The discovery of sodium D-line emission from Io (Brown 1974) and that these atoms form an extended cloud (Trafton et al. 1974) was the first evidence that the surface of Io might supply material to the Jovian magnetospheric plasma. It is now clear that the material which makes up the plasma, predominately sulfur and oxygen ions, was in fact originally ejected from Io as neutrals (Matson et al. 1974, Haf t et al. 1981, Cheng 1982). Although the alkali metals, sodium and potassium, are not the major species in the plasma (Bagenal and Sullivan 1981, Gehrels and Stone 1983, Bel- cher 1983), and are often left out of large- scale material injection processes (Kumar 1984, Cheng 1984), they are easily observed
i This work was supported by the NSF Astronomy Division under Grant AST-85-11391 and the NASA Geology and Geophysics Division under Grant NAGW-186.
2 Current address: Code 4673, Radiation Effects Dept., Naval Research Laboratory, Washington, DC 20375-5000.
233
and features of their existence can provide strong constraints on larger scale magneto- sphere-satel l i te interactions (Schneider et al. 1987).
A. Impl ica t ions f o r A t m o s p h e r i c Co lumn Dens i t y
The injection of material from Io into the torus region requires that at some point ma- terial be ejected from the surface of Io ei- ther directly into the magnetosphere or into an atmosphere requiring subsequent ejec- tion (Sieveka and Johnson 1984). In either case, material must be ejected from the sur- face and this presents a dilemma due to the low volatility of possible sodium-containing compounds (Matson et al. 1974). Since so- dium is usually contained in the form of low vapor pressure compounds, at the surface temperatures characteristic of Io, these compounds will not be present in an atmo- sphere that is not, at least partially, sputter produced (Fanale et al. 1977, Haf t et al. 1981, Summers et al. 1983). Therefore, sputtering of sodium-containing com- pounds on the surface of Io is the favored
mechanism to explain the presence of the neutral sodium cloud (Matson et al. 1974). This mechanism, though, is improbable if there is a collisionally thick atmosphere which does not allow the majority of the plasma ions to penetrate to the surface. The sputtering mechanism, therefore, puts con- straints on the atmospheric column density (Haft et al. 1981, Matson and Nash 1983) and thus the means of supplying the sulfur and oxygen required to maintain the plasma torus (Sieveka and Johnson 1985, McGrath and Johnson 1987).
B. Implications f o r Surface Composit ion
The existence of the sodium cloud, in particular its size and stability, also im- poses strong constraints on Io's surface composition. The alkali metal-containing material must be sufficiently plentiful in or- der to supply enough material to maintain the neutral cloud in a steady state (Nash et al. 1987). By stepping backward in the ejec- tion process, this material must then also be a participant in the rapid volcanic resurfac- ing that occurs on Io (Johnson and So- derblom 1982). Furthermore, if we assume, as is generally accepted, that the surface is ultimately the source of all plasma material (Matson et al. 1974), then this requires that the host contain only atoms which are con- tained in the plasma. This qualitative rela- tionship between the surface composition and plasma composition can also be quanti- tatively correct if the volcanic resurfacing rate in the regions which supply most of the sodium is sufficiently slow so that sputter- ing of a number of monolayers of material has occurred. That is, in spite of the possi- ble initial nonstoichiometric sputtering of sodium-containing compounds on the sur- face, in time the sputtering rate for individ- ual components must approach the stoi- chiometric composition of the original material (Sigmund 1981, Haff et al. 1981).
For example, the absence of a signature for chlorine eliminates NaCl as a possible surface host and the detection of carbon was only consistent with it being of solar
wind origin rather than from Io, thus elimi- nating the carbonates (Fanale et al. 1982, Gehrels and Stone 1983). Therefore, in or- der to be consistent with plasma data, the alkali metals are likely to exist on the sur- face in a compound containing sulfur or ox- ygen or both. Possible examples of these compounds must also satisfy available spectral data. The sulfates would supply Na, S, and O to the magnetosphere but are unlikely surface constituents since their deep absorption band at 4.4 /zm is absent from Io's spectrum (Fanale et al. 1982). Al- ternatively, sodium (and potassium) can be an adsorbed species (Hapke 1979), depos- ited from the volcanic gases or produced by ion bombardment of the surface (Hapke 1986, McGrath et al. 1986). The important difference is that adsorbed sodium is thought to be weakly bound and thus should sputter monatomically with a larger yield than chemically bound sodium. Ex- perimental results for the sputtering of weakly bound adsorbed sodium do not ex- ist.
