Charged particle alteration of ices: Applications to solar system astrophysics

412

Nuclear Instrumentsand Methodsin PhysicsResearch B32 (1988) 412-418 North-Holland,Amsterdam

CHARGED PARTICLE ALTERATION OF ICES: APPLICATIONS TO SOLAR SYSTEM ASTROPHYSICS L.J. LANZEROTTI AT&T Bell L.&oratories,

Murray Hill, New Jersey 07974, USA

The outer solar system consistsof many objects whose surfacesare coveredwith condensedvolatilesor which contain condensed volatiles as major constituents. These volatiles are insulator materials. Such objects include planetary satellites, comets, and interplanetary grains. The solar system also contains varied charged-particle (plasma) environments which can interact with the volatile-containing objects, producing physical and chemical alterations. This paper briefly summarizes some of the interesting astrophysical problems of present-day interest which are related to charged-particle modification of ices.

1. Introduction A number of factors have contributed in the last decade toward a growing interest in the effects of charged particles on ices - simple insulator systems. Both ground-based and satellite-based sensors have provided ever increasing detail on the nature of the ice surfaces and compositions of outer solar system objects. Spacecraft instrumentation has provided accurate, in situ data on the intensities and compositions of the different charged particle environments in the solar system. Careful laboratory experiments, whose results can be applied to astrophysical problems, have been carried out. Continued interests in the use of ion and electron beams for integrated circuit lithography and other purposes have encouraged a generalization of laboratory results from particle impacts on organic ices to the more complex organic systems used in practical applications, and vice-versa. This growing interest in the subject has prompted a number of review papers in recent times, reviews which cover essentially all aspects of the subject. The most recent of these include one dealing primarily with laboratory results [l], and several which are more oriented towards a use of the laboratory results for astrophysical - primarily solar system - problems [2-81. Thus, this paper presents only a short summary of selected astrophysical considerations. A special note is made of new results from the Voyager spacecraft encounter with the planet Uranus and the implications of charged particle interactions with organic ices and organic/water ice mixtures in the outer solar system. 2. Solar system objects and their environments The planets and moons of the outer solar system are composed of constituent materials vastly different from

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 distance from the sun) 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 with distance from the sun and volatiles such as methane can be condensed on the rings and moons of Uranus, and probably on the surfaces of the planet Pluto and its satellite Charon. Physical parameters of outer solar system objects are given in table 1. Characteristic temperatures in the solar system as a function of distance from the sun are shown in fig. 1 [2]. The solid line is the temperature profile expected from simple black-body equilibrium with solar photon radiation incident on a rapidly rotating object [9]. The solid point for each planet indicates the effective average temperature of the respective planet using measured albedos (reflectance coefficients). On the right-hand scale are indicated the melting points for various molecular species found in the solar system and the effective average temperatures (broken lines) of bodies composed of the indicated ices exposed to solar photon irradiation [lo]. It is clear that in the outer solar system the condensed-gas frosts will sublime very slowly. Comets and ice grains are also constituents of the outer regions of the solar system and, in particular, it is believed, of vast regions beyond the orbit of Pluto (aphelion - 50 AU). Even through its orbit period is more than 75 years, Halley’s comet is actually a quite nearby resident of the sun. Its aphelion distance, - 35 AU, is just beyond the orbit of Neptune. Virtually all of the nonperiodic comets which enter the solar system are

LJ. Lmuerottr Table 1 Physical properties Object

Earth Moon Jovian satellites 10 Europa Ganymede Callisto Saturnian satellites Mimas Enceladus Tethys Dione Rhea A-ring Object Uranian satellites Miranda Ariel Umbriel Titania Oberon

413

/ Charged particle alteration of Ices

and escape energiesand velocitiesfrom the surfacesof various solar system bodies. Radius

Mass

Density

(km)

(lO23 g)

(g/cm3 )

6378 1738

59.760 735

1820 1500 2640 2420

891 487 1490 1070

195 250 525 560 765 - 0.001

0.37 0.72 6.10 10.3 24.4

242 580 595 805 775

0.75 13.5 12.7 34.8 29.2

Distance from planet

Escape velocity (m/s)

