The presence of condensed SO2 on Io mandates a finite abundance of SO2 vapor which must be present, regardless of plume activity. The absorption of SO2 was measured on particulate sulfur and the equilibrium between absorbed SO2, SO2 vapor, and SO2 ice examined, based upon measurements and simple thermodynamic considerations

Fanale, Fraser P.

Zent, Aaron P.

English

NASA Technical Reports Server

mON190:A O D m M C  
Aaron P. Zent and Fraser P. Fanale, Planetary Geosciences Division, Hawaii Institute 
of Geophysics University of Hawaii, Honolulu, Hawaii 
The presence 01 condensed SO2 on 10 mandates a finite abundance of SO2 vapor 
which must be present, regardless of plume activity. Currently, even the order of 
magnitude of the ambient SOz pressure is unknown. However, a number of models 
indicate that the pressure may be near saturation much of the time. Among the 
models that suggest atmospheric pressure does not approach saturation is that of 
Matson and Nash, [1983], who base their model in part on the argument that 10's 
surface must have a very high porosity, and in part on the argument that the parti- 
culate material in 10's volcanic plumes has a very fine grain size. This implies that a 
significant amount of cold, particulate, surface area is likely to be in diffusive con- 
tact with an SO2 atmosphere that probably a t  least approaches saturation locally. 
These conditions indicate that adsorption of SO2 may be important SO2 adsor- 
bate. We havs measured the adsorption of SO2 on particulate sulphur, and exam- 
ined the equilibrium between adsorbed SOz, SOZ vapor, and SO2 ice based upon our 
measurements and simple thermodynamic considerations. 
Measurements of SO;! adsorption on partic-llate sulphur a t  conditions near 
those of Io were made on a Nulninco Model MIC 103 OOR Surface-Area Pore-Volume 
Analyzer. Adsorption data were gathered a t  178K, 193K and 225K. These are. higher 
than surface temperatures on lo, which peak wound 130K. The SOz vapor pressure 
(1 to lospa) also exceeds lo surface pressures. The adsorptive behavior of the sys- 
tem must be extrapolated to Io conditions. 
The most rigorous extrapolation method of which we are aware is one 
presented by Anderson e t  al. [1967]. They showed that it is possible to compute 
adsorption isotherms for temperatures below those covered in adsorption measure- 
ments via the equation 
In - (AH, - AH,) (T2 - TI) - 
RTlT2 
Where Pa and pi are the vapor pressures over the adsorped phase and the solid 
phase, respectively. The subscript 1 refers to state variables at condition 1, where 
data are available, and 2 refers to state variables at condition 2 for which we would 
like to  calculate the relative vapor pressure. AH,., is the change in partial molar 
enthalpy in going from the vapor to the adsorbed phase, and  AH^ is the same quar- 
tity for the vapor to ice phase change. 
In order to  use Eq.(l), we must know AH~(T) and, from the data, we must find 
AH,,(T). AH, is found by solving the Clausius-Clapeyron equation a t  points of con- 
stant adsorptive coverage. Since the data are not a t  constant adsorptive coverage 
we must fit a curve to the data and read equilibrium pressures a t  constant adsorp- 
tive coverage from that. 
We are then able to extrapolate our adsorption isotherms to lower tempera- 
tures via Eq.(l). In all cases, our isotherms agree with the data to better than a 
factor of 2.5; the average precision of the data is *50%. 
At a fixed temperature, the equilibrium vapor pressure over adsorbed SO2 may 
be thought of as an extensive property of the system; it increases as the mass of 
SOz in the system increases. The equilibrium vapor pressure over ice however is 
intensive; it does not matter. how much SOz is in the solid phase, the vapor pressure 
depends only on temperature. That means that at a fixed temperature, there is a 
maximum adsorptive coverage, which occurs at the point a t  which the equilibrium 
https://ntrs.nasa.gov/search.jsp?R=19870013926 2020-03-20T10:36:57+00:00Z
vapor pressure over the adsorbed phase is equal to the vapor pressure over ice. 
Thereafter, any additional SO2 present in the system must exist as ice. 
We should riow be able to calculate the maximum adsorptive coverage in terms 
of volume of SO2 per unit mass of sulphur, although we consider this an intermedi- 
ate data product. We would really like t o  know the maximum coverage of SO2 in 
t,errns of number density p e r  unit sxrface area of sulphur (i.e. the rnonolayer cover- 
age). We need to represent our results in this way 5ecause the specific surface area 
of S on lo is unconstrained, but the upper lbnit on the number density uf the cover- 