C. Na2S as a Surface Component
An often-mentioned family of com- pounds in which the sodium may exist on the surface are the sulfides, Na2Si (Nash and Fanale 1977, Fanale et al. 1977, Lunine and Stevenson 1985). The monosulfide of sodium, Na2S, is especially promising since it also exhibits several important features found in Io's spectrum (Fanale et al. 1979, Nash and Nelson 1979, Nash 1987, Nash and Van Hecke 1987). The most common method of preparation of Na2S is by react- ing the elements, although it is also possible to dehydrate the hydrate (Brauer 1963, Walker 1979). The production of sodium sulfide on the surface of Io could take place within a hot spot through a similar reaction of the elements. This scenario not only pro- vides a mechanism of production, but in- corporates itself into the volcanic resurfac- ing process as required.
To date little has been written about the chemical or physical properties of the so-
THE SPUTTERING OF SODIUM SULFIDE ON IO 235
dium sulfides, possibly due to the extreme difficulty involved in handling these unsta- ble and hygroscopic compounds. The most comprehensive investigation of the chemis- try of sodium polysulfides of any impor- tance to these measurements was done by Oei (1973a, 1973b) in investigating the reac- tive species in a sodium-sulfur battery. At room temperature, NazS is a white crystal- line solid, as are all alkali metal sulfides (Tegman 1974). Its high melting point (mp = 1453°K) is evidence for the strong bind- ing of the molecules occurring within the solid and its unknown boiling point is evi- dence for its chemical instability. It decom- poses to the elements upon heating (elimi- nating the possibility of evaporating thin films), rapidly hydrolyzes to NaOH and NariS upon exposure to moisture, and is converted to H2S and Na2CO3 upon expo- sure to air (Nash 1987).
D. Sputtering of Na2S
In this paper we describe the sputtering of NazS. The production of thin films of Na2S, which are necessary to produce any meaningful sputtering measurements, ac- counted for the majority of time spent in researching this system. We chose NazS in this study because it is the least volatile of the sulfides and because of its commercial availability and purity as compared to those of the other polysulfides. The experimental results are used to explore the possibility of ion-induced ejection of sodium from NazS in terms of supplying the necessary injec- tion rate of sodium atoms. We also incorpo- rate our experience with sulfur sputtering in order to consider other possible sulfides or sodium-containing surface materials.
II. EXPERIMENT
The production of thin films of NazS was accomplished by a spray coating proce- dure. Due to the extreme hygroscopic na- ture of sodium sulfide any handling of the sodium sulfide powder, the target sub- strates, or the final targets took place in a dry nitrogen atmosphere. The target forma-
tion procedure began with producing the spray coating solution. In a dry nitrogen filled glove box approximately 1 cc of NazS powder (Aesars 99.9% pure) was mixed in a beaker with about 50 ml of anhydrous meth- anol. Due to the solute's polar nature most of the powder dissolved easily. To remove any large particles which did not go into solution the mixture was then filtered once. With a small hobby spray painter or atom- izer this solution was then sprayed onto a nickel substrate. The spraying process took on the order of 1 sec to complete and re- sulted in a very thin film of solution on the substrate. The solvent then quickly evapo- rated leaving behind the Na2S solute as a thin film. Approximately 900 such "coats" were applied over a period of 90 min. The film formed slowly and in small regions. The film was considered complete when the regions overlapped and the substrate was completely covered. Once the spraying was completed the targets were dried for 2 hr by warming slightly above room temperature and increasing the dry nitrogen flow rate on the target. The final targets were then mounted in a vacuum chamber and pumped on at a pressure of 10 -6 Torr for 1 hr before cooling to low temperatures. As a check of the stoichiometry of the NazS films quanti- tative X-ray analysis was done on the Na2S powder and a Na2SO4 standard, verifying the elemental composition (two Na to one S).