5.5 3.34

0.64 0.029

11,200

2,400

60.3 R,

3.5 3.0 1.9 1.8

0.035 0.022 0.040 0.029

2580 2080 2790 2390

5.95 R, 9.47 R , 15.1 R, 26.6 R,

1.2 1.1 1.0 1.4 1.3

0.00013 0.00022 0.00083 0.0013 0.0023 - 2.6x10-‘s

-1

believed to originate in a large “cloud” residing at distances up to - 5 x lo4 AU (the Oort cloud of comets). These objects, 1012 to 1013 in number, are believed to

Escape energy (eV/amu)

1.26 1.65 1.44 1.59 1.50

of ice bodies from the sun perhaps some surround the

‘03L

150 190 400 500 660 - 7.2~10-~ 205 563 539 767 716

0.00022 0.0016 0.0015 0.0031 0.0027

53

R, R, R, R, R,

R,

4.95 R, 7.3 R, 10.15 R, 16.64 R, 22.24 R,

solar system. An individual member, perturbed by the gravitational force of a passing star, can enter the inner solar system where it becomes identified as a comet as its ices begin to sublime. In addition to the solid objects, the solar system is filled with photons from the sun and other stars, and with charged particles (plasma populations). These

PLANETARY MAGNETOSPHERES

DISTANCE FROM SUN (AU)

Fig. 1. Temperature in kelvin 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 known nine planets (M, V, E, M, J. S, U, N, P = Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, respectively). 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].

3.08 3.97 4.91 6.29 8.78

‘\ ‘\ ‘\

/

*&LACTIC ~/cOSMlC RAYS

Fig. 2. Sketch of regions in the solar system with various types of particle populations (lines with arrows). The surface and outer regions of the sun contain complex magnetic field topologies (solid lines around and extending from the sun). The sun continuously emits the solar wind (solid lines with arrows) and solar flares eject much higher energy particles (dashed lines with arrows). Cosmic rays from the galaxy enter the solar system (dash-dot lines with arrows). Particles are trapped in planetary magnetospheres (spiral solid lines with arrows). VIII. DEVELOPING

TRENDS

414

L..J. Lanzerotti

/ Charged particle alteration of ices

charged particles are primarily electrons and protons and comprise the solar wind, solar flare-produced cosmic rays, cosmic rays from other stars in the galaxy, and the magnetospheres of many of the planets (fig. 2). If the surfaces of planets and planetary satellites are not shielded by suitable atmospheres or magnetic fields, plasma populations can impact the icy surfaces, physically eroding and chemically altering them. 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 any particle environment in which they exist. Solar system plasma environments cover a wide range of particle energy spectra and intensities. For example, the solar wind, continually flowing out from the sun with a density of about ten 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, which are of central importance for alterations of the surfaces of planetary satellites, vary considerably from planet to planet and have major spatial and temporal dependencies at each planet. The planetary magnetospheres themselves vary vastly in size among the planets [ll]. The controlling factors in determining a magnetosphere boundary in the sun-ward direction are primarily the planetary magnetic moment and the solar wind plasma pressure at the planet’s orbit. In the anti-sunward direction, a planetary magnetotail can extend further than 1000 Earth radii (in the case of measurements made to date for the Earth) to more than 5 AU (in the case of Jupiter for measurements made to date). For Jupiter’s magnetosphere, the internal plasma pressure also plays a key role in determining the magnetosphere configuration and size. A schematic comparison of planetary magnetosphere sizes is shown in fig. 3 for Earth, Uranus, Jupiter, and Saturn. The diameter of the sun (- 1.4 X lo6 km) is somewhat larger than the scale marker in the bottom part of the figure and is much smaller than the size of the plasma environment surrounding Jupiter, the largest planet in the solar system. If any planetary magnetosphere were visible to the eye, it would resemble a comet; if Jupiter’s magnetosphere were visible, from Earth it would be the largest astronomical object in the sky. In the case of Earth, Jupiter and Saturn, each planet’s rotation axis is approximately perpendicular to the planet’s orbital plane. Uranus’ rotation axis lies in the orbital plane; the orbit plane of the moons of Uranus is thus approximately perpendicular to the planet’s orbital plane and, during the present epoch, looks like a bulls-

EARTI

\

\

,

/

/

/ JUPITER

*F.406km

SATURN

Fig. 3. Schematic comparison of the relative sizes of several planetary magnetospheres which exist within the solar system. In each case, the solar wind is incident from the left.

