M. Sieveko, J. W. Garrett, K. R. Farmer, W. L. Brown, and L. J. Lanzerotti, Geo. Res. Lett., submitted for publication. (31)Y. L. Yung and M. B. McElroy, Icarus, 30,97 (1977).
J. Phys. Chem. 1083,87,4220-4229
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"atmosphere" and adsorbed O2 in the colder regions on Europa and, possibly, Ganymede.16 As this summary indicates, the implications of charged-particle irradiation of ice in the solar system and in the interstellar medium are extremely broad. The quantitative evaluation of the importance of chargedparticle bombardment for astrophysical purposes depends on obtaining a much more comprehensive understanding
of the physics and chemistry of the erosion and modification processes along with a better understanding of the charged-particle fluxes in various regions of space.
Acknowledgment. The work at the University of Virginia is supported by NSF Astronomy Division Grant AST-82-00477. Registry No. Water, 7732-18-5.
Interstellar Ice A. 0. 0. M. Tlelens,' W. Hagen,+and J. M. Greenberg '.aboratory Astrophysics Group, Huygens Laboratorium, Rlks Unhersiteit, 2300 RA LeMen, The Netherlends (Received: August 23, 1982; I n Final Form: February 7. 1983)
The 3250-cm-' "ice" band observed in absorption and polarization in interstellar spectra is studied in the laboratory in low-temperaturesolid mixtures of H20 and other molecules. General empirical rules are derived which relate the shape and relative intensity of the HzO infrared absorptions to the degree of HzO dilution and hydrogen-bonding capacity of the dilutant. The chemical composition of mantles accreting on interstellar grains inside dense (103-105~ m -molecular ~) clouds has been calculated numerically. The reaction scheme comprises gas-phase as well as grain surface reactions. The results show that in most circumstancesgrain mantles consist of the molecules HzO,HzCO,NH3,Nz, 02, CO, and COP The relative concentrations of these species depend strongly on the physical conditions in the gas. The expected infrared characteristics of the calculated grain mantles are discussed with an emphasis on the observed 3250-cm-' "ice" band. Grain mantles accreted at a contain large concentrations of HzO (-60%) and produce a broad 3250-cm-' "ice" density of about lo4 band. It is suggested that the low-frequency wing on this feature observed in interstellar spectra is due to absorption by HzCO. A small contribution from HzO hydrogen bonded to NH3 is probably also present.
well as grain surface reactions. The aim of this study is I. Introduction to identify under what interstellar conditions a broad The spectra of many interstellar infrared sources show 3250-cm-' "ice" band is formed. These calculations will a prominent absorption feature at 3250 cm-l, the so-called also help to distinguish between the two possible causes "3-pm band", which is generally attributed to absorption of the observed low-frequency wing. Finally, this study by solid H20 in the mantle of small, low-temperature dust may suggest laboratory experiments and interstellar obparticles (0.3 pm diameter, 10 K).l The present study is servations which can discriminate between various theopart of an ongoing program to investigate the physical and retical possibilities. chemical properties of these interstellar dust particles by This article is organized as follows. The experimental employing laboratory analogues.24 Previous work on pure setup is briefly summarized in section 2. The laboratory amorphous solid water H20(as)failed to match the detailed shape of the observed interstellar 3250-cm-l f e a t ~ r e . ~ ? ~spectra are presented in section 3. These results are interpreted in section 4 with an emphasis on the OHThe observed feature is broader than the laboratory feastretching vibrations. The theoretical calculations of the ture and shows a low-frequency wing. chemical composition of interstellar grain mantles are It has been shown that this low-frequency wing can be discussed in section 5. In this section we also compare the due either to absorption by molecules containing CH infrared characteristics of the calculated mixtures to the groups also present in grain mantles3 or to absorption by interstellar observations. In section 6 the main results of H 2 0 molecules hydrogen bonded to strong bases.6 In the our study are summarized. present work we investigate the origin of this low-frequency wing. Systematic laboratory studies of the influence of 2. Experimental Procedures dilution with inert as well as hydrogen-bonding species on Details of the experimental setup, spectroscopic meththe spectrum of H 2 0 are reported. Differences in these ods, and method of calculation of optical constants have spectra with respect to the spectrum of pure HzO(as) are been described el~ewhere.~ interpreted in terms of size, shape, and hydrogen-bonding capacity of the dilutant. Such a study may also help to (1) K. M. Merrill, R. W. Russell, and B. T. Soifer, Astrophys. J.,207, elucidate the structure of liquid water. 763 (1976). The chemical composition of mantles growing on in(2)W.Hagen, L. J. Allamandola, and J. M. Greenberg, Astrophys. Space Sci., 66,215 (1979). terstellar grains has been calculated numerically with a (3) W. Hagen, L. J. Allamandola, and J. M. Greenberg, Astron. Aschemical reaction scheme which comprises gas-phase as trophys., 86,%3 (1980). *Present address: NASA Ames Research Center, M.S.245-6, Moffett Field, CA 94035. t Present address: Koninklyke/Shell-laboratorium, Amsterdam, The Netherlands. 0022-3654/83/2087-4220$01.50/0
(4) W. Hagen, A. G. G. M. Tielens, and J. M. Greenberg, Chem. Phys., 56,367 (1981). (5)A. L&er. J. Klein, S. de Cheveime, C. Guinet, D. Defourneau, and M. Belin, A&&. Astrophys., 79,2 6 (1979). (6)W.Hagen, L. J. Allamandola, J. M. Greenberg, and A. G. G. M. Tielena, J. Mol. Struct., 60, 281 (1980).
0 1983 Amerlcan Chemical Society