The nickel substrate, on which the NazS film was deposited, had 1 /.tCi of Po-210 electrochemically deposited on the center. By measuring the energy loss of the 5.305- MeV a-particle emitted by the Po-210 as it passed through the NazS film a density thickness could be measured. The sputter- ing yield could then be determined by mea- suring a change in film thickness for a given amount of integrated beam current. The above procedure of target formation re- sulted in an extremely nonuniform film. The variation in the thickness, as deter- mined by the width of the a-particle spec- trum, was on the order of 3/zm for a 8-/zm-
236 CHRISEY ET AL.
thick film. Because of the variation in thickness we determined the yield below by measuring the change in the centroid of the a -peak . Fur ther details of the sputtering ap- paratus, the thickness measurement tech- nique, and the neutral mass spec t rometry are descr ibed e lsewhere (Chrisey et al. 1987, 1988).
III. RESULTS
A. Sputtering Yield Results
The sputtering yield for 34-keV Ar + inci- dent on NazS at 15°K was measured for two separate films. Typical particle fluxes dur- ing the irradiation were - 5 / ~ A / c m 2 (or 3 x 1013 ions/cm2-sec). The total particle flux hitting the target or dose was - 5 x 1018 ions /cm 2. The total amount of material re- moved , as described above, was found to cor respond to 0.5 NazS molecules per inci- dent ion. This yield value, expressed in terms of the number of parent species ejected, assumes stoichiometric sputtering and is very small compared to solid sulfur, as is to be expec ted due to its large surface binding energy.
B. Mass Spectrum of Ejected Species
Since Na2S is a two componen t molecu- lar target there can be a variety of molecu- lar species ejected. The measured values for different neutral species detected by a quadrupole mass spec t rometer (QMS) are given in Table I and normalized to S, the largest signal measured. It is seen that sig- nificant quantities of molecular Na2S are ejected, which was not initially expected for this material . I t is important to note that the values given in Table I are not relative yields but ra ther an uncorrected counting rate f rom the QMS. In order to make a quanti tat ive compar ison of different spe- cies ejected, these signals must be cor- rected for various instrumental sensitivities such as the cracking fractions of the species ion the ionizer and the mean time spent in the ionizer. These data do not exist for NazS.
Molecular Na2S, upon entering the elec-
TABLEI
RELATIVE QMS SIGNALS FOR EJECTED MASSES
FROM SPUTTERING OF A NaES TARGET BY 3 4 - k e V Ar t IONS
Note. The QMS signal shown is the relative number of counts the QMS detects for a given number of inci- dent ions hitting the target. Also shown is the escape energy, Eesc, in electronvolts for the various species calculated for the escape velocity of Io (Eeoc = 1/2 M (Ve~) 2, where V~s~ 2.6 km/sec).
tron impact ionizer, can produce fragment ions such as N a ] , S +, Na +, and NaS + in addition to NazS + ; the exact composi t ion of the sputtered ejecta is not clear f rom the values given in Table I. Some of the species of ions observed can be fragments of one or more parents. However , a species such as $2, which based on previous measurements of the sputtering of sulfur-containing com- pounds always appears (Boring et al. 1986), must be produced in the ion bombardment process . Thus the detect ion of $2 indicates chemical modification induced by the inci- dent ions. I f we ignore these correct ions and interpret the numbers in Table I as approximate relative number densities present in the sputter ejecta, then there is an apparent overabundance of sulfur ejection. This may be due to the aforemen- tioned instrument sensitivities or a beam- induced migration phenomenon in which a slightly charged surface can cause the Na + ions in the solid to migrate (Miotello and Mazoldi 1985). However , since the thin films were originally composed of Na2S and many monolayers of material were re- moved, stoichiometric sputtering is as-
THE SPUTTERING OF SODIUM SULFIDE ON IO 237
.._=
-g ,g
v
Z
>-
4 0 0 0
2 0 0 0
1 0 0 0
8 0 0
6 0 0
4 0 0
= = I i J i
/ / / / 5 2 0 e V S +
/ /
/ / / 2 6 0 e V O ÷
q / i o]o 4o 6o 8'o, 2 0 0 210 i i 10 2 0 0 4 0 0
F D ( ~ ) ( e V ~ 2 )
1.0
0 . 8
0 . 6
0 . 4 ¢~
0.2 ~ >-
0.1
0 . 0 8
I 600 800
FXG. I. The relative yield of neutral sputtered species detected as m/q = 23 versus the surface deposited energy, FD(0), for incident 34-keV Ne +, Ar +, Kr +, and Xe ~ from left to right, respectively. The fitted line is for a linear dependence as expected for collision cascade sputtering. The values of FD(0) have been divided by 3 to represent the target as a "quasi-monatomic" solid. The values of FD(0), and thus the yield, expected for corotating ions (520-eV sulfur and 260-eV oxygen ions) are indicated. On the right-hand axis we give the sputtering yield which would be expected for collision cascade sputtering and normalize vertically based on the measured 34-keV Ar + sputtering yield value.