URANUS

MAGNETOSPH

Fig. 4. Sketch of the magnetosphere of Uranus as derived from measurements provided by instrumentation on the Voyager 2 spacecraft during its encounter with the planet in January 1986.

LJ. Lnnzerotti

I

I

I

ASTRONOMICAL

UNITS

Fig. 5. Locations of the maJor moons of several of the planets expressed in units of planetary radii (vertical scale). The sunward boundaries of the planetary magnetospheres are shown in the same units.

eye target from the perspective

of Earth or Sun. Whereas for Earth, Jupiter, and Saturn, the planet’s magnetic dipole axis is oriented approximately parallel (within 10°-150) to the planet’s rotation axis, the dipole axis of Uranus is tilted approximately 60° with respect to the rotation axis [12]. Thus, the magnetosphere of Uranus presents a constantly changing aspect to the incident solar wind flow. A schematic drawing of the magnetosphere of Uranus as ascertained from Voyager measurements made during the encounter with the planet in January 1986, is shown in fig. 4 [13]. The orbital locations of the major moons of Earth and the outer planets, relative to the sunward boundaries of the respective magnetospheres, are shown in fig. 5. The vertical bar for each planet is the ratio of the sunward magnetosphere boundary to planetary radius. For Jupiter and Saturn the major moons are imbedded in their respective magnetospheres for their complete orbits, unlike the case of Earth, where the Moon only

o+ts+? $08

A molecule or atom eroded from a surface in space by a charged particle can undergo a variety of fates. The resultant effects will depend upon the physical characteristics (size, density, gravitational attraction) of the impacted body (ice grain or planetary satellite, for

(a) 102 t

t,

RJ=~

?I a :: E-s+ 104

0’~:~~ ’

d

E

3. Outer solar system

S+ 4

k y

passes through the magnetotail and not through the near-Earth magnetosphere. Because of the tilt of the magnetosphere relative to the rotation axis of Uranus, the Uranus moons all pass in and out of the magnetosphere each orbit. Thus, these moons can be subjected to bombardment by solar wind and solar cosmic ray particle fluxes, as well as by magnetosphere radiation. The magnetosphere of Jupiter contains intense fluxes of keV-energy sulfur and oxygen ions, as well as protons. Saturn has populations of nitrogen and oxygen ions. The ion population of Uranus is essentially all protons. The Pioneer and Voyager spacecraft missions to Jupiter, Saturn, Uranus, and the outer solar system have been crucial in providing the necessary data to assess quantitatively the effects of charged particles on the icy bodies in the distant heliosphere. Examples of magnetosphere particle spectra as obtained by instruments on the Voyager spacecraft are shown in fig. 6. Ion spectra measured by the plasma (PLS) and Low Energy Charged Particle (LECP) instruments at the orbit of Jupiter’s moon 10 are shown in fig. 6a [14]. Fig. 6b contains spectra measured by the LECP instruments on both Voyager 1 and Voyager 2 in the vicinity of the Saturn moon Dione [5]. Because of the configuration of the magnetosphere of Uranus relative to the orbital plane of the moons, the particle fluxes measured by Voyager 2 during its encounter of the planet must be integrated over the orbits of the moons. Such orbit-integrated proton spectra for Ariel are shown in fig. 6c [15]. These spectra do not contain any contributions from possible solar flare or solar wind particle fluxes. Inclusion of these particle sources would raise the spectral intensities.