age, which is more closely related to the chemical potential, must be the same in 
the laboratory as on lo. 
Fortunately, adsorption measurements provide concurrent measurements of 
specific surface area through application of BET theory [Brunauer et  al., 19381. We 
calculate BET surface areas, assuming the size of the adsorbed SO2 molecule is - 
30A2. We use the mean specific surface area of 5 x m2 g-I indicated in our 
analysis to calculate maximum coverage for sulphur at  lo conditions. 
This relationship is shown in Figure 1, along with the data. Each of the isoth- 
erms is truncated against the vapor pressure curve for SO2 ice (solid line). A very 
simple expression can be written to describe the maximum amount of adsorbed SO2 
per square meter of sulphur, at  any temperature. 
Max p, = exp(aT-b) ( 2 )  
where a = 0.1673 K-', and b = 33.89, and p, is the density of adsorbed SO2 in terms 
of g SOz rn-2~.  If the maximum surface temperature on 10 is around 130 X, the max- 
imum abundance of adsorbed SO2 on 10 is around 5 X lo-' g SOz m-2 S. At disk aver- 
age temperatures of 90 K, the maximum adsorbed SO2 capacity cannot be more 
than 6.6 X g SOz rnb2 S. If more SOz is present in the system at  these tempera- 
t.ures, it will exist as ice. 
To understand why our results constitute a new perspective, we must examine 
the previous estimates of SOz adsorption on lo. 
Unpublished adsorption measurements of SO2 on S obtained by Fanale and 
Laue, a t  warmer, higher pressure, conditions suggested that up to a monolayer of 
SOz could be adsorbed onto S at  low relative pressures. I t  was more or less 
accepted that at lower pressures and temperatures characteristic of lo, the adsorp- 
tive coverage should be about the same, around a monolayer [e.g. Fanale e t  al., 
19821. Let us examine how many rnonolayers our current analysis indicates. The 
upper limit of adsorptive coverage is around 5 X g SO2 m-zsulphur a t  130K (Fig. 
1). This mass represents around 4.7 X 10le molecules 'per square meter. If we 
assume that each adsorbed molecule occupies 30A2, the maximum number of 
adsorbed SO2 molecules occupies no more than 1.4 X m2. Thus 1.4 X is 
the maximum number of adsorbed rnonolayers, well over an order of magnitude less 
than previously believed. Nash [I9831 calculated that 0.5 monolayers of adsorbed 
SO2 would be required to create the 30% 4-pm absorption. We argue that the max- 
imum adsorbed SO2 coverage must be significantly less than 0.5 rnonolayers, mak- 
ing spectroscopic detection of adsorbed SO2 highly problematic. 
The question of how important the adsorbed phase on lo is, now becomes 
largely a question of what the adsorbent is. 
We selected sulphur as our adsorbent because it is clear that sulphur allo- 
tropes constitute a significant fraction of 10's surface.. To the extent that elemental 
sulphur dominates the Ionian surface, we have excellent confidence in our results. 
However, the Na and I< in the magnetosphere require the presence of other phases 
on the surface. 
The i m ~ r t a n c e  of the adsorbed phase in suppling the magnetosphere, and in 
diurnal exchange of SO2 with the atmosphere, depends on the abundance and 
adsorptive capacity of other surface materials, as well as on :he specific surface 
area of the sulphur on the 10 surface. Nash f 19861, in experirnonts on vacuum subli- 
mation of solid sulphur, identified a very fluffy form which results from preferential 
sublimation of ring sulphur (Sa) over polymeric sulphur. Such a filamentary residue 
might be expected to have a very high surface area, and hence render the surface 
adsorbed phase a significant sink by mass, although monolayer coverage would 
remain very small. 
REFERENCES 
1. Anderson, et  al., Science,  155, 319-322, 1967. 
2. Brunauer, S., et al., J. Am. C7Lern. Soc . ,  60, 309-319, 1938. 
3. Fanale, F. P., et  al., In: Sate l l i t es  of Ju@~T (Morrison, D. ,  ed.), 756 - 781. 1982. 
4. Matson, D. L., and Nash, D. B... J. Geophys. Res., 88, 4771-4783, 1983. 
5. Nash, D. B., I c a w ,  54, 511-523, 1983. 
6. Nash, D. B., In: Lunar and  Plane tary  Sc ience .  XITI, pp. 603-604, 1986. 
Pressure (Pa) 
Q, 
Fqg. 1 - The pressure temperature fleld tangential to that of 10. The isotherms are found from Eq. (1) 
(dashed lines) and each is truncated at the vapor pressure over So2 ice (solid line). Coverage 
corresponding to one monolayer is indicated by the arrows (@). On 10, (T< 130K), adsorption is limited to 
a fraction of a monolayer by equilibration with ice. 


SO2 on Io: A thermodynamic perspective

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