sumed. Table I simply indicates that both molecular and a tomic species occur and be- low we use one of the species (Na) to moni- tor the sputtering yield.
C. Mechanism o f Ejection
In order to understand the behavior of the sputtering yield as a function of ion en- ergy and ion type we measured the signal for m/q = 23 (Na) with the QMS for four different ions (34-keV Ne +, Ar +, Kr +, and Xe +) and plot the results in Fig. I versus the surface-deposi ted energy, FD(0). The surface-deposi ted energy is often writ ten as aSh. In this express ion o~ describes the an- isotropy of the cascade and S, is the nuclear stopping cross section. For collisional sput- tering of mona tomic solids (Sigmund 1969, 1981) the sputtering yield is proport ional to FD(0) (see Eq. (2)). The linear dependence of the sputtered signal as a function of the surface-deposi ted energy, as indicated by the line in Fig. 1, is like that in the standard theory for the collisional sputtering of mon- atomic solids. This justifies the method of extrapolat ing the expected sputtering yield
for corotat ing sulfur and oxygen ions used here and also used for the sputtering of sul- fur (Chrisey et al. 1987). Assuming the sputtering yield (Y) also has a linear depen- dence like that seen for m/q = 23 we plot on the right-hand axis the sputtering yield which would be expected for collisional sputtering based on the measured 34-keV Ar + sputtering yield value. The arrows in Fig. 1 indicate the values of the surface- deposited energy expected for corotating sulfur and oxygen ions. In Table II we present values of the surface-deposi ted en- ergy and sputtering yield that were mea- sured for 34-keV Ar + bombardmen t and also those which would be expected for corotat ing sulfur- and oxygen-ion bombard- ment.
The mechanism of sputtering ocurring here must be of collisional origin (Chrisey et al. 1987) and not of electronic origin. In order for electronic sputtering of insulators to be efficient, the surface binding energy must be small compared to the stored elec- tronic excitat ion energy (i.e., the band gap in solids), as is the case for the condensed
238 CHRISEY ET AL.
TABLE II
NUCLEAR STOPPING CROSS SECTIONS, Sn, AND YIELD VALUES FOR 34-keV Ar + AND COROTATING
SULFUR AND OXYGEN IONS
Ion Ion a" S, FD(0) Yield energy (eV /~k 2) (eV ,~2) (Na,S/ion) (keY)
34 Ar 0.23 1073 247 0.5 0.52 S 0.24 419 103 0.2 0.26 O 0.32 182 58 0.1
Note. The nuclear stopping cross sections are from Ziegler (1984) and are given as a mean cross section per atom. The extrapolated yield values were obtained by assuming the total yield is proportional to the sur- face-deposited energy FD(0) (where FD(0) = aS,).
Values of a for various species were obtained from an empirical curve by Sigmund (1981).
gas solids (Brown et al. 1984). For other expected surface const i tuents of Io, such as SO2 and sulfur, electronic sputtering was possible (Lanzerot t i et al. 1982, Torrisi et al. 1987, Chrisey et al. 1987). For the tightly bonded sys tem Na2S, it seems very unlikely that any significant electronic sput- tering could take place at the electronic excitation densities produced by typical plasma-ion bombardment .