104

10’0 7 > 2 7

415

/ Charged parttcle alteration of ices

H+

102

2 100 i 10-2

10-I 100

102

E(keV)

404

10’

I II/l I III 1 III 102 103 104 E (keV)

10-31 10’

ARIEL ORBIT- INTEGRATED FLUX INITIAL FLUX OUTBOUND --I NBOUN? 102

103

PROTON ENERGY, keV

Fig. 6. Sample magnetosphere particle spectra acquired by Voyager spacecraft in the magnetospheres of Jupiter, Saturn, and Uranus. VIII. DEVELOPING

TRENDS

416

L.J. Lanrerotti / Charged particle alteration of ices

example) and the mechanism(s) of the sputtering process. An eroded particle can escape directly or can reimpact the surface, 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, diffuse in the atmosphere, and ultimately be recondensed on the surface. A molecule or atom eroded from a body can be ionized by solar or stellar photons or by an external astrophysical plasma, with the ionized species probably escaping, particularly since there is generally an externally-imposed magnetic field (such as a planetary or interplanetary magnetic field). Jupiter: lo. The effects of Jovian magnetosphere particles on the moons of Jupiter and Saturn have been summarized most recently in ref. [7]. The intensely volcanic satellite IO has been a subject of much study since the discovery of the sodium emission accompanying the moon in its orbit 1161. A sputtering hypothesis to remove sodium from the surface was proposed [17]. Considerable subsequent work has been devoted to studies of the energetic particle erosion of SO, frost layers on colder portions of the satellite [14,18] in order to understand the complicated plasma torus discovered by Voyager instrumentation [19]. Considerations for formation of the torus and various atmosphere conditions around the moon have become very detailed, and are reviewed in Cheng et al. [7]. Jupiter: Europa, Ganymede, and Callisto. These three moons are all water-ice covered, and water is a major bulk constituent of each. Papers studying the effects of charged particle bombardment of these bodies, using particle fluxes measured in the magnetosphere by the

Table 2

Satellite erosion by LECP-measured ion fluxes

Satellite

Jupiter Europa Ganymede Callisto Saturn

Tethys Dione Rhea Uranus Miranda Ariel Umbriel Titania Oberon

Escape flux (H,O cm -Zs-1=10-”

/&myr-‘)

Proton incident

Oxygen incident

5x106 2x106 2X10S

2x109 6x10’ 7x10’

5x106 6~10~ 2x106

4x10* 5x10* 2x10*

2x105 3x104 4x104 9x102 3x102

_ _

Voyager spacecraft, are reviewed in [7]. Laboratory measurements of the velocity distributions of the ejected species suggest that, because of the gravitational attraction of these moons, less water loss will occur than was previously estimated, with more redistribution and production of thin “atmospheres” [20]. Loss rates of water by sputtering for the three ice-covered major Jovian moons are listed in table 2. Saturn’s moons and rings. Studies similar to those for Jupiter’s moons have been applied to Saturn’s, using particle fluxes measured in the Saturnian magnetosphere by Voyagers 1 and 2. Escape of the eroded species is more probable because of the reduced gravitational attraction of these moons (table 1, ref. [21]). The loss rates of water are listed in table 2 for several Saturnian satellites. The eroded water products can form a heavy ion (oxygen) torus in the inner magnetosphere [4,22]. It has also been proposed that ammonia ice, suggested to be important in the internal dynamics of, for example, the moon Enceladus [23,24], was not observed by Voyager remote sensing instruments because of the preferential erosion of NH, compared to Hz,0 [25] from the moon’s surface. The magnetosphere particles fluxes decrease in intensity at the edge of the major A ring of Saturn. Thus, while the rings are undoubtedly the reason for the termination of the particle fluxes, there do not appear to be significant effects by the particles on the interiors of the principal rings. However, galactic cosmic rays do strike the ring objects, producing nuclear interactions and high energy protons and electrons from the decay of the produced neutrons (261. The ions in the magnetosphere can interact with the tenuous E ring (which exists in the region of 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 - 102-lo4 years for this ring if it is not replenished [7,27] by some mechanism. Uranus. Cheng and Lanzerotti [28] originally proposed, on the basis of 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. Subsequently, a number of other laboratory experiments have been performed which have provided additional information on the dependence of the polymerization on incident particle species and energy (e.g., refs. [29,30]). Furthermore, the Voyager 2 spacecraft encounter with Uranus provided measurements of the magnetosphere particle fluxes of the planet. These Voyager measurements were used to estimate the erosion rates of any water ice from the satellite surfaces and the polymerization rates of any methane ice. The erosion rate for pure water-covered satellites are given in table 2. The time interval for the deposition of 3000 eV/cm3 energy