D. Appl ica t ion to the Produc t ion o f the S o d i u m Cloud
The amount of material escaping the gravitational field of Io necessary to main- tain the neutral cloud in a s teady-state con- dition has been est imated to be about 10 7
sodium atoms/cm2-sec (Matson et al. 1974). The amount of material which would be ejected f rom the surface due to the entire corotat ing p lasma ion population bombard- ing the surface is given by
ejected sodium = 4~ Ym~c, (I)
where ~b is the corotat ing ion flux, Ym is the mean yield, and ~c is the fractional coverage of NazS. Substituting q~ --- 10 l° ions/cm2-sec, the mean yield of sodium Ym = 0.2 sodium a toms /p lasma ion f rom Table II, and as- suming a rough lower limit, [~c = 0.01, we
obtain 2 x l 0 7 sodium atoms/cm2-sec leav- ing the bombarded surface. Note that the value of 1% used for ~c is the same as that of H a f t et al. (1981) but is less than that ob- tained f rom measurements of the sodium mixing ratio in the cold p lasma torus near Io or < 5 % (Bagenal and Sullivan 1981) and 3% (Gehrels and Stone 1983). The number presented above corresponds to the number of sodium atoms which leave the surface, regardless of the form in which they leave. As the observed cloud is a tomic sodium, sodium ejected in a molecular form must dissociate following ejection in order to contribute.
The previous results for the sputtering of SO2 and sulfur showed that only a small percentage ( - 5 % ) of the species ejected would escape Io directly (Johnson et al. 1984, Chrisey et al. 1987). That is, the dis- tribution in energy of species sputtered f rom SO2 and sulfur indicated that the ma- jori ty of the species ejected f rom the sur- face of Io would be gravitationally bound and thus form a sput ter-produced corona. I f we were to assume that the escape fraction was nearly the same for Na2S (Thomas 1986) then subsequent ejection and dissoci- ation f rom this sodium-containing corona would have to be very efficient in order to maintain the sodium cloud. But, as pointed out in the Introduct ion, Na2S is a solid very different f rom SO, and sulfur. There are strong theoretical reasons to expect the en- ergy spect ra of Na2S should also be differ- ent f rom those of SO2 and sulfur, as dis- cussed below.
IV. DISCUSSION
The model for the product ion of the so- dium cloud to be explored in this paper is the direct ejection of sodium from some form of sodium sulfide on the surface of Io. Although the ejected sodium yield calcu- lated above is larger than that required for an injection rate of 107 sodium a toms /cm 2- sec, the fraction of this ejected sodium which escapes the satellite has not been in- cluded. Fur thermore , f rom Table I it ap-
THE SPUTTERING OF SODIUM SULFIDE ON IO 239
pears that most of the sodium is ejected in a molecular form which will require later dis- sociation. An estimation of the escape frac- tion, its implication for the production of the sodium cloud, and a reevaluation of Na2S as a possible sodium-containing sur- face material are discussed below.
A. Collisional Sputtering
The observed yield for 34 keV Ar ÷ is small compared to that usually measured for much more volatile targets such as con- densed SOz (Johnson et al. 1984). This value can be put in perspective if we ignore the difference in the Na and S masses and treat solid NazS as a monatomic target. The aforementioned yield of 0.5 Na2S mole- cules/ion would then correspond to 1.5 at- oms/ion. With the yield expressed in these units we substitute an average nuclear stop- ping cross section (S,) per atom in the stan- dard sputtering yield formula (Sigmund 1969, 1981) for monoatomic solids,
Y = 0.042 o~S,/(U ~2). (2)
This can be used to give an effective sur- face binding energy per atom or U in the above expression. The result of applying Eq. (2) to solve for U is a value of 6.9 eV/ atom (or 21 eV/NazS molecule). This value is very large but agrees well with the calcu- lated value of the cohesive energy by Agarwal et al. (1977) of 23 eV/Na2S mole- cule. The cohesive energy of a monatomic material is often used as a reasonable esti- mate for U in Eq. (2). A similar agreement between this "quasi-monatomic" sputter model and experiment was previously ob- tained for $8 sputtering. The effective bind- ing energy as calculated from a fit of the monatomic sputter yield formula to experi- mental data (as above) gave a value of 0.19 eV/S atom or 1.5 eV/S8 molecule. This value also agreed reasonably well with the measured value of 1.1 eV/S8 molecule for the sublimation energy (Chrisey et al. 1987). These results are somewhat surpris- ing as the expression was developed for monatomic solids and not molecular solids.
B. Electronic Sputtering
The conclusion that electronic sputtering on Io will not produce a significant sputter- ing yield is an extremely important result in terms of determining the energy and type of plasma ions which might reach Io's surface. In order to remove the problem of Io having a thick atmosphere and also having ions reach the surface it has been suggested that the plasma ions which produce the sputter- ing are high energy protons (Matson et al. 1974, Macy and Trafton 1975). This is con- sistent, for example, with a 1016/cm 2 SO2 atmosphere (Sieveka and Johnson 1985; McGrath and Johnson 1987) because a 1- MeV H + ion can penetrate this atmosphere, losing only about 200 eV. A l-MeV H + ion incident on an SO2 target will produce sig- nificant sputtering and chemical alteration (Lanzerotti et al. 1982, Johnson et al. 1984, Moore 1984). An NazS surface, on the other hand, requires a heavy low energy ion, like corotating oxygen and sulfur ions, to pro- duce significant sputtering. Furthermore, in the absence of a secondary mechanism such as plasma ejection from an atmo- sphere (Pilcher et al. 1984), the velocity of the sodium atoms in the cloud is consistent with the mechanism of sputtering being col- lisional rather than electronic in origin (Carlson et al. 1978). This is because the distribution in energy of species sputtered by an electronic mechanism has a sharp cutoff at high energy corresponding to the maximum impulsive energy input in the electronic relaxation process less the sur- face binding energy (Pedrys et al. 1984).
C. Energy Spectra
In considering the sputtering of Na2S on the surface of Io in terms of contributions to the injection rate, it has not yet been determined whether the majority of sodium is ejected directly into the magnetosphere or later during its gravitationally bound tra- jectory (Sieveka and Johnson 1984, Mc- Grath and Johnson 1987). We assume here that the energy distribution of ejected spe-
240 CHRISEY ET AL.
TABLE 1II
VALUES OF THE SURFACE BINDING ENERGY (U) FOR
VARIOUS PROPOSED SURFACE CONSTITUENTS OF 10,
GIVEN PER MOLECULE
Species U Source (eV)
SO2 0 .37 Sublimation energy $8 1.1 Sublimation energy Na/lunar rock ~ 4.5 Sublimation energy Na2S 23 Cohesive energy
Note that this value is thought to correspond to the sublimation energy of Na20 (De Maria et al. 1971).
cies from an NazS target displays a Thomp- son distribution (Sigmund 1969), as ex- pected for collisional-induced sputtering,
d N E dE - K (E + U) 3' (3)
where d N is the number of a given species ejected from the surface with an energy be- tween E and E + dE, U is the surface bind- ing energy of a given species, and K is a normalization constant. The fraction of the total amount sputtered which can escape the gravitational field of a satellite, ~esc, is the integral of Eq. (3) for energies greater than the escape energy, Eesc, divided by the integral of Eq. (3) over all energies. The result is
2 x + 1 l~esc -- (X + 1) 2' (4)
where x = Eesd U. From the above equation it is obvious that the number of a given spe- cies which can escape directly is dependent on the relative magnitude of U to Ee,c. Val- ues of U, known for various molecular spe- cies relevant to Io, are given in Table lII. If we choose for the surface binding energy values of 7 and 21 eV (from the above quasi-monatomic model) for Na and Na2S, respectively, we find the fraction for which escape is nearly 100%. A similar result was used earlier for sodium by Sieveka and Johnson (1984). This fraction is much larger than that found for sulfur and SO2 because
of the shift toward higher energy of the dis- tribution in Eq. (3) (Sieveka and Johnson 1984). This value also cautions against the use of values for the escape fraction and sputtering yield from the SO2 and sulfur system for the sputtering of Na2S (Carlson et al. 1978, Sieveka and Johnson 1984, Thomas 1986). With this value of about 100% we find from the result of Eq. (1) that in terms of jus t material injection, direct sputtering on the surface can account for 2 x 10 7 sodium atoms/cm2-sec even with a modest 1% concentration. This is equal to the lowest estimate for the supply rate (Matson et al. 1974) but below the largest more recent estimates of 109 sodium atoms/ cm2-sec (Pilcher et al. 1984, Nash 1987).