L.J. Lanzerotti / Charged particle alteration of ices 10'0 ,

I

I

r-----___

URANUS SATELLITES

; :

109

!

t

OBERON

I’

,-

_H

’

i

Y 405 F

104

103

I

10-l

I

I

100

IO’

MODIFICATION

DEPTH,

102 /.m

Fig. 7. Time for accumulating a dose of 3000 eV/nm’ on the Uranian satellites as a function of the depth of penetration of the normally-incidentprotons. No corrections have been made for simultaneoussputteringof the surface material.

by magnetosphere protons into the surfaces of the five major Uranian moons as a function of depth of penetration is shown in fig. 7 [15]. An energy deposition of 3000 eV/nm3 by protons will polymerize (decompose) pure methane to the point where the hydrogen to carbon ratio is about 2. Pluto and other bodies. Laboratory results on the polymerization of methane ice by charged particles have also been applied to Pluto, the outer-most planet in the solar system [31]. Solar wind and magnetosphere particles could also produce polymerization of organic ices and materials on other moons in the solar system, such as Iapetus (around Saturn), which has an unusual large, dark surface feature. A proposal has been made that the Iapetus feature could also be produced by polymerization by solar UV photons [32]. Comets and icy bodies. A number of icy bodies the size of a comet (nucleus a few to several tens of km in diameter) probably reside in stable elliptical orbits with large ellipticities in the outer solar system. Even Halley’s comet spends - 75% of its - 76 year orbital period beyond the orbit of Uranus (- 20 AU). The amount of matter lost from solar wind erosion on a comet such as Halley’s is small, though non-negligible, - 10e5 g/cm’ of Hz0 or - 2 X 10v4 g/cm* of CO, per orbit for an object in a near-circular orbit at - 50 AU [33].

417

The encounter of the Vega and Giotto spacecraft with Comet Halley in March 1986 greatly increased awareness of the importance of possible modifications of fhe surface layers of comets by charged-particle impacts on ice mixtures [33-391, the eroded species (e.g., refs. [40,41]), and the formation of a nonvolatile crust [42,43]. A major problem is that the cosmic ray fluxes are unknown beyond the orbit of Pluto, so that reliable estimates of the irradiation history of objects in the Oort cloud cannot be made. Nevertheless, further laboratory work on the effects of charged particles on ice mixtures comprised of water, ammonia and organic species such as methane, CO, and CO, are needed in order to provide basic inputs to future theoretical considerations related to comets. This is of special importance if future spacecraft missions to comets, such as the CRAF mission to orbit a comet and send a penetrator into the object, come to fruition. Interplanetary grains. At solar system distances beyond - 2 AU the solar wind and solar energetic particles dominate sublimation, and thus are the principal determinants of the lifetimes of water ice grains [33,44,45]. For larger surface areas (such as comets) in interplanetary space, the solar wind dominates the erosion of H,O ice surfaces beyond - 5 AU and the erosion of COa ice surfaces beyond - 20 AU [33]. Ion tracks have been found in interplanetary dust grains collected at high altitudes [46], confirming that charged-particle impacts with such objects are important physical processes in the solar system. Considerable recent discussion has centered on the ion processing and enhanced adhesion of grains which have volatile and/or organic mantles [42,47,48], mechanisms which might compete with high temperatures (as occur in a nova or supernova) in the formation of such dust.

4. summary The astrophysical implications of charged particle irradiation of ices can be very extensive. Recent considerations conclude that under reasonable assumptions charged-particle erosion of the satellites of Jupiter and Saturn, and probably Uranus, dominates erosion by micrometeorite bombardment of the objects [49], even though the micrometeorite fluxes are only poorly known. Most considerations of the effects of particle bombardment have been done for solar system environments because knowledge of the objects and their plasma surroundings are most well characterized. However, the physical and chemical alterations produced by charged particles are undoubtedly of importance for the interstellar medium as well, although they are more speculative in this case. It is therefore clear that charged-particle effects on astrophysical objects must be taken into VIII. DEVELOPING TRENDS

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L J. Lmrerottr

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Chargedparticle alteration of ices

account in order to continue to develop an understanding of phenomena observed in many regions of space. This summary is taken from an invited talk presented at the 4th International Conference on Radiation Effects in Insulators, Lyon, France, July 1987. I thank W.L. Brown for helpful comments.

References R.E. JohnHI W.L.Brown, L.J. Lanzerotti, K.J. Marcantonio, son, and C.T. Reimann, Nucl. Instr. and Meth. B14 (1986) 392. 121 R.E. Johnson, L.J. Lanzerotti, and W.L. Brown, Nucl. Instr. and Meth. 198 (1982) 147. and R.E. Johnson, Science 131 W.L. Brown, L.J. Lanzerotti, 218 (1982) 525. L.J. Lanzerotti, and W.L. Brown, Adv. [41 R.E. Johnson, Space Res. 4 (1984) 41. 151 L.J. Lanzerotti, W.L. Brown, and R.E. Johnson, in: Ices in the Solar Systems, eds. J. Klinger et al (Reidel, Dordrecht, 1985) pp. 317-335. W.L. Brown, and R.C. Johnson, Nucl. [61 L.J. Lanzerotti, Instr. and Meth. B14 (1986) 373. [71 A.F. Cheng, P.K. Haff, R.E. Johnson, and L.J. Lanzerotti, in: Natural Satellites, ed. J. Bums (Univ. Arizona, Tucson, 1985) p. 403. [8] L.J. Lanzerotti and R.E. Johnson, in: Ion Beam Modification of Insulators Materials, eds. P. Mazzoldi and G.W. Arnold (Elsevier, Amsterdam, 1987). (Prentice191 R.M. Goody and J.C.G. WaIker, Atmospheres Hall, Inc. NJ, New Jersey, 1972) pp. 46-48. to Space Science, ed. W.N. HOI B. DOM, in: Introduction Hess (Gordon and Breach, New York,1965) p. 485 and S.M. Krimigis, Physics Today 38 [ill L.J. Lanzerotti (October 1985) 25. L.F. Burlaga, I121 N.F. Ness, M.H. Acuna, K.W. Behannon, J.E.P. Connemey, R.P. Lepping, and F.M. Neubauer, Science, 233 (1986) 85. W.I. Axford, A.F. Cheng, ]131 S.M. Krimigis, T.P. Armstrong, G. GIoeckIer, D.C. Hamilton, E.P. Keath, L.J. Lanzerotti, and B.H. Mauk, Science 233 (1986) 97. W.L. Brown, WM. Augustyniak, R.E. [141 L.J. Lanzerotti, Johnson, and T.P. Armstrong, Astrophys. J. 259 (1982) __^ YZIJ. [15] L.J. Lanzerotti, W.L. Brown, C.G. Maclennan, A.F. Cheng, S.M. Krimigis, and R.E. Johnson, J. Geophys. Res. 92 (1987) 14949. (161 R.A. Brown, in: Int. Astron. Union Symp. No. 65, eds. A. Woszczyk and C. Iwaniszewska (1974) 527. [17] D.L. Matson, T.V. Johnson, and F.P. Fanale, Astrophys. J. 192 (1974) L43. [18] C.L. Melcher, D.J. LePoire, B.H. Cooper, and T.A. TombrelIo, Geophys. Res. Lett. 9 (1982) 1151. [19] F. BagenaI and J.D. Sullivan, J. Geophys. Res. 86 (1982) 8447.