D. An Alternative Scenario
Na2S contains the most sodium of the so- dium polysulfides known to exist. Unfortu- nately this abundance of sodium, coupled with the favored size and oxidation state of - 2 for S, causes the strong ionic bonding in NazS and thus the relatively low sputtering yield. Although the larger sulfides contain less sodium their increased sulfur coordina- tion leads to a considerably lower binding energy (as indicated by their melting points). Values of the melting point for vari- ous sodium and potassium polysulfides, thought to exist in the melt, are given in Table IV. This t radeoff of lower sodium content per molecule for increased volatil- ity, and thus increased sputtering yield, makes the larger sodium polysulfides at- tractive in terms of meeting the required injection rates.
As an example of this idea of lower bind- ing on increased sulfur coordination we plot in Fig. 2 the melting points of various known sodium and potassium polysulfides versus the ratios of Na or K atoms to S atoms in the molecule. This plot starts with $8 since it contains the lowest number of sodium atoms (zero) and ends with Na2S and K2S since they contain the most (two) per molecule. The large jump in melting points from Na2S to Na2S2 is indicative of
a When NazS3 approaches a melting point it quickly changes to equal parts of Na2S4 and Na2S2 (Oei 1973a).
the favored size of S 2 versus $22 and larger ions in the lattice.
At this point it is worth reevaluating the aforementioned sodium sulfide production scenario in order to explore the feasibility of larger sodium polysulfides. The chemical composition of various amounts of sodium and sulfur at various temperatures is pre- sented most clearly in the phase diagram results of Oei (1973a). A mixture of sodium and sulfur, when heated, will first form Na2S,
2Na + S ~ Na2S. (5)
Depending on the stoichiometric ratio of sulfur to sodium which exists in the melt, subsequent formation of larger polysulfides can proceed not through a single reaction as shown above but rather through an interme- diate polysulfide. If additional sulfur exists, as is likely, sodium pentasulfide is formed,
2Na2S + 4S ~ Na2S + Na2S5--~
Na2S2 + Na2S4. (6)
Further reactions among these products with sulfur then proceed,
3Na2S2 + 6S ~ 2Na2S5
+ Na2S2 ~ 3Na2S4. (7)
The times necessary for these reactions (hours) are negligible compared to that for which sodium and sulfur will be in contact with one another in a hot spot on Io. There- fore, for our purposes the final composition can be determined by the phase diagram of Oei (1973a) for a given ratio of S to Na. Although Oei's diagram ends with Na2S5 being the largest polysulfide formed, subse- quent work associated with the sodium- sulfur battery state that larger polysulfides
o-
1500
1 0 0 0
5 0 0
D K 2 S i
• Na 2 S i
D
S 8
I I I
I I I 0.5 1.0 1.5
÷ N a , K - - / m o l e c u l e
÷ S
2.0
FIG. 2. The melting points of various sodium and potassium sulfides versus the ratio of sodium or potassium atoms to sulfur atoms per molecule. Also shown in this plot is $8.
242 CHRISEY ET AL.
up to Na2S20 exist in the melt (Kn6dler 1985).
The method of Na2S formation within a hot spot given in the Introduction never stated the stoichiometric ratio of Na to S which exists in the melt. Instead the only estimate of sodium presented was for the Na2S surface coverage, based on sodium mixing ratios in the plasma torus (Bagenal and Sullivan 1982, Gehrels and Stone 1983, Schneider et al. 1987). Clearly, if excess sulfur exists in the melt, the larger poly- sulfides will be produced (Lunine and Stevenson 1985), in which case the amount of sodium existing on the surface will stay the same ( - 1 % in our estimates), but its effective surface binding energy will de- crease significantly. That is, we expect that the effective surface binding energy in the aforementioned quasi-monatomic model will decrease as the melting point de- creases. Then as the sodium bonds to more and more sulfur atoms the average binding per atom which occurs in the solid should approach that of pure sulfur. As previously stated, the effective sputtering binding en- ergy of sulfur determined from our earlier results is 0.19 eV/S atom.
If, on increased sulfur coordination, the sodium binding energy approaches that of pure sulfur, then the mean yield, Ym in Eq. (1), due to a plasma made up equally of oxygen and sulfur ions would be about 20 sodium atoms/plasma ion, i.e., the same sputtering yield as for pure sulfur (Chrisey et al. 1987). However, as the yield in- creases the escape fraction decreases due to the large number of slower species. As- suming an idealized situation where the sputtering yield and escape fraction are given by Eqs. (2) and (4), respectively, the direct injection rate for sputtering from the surface (which is the product of Eqs. (2) and (4)) becomes independent of U when the surface binding energy per atom de- creases below Ecsc per atom. Based on the above, a significant amount of material goes on to form a sputter-produced coronal atmosphere similar to the case of sulfur (Chrisey et al. 1987) and SO2 (Sieveka
and Johnson 1985) sputtering. This atmo- sphere is then a potential source of material for the torus containing atoms and small molecules (e.g., NaS). In fact, previous cal- culations of the sodium supply rate sug- gested that ejection and dissociation of so- dium from a molecule in an atmosphere was favorable for producing the observed spa- tial distribution of fast sodium ejection as well as the correct ratio of fast to slow so- dium ejection (Sieveka and Johnson 1984, Pilcher et al. 1984). Using the sulfur sput- tering rates from Chrisey et al. (1987), the yield in Eq. (i) becomes Ym -- 20 sodium atoms per plasma ion, giving a surface source strength of - 2 × 10 9 sodium atoms/ cmZ-sec using a 1% coverage.
The distribution of masses ejected from the larger polysulfides are unknown. Pre- vious work on the sputtering of sulfur indi- cated that smaller species (in particular $2) were primarily ejected from a target origi- nally composed of $8 molecules (Chrisey et al. 1988). These results also showed that correlations in the cascade (e.g., multiple hits) were increasingly necessary to eject larger whole species. Therefore, we expect that corotating ion bombardment will eject primarily small molecules containing so- dium with some atomic sodium.
V. CONCLUSION
We have presented measurements of the collision cascade sputtering of Na2S and ex- trapolated the yield to that for the corotat- ing ions (stopping powers) relevant to Io. The yield is much less than that for other more weakly bound species. As sodium may appear in the form Na2Si (where i > 1), then based on our description of the sputter yield for Na2S and on our previous mea- surements for $8 and using a rough lower limit of 1% atomic composition for sodium, we can constrain the sodium supply to the sputter corona by corotating ions as 2 × 107-2 x l09 sodium atoms/cm2-sec depend- ing on the form of sodium on the surface. These numbers are now available for de- scribing the Io atmospheric corona. It is in-
THE SPUTTERING OF SODIUM SULFIDE ON IO 243
teresting that a large fraction of sodium is ejected in molecular form (e.g., NaS, Na2S). This is in contrast to assumptions made earlier for evaluating the sodium sputter supply rate (Haft et al. 1981). How- ever, it is consistent with the calculation of Sieveka and Johnson (1984) and Pilcher et
al. (1984) in that aspects of the fast sodium supply rate suggested collisional ejection of sodium contained in a molecular form from the upper atmosphere. The mass spectra also suggest that a fraction of the sodium is ejected in an atomic form. Because of the nature of the collision cascade spectrum, these neutral sodium atoms have large av- erage energies so that in a collisionless re- gion (e.g., polar region, nightside) much of the sodium so produced escapes. The fact that some of the ejected sodium occurs as atomic sodium is evident from the observa- tions of Schneider et al. (1987). That sup- plied to the atmosphere in molecular form requires subsequent photo- or plasma-in- duced dissociation and collisionai ejection by the incident plasma. Although the amount of Na required from the surface is not well established the largest suggested values required to supply the torus are - 1 0 9
sodium atoms/cm 2. The present study shows that more than this upper limit of sodium can be supplied to the coronal at- mosphere if Na is ejected from a polysulfide and if the surface concentration is larger than the 1% used here. The subsequent ejection into the torus is also determined by the incident plasma (Sieveka and Johnson 1984, McGrath and Johnson 1987).
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