[20] R.E. Johnson, J.W. Boring, C.T. Reiman, L.A. Barton, E.M. Sieveka, J.W. Garrett, K.R. Farmer, W.L. Brown, and L.J. Lanzerotti, Geophys. Res. Lett. 10 (1983) 892. [21] L.J. Lanzerotti, C.G. Maclennan, W.L. Brown, R.E. Johnson, L.A. Barton, C.T. Reiman, J.W. Garrett, and J.W. Boring, J. Geophys. Res. 88 (1983) 8765; ibid 89 (1984) 9157. [22] R.E. Johnson, in: Ices in the Solar System, eds. J. Khnger et al. (Reidel, Dordrecht, 1985) pp. 337-339. [23] D.J. Stevenson, Nature 298 (1982) 142. [24] S.W. Squyres, R.T. Reynolds, P.M. Cassen, and S.J. Peale, Icarus 53 (1983) 319. [25] L.J. Lanzerotti, W.L. Brown, K.J. Marcantonio, and R.E. Johnson, Nature 312 (1984) 139. [26] J. Cooper, J. Eraker, and J.A. Simpson, J. Geophys. Res. 90 (1985) 3415. [27] A.F. Cheng, L.J. Lanzerotti, and V. Pirronello, J. Geophys. Res. 87 (1982) 4567. [28] A.F. Cheng, and L.J. Lanzerotti, J. Geophys. Res. 83 (1978) 2596. [29] L. Calcagno, G. Foti, L. Torrissi, and G. Strazzulla, Icarus 63 (1985) 31. [30] L.J. Lanzerotti, W.L. Brown, and K.J. Marcantonio, Astrophys. J. (1987) 910. [31] G. Strazzulla, L. Calgagno, and G. Foti, Astron. Astrophys. 140 (1985) 441. 1321 S.W. Squyres, and C. Sagan, Nature 303 (1983) 782. [331 R.E. Johnson, L.J. Lanzerotti, W.L. Brown, W.M. Augustyniak, and C. Musil, Astron. Astrophys 123 (1983) 343. (341 V. Pirronello, G. StrazzuIIa, and G. Foti, Astron. Astrophys. 118 (1983) 341. 1351 V. Pirronello, G. Strazzulla, G. Foti, W.L. Brown, and L.J. Lanzerotti, Astron. Astrophys. 134 (1984) 204. ]361 G. Strazzulla, V. Pirronello, and G. Foti, Astron. Astrophys. 123 (1983) 93. 1371 M.H. Moore, B. DOM, R. Khanna, and M.F. A’Heam, Icarus 54 (1983) 388. and K. Fredga, Astrophys. Space Sci. 50 1381 L. Kristoferson, (1977) 105. [391 M.H. Moore, and B. Dorm, Astrophys. J. 257 (1982) L47. J. B&nit, J.-P. Bibring, D. Ledu, and R. vO1 F. Rocard, Meunier, Radiat. Eff. 99 (1986) 97. ]411 J. B&t, J.-P. Bibring, S. Della Negra, Y. LeBeyec, and F. Rocard, Radiat. 99 (1986) 105. Icarus 66 (1986) 619. (421 R.E. Johnson and L.J. Lanzerotti, J.F. Cooper, and L.J. Lanzerotti, ESA 1431 R.E. Johnson, SP-250 (1986) 269. W.L. Brown, J.M. Poate, and W.M. 1441 L.J. Lanzerotti, Augustyniak, Nature 272 (1978) 431. 95 (1981) I451 T. Mukai, and G. Schwehm, Astron. Astrophys. 373. Science I461 J.P. Bradley, D.E. Brownlee, and P. Fraundorf, 226 (1984) 1432. 1471 G. Strazzulla, Icarus, 67 (1986) 63. (481 K. Rossler, H.-J. Jung, and B. Nebeting, Adv. Space Res. 4 (1984) 83. [491 P.K. Haff and A. Eviatar, Icarus 66 (1986) 258.