Spectra of Io obtained during eclipse show a narrow deep absorption feature at 4.871 microns, the wavelength of the Nu sub 1 + Nu sub 3 band of solid SO2. The 4 micron radiation comes from volcanic hot spots at a temperature too high for the existence of solid SO2. It is concluded that the spectral feature results from SO2 particles suspended in plumes above the hot spots. The derived abundance of approximately 0.0003 gm/sq cm may imply an SO2 solid-to-gas ratio of roughly one for the Loki plume, which would in turn suggest that it is driven by the SO2 rather than by sulfur

Cruikshank, D. P.

Geballe, T. T.

Howell, R. R.

English

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https://ntrs.nasa.gov/search.jsp?R=19840014455 2020-03-20T23:54:05+00:00Z
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THE PLUMES OF 10:
A DETECTION OF SOLID SULFUR DIOXIDE PARTICLES
R. R. Howell* and D. P. Cruikshank*
Institute for Astronomy, University of Hawaii
T. R. Geballe
Kapteyn Astronomical Institute
yl^ijksuniversiteit to Groningen
and
United Kingdom Infrared Telescope
*Visiting Astronomer at the United Kingdom
Infrared Telescope, Hilo, Hawaii
WW	
D+,i
2ABSTRACT
Spectra of to obtained during eclipse show 'a narrow deeo absorption feature at
4.471 µm, the wavelength of the Y l + v S band of solid SO,. The 4-µm
radiation comes from volcanic hot spots at a temperature too high for the
existence of solid SO 2 . We conclude that the spectral feature results from
SO 2
 particles suspended in plumes above the hot spots. The derived abundance
of approximately 3 x 18 -4 gm/cm 2
 may imply an SO 2
 solid-to-gas ratio of
roughly one for the Loki plume, which would in turn suggest that it is driven
by the SO 2 rather than by sulfur.
JP
3INTRODUCTION
Io appears to be the most volcanically active body in the solar system.
The first recognition of that activity came through the Voyager 1 images in
1979. In light of that discovery, it has been possible to interpret several
phenomena observed telescopically from Earth as due to the volcanoes, and to
use those phencmena to monitor the volcanism. In particular, a significant
fraction of the flux in the 4 to 19 µm region has been found to come from the
thermal hot spots on Io's surface ( 2).
The eclipses that occur as Io traverses the shadow of Jupiter provide an
opportunity to measure the hot spot emission- in the absence of reflected
sunlight. Observations at such times imply that typically one-third of the
total out-of-eclipse flux at 4.3 µm comes from the hot spots, decreasing to
one--tenth at 3.3 µm. Because of the limited observing time available during
eclipse, studies of the hot spot flux have until now been made only with broad
spectral bandpasses. However, the recent development of multidetector cooled-
grating spectrometers has made more sophisticated measurements possible.
OBSERVATIONS
He report here 'high" resolution 598) observations of Io
obtained during the eclipse of 2 August 1933 using the new 7-detector, cooled-
grating spectrometer t3) at the United Kingdom Infrared Telescope on
Mauia Kea. Eight independent spectra were obtained over an interval of
14 minutes, beginning when the satellite was 19 areseconds from the limb of
Jupiter and ending 11 minutes before it left the shadow. They result from the
seven channels obtained at a single setting of the grating. To remove
instrumental and atmospheric effects, the spectra were divided by that of the
n
F
I
•lIN,11W.
-0
4nearby star a Libras. An out-of-eclipse spectrum was obtained on 30 June ►
using • Ophiuchi as a standard.
Figure la shows the average of all of the eclipse spectra. Because of
guiding errors introduced while the satellite was invisible on the television
monitor, the absolute signal level fluctuated. However, the 7 data points won
measured simultaneously, so the shapes of the spectra were not affected. In
order to estimate the errors, we first normalized each of the 8 separate
observations by the total flux, then found the standard deviation of the mean
for the normalized spectra.
DISCUSSION
A feature is present at 4.071 ± 0.8eS µm, the wavelength of the
Y i + v 3
 band of solid SO 2 (4,S). This band also appears in Io
reflection spectra (Figure 2), but is then saturated and extends from
approximately 4.0 to 4.1 µm. In reflection it arises predominantly from SO2
frost found on the surfac,t; however, this cannot be the source of the band
seen in eclipse. At such times the radiation comes from surface hot spots at a
temperature of several hundred degrees Kelvin, and solid SO 2 cannot exist at
these temperatures. Its melting point is 198 0X, and eve;. at somewhat lower
temperatures it would quickly sublimate. SO 2 gas cannot produce the feature;
in that phase the v i -+ v 3 band is centered at 4.00 µm.
We propose that the solid SO 2 which produces the observed band in
eclipse must be present as small particles in volcanic plumes above the hot
spots. We can find no other mechanism that allows it to be in the line of
sight to the hot spots but isolated thermally from them.
Such solid SO, in the plumes has been suggested by three earlier
observations. The most direct was the Voyager i IRIS observation of the plume
5at Loki (6). It observed the 7.4-µm v 3 fundamental of SO 2 gas in the
line of sight to the hot spot. A feature on the long wavelength side of the
band could not be fit well by gas alone; the IRIS team and later Slobodkin
et al. (7) suggested that it could be due to solid SO 2 . In addition to
the infrared observations, Voyager visual images of the scattered light from
the plumes were interpreted as implying the existence of a considerable amount
of submicron size particles (3). Finally, the spectral characteristics of
parts of Io's surface suggest that SO 2
 is deposited by the plumes.
Although the central point in Figure is is only 1.7 standard deviations
below the apparent continuum, other evidence convinces us that the feature is
real and suggests that it may be variable. The central channel produced the
lowest signal on four of the eight individual spe ,.tra obtained. Even
individually, these four suggest the presence of the band. However, three
others show no evidence of the band. Excess noise, which was present in the
middle spectrometer channel, could explain some of this, but does not seem
large enough to account for it entirely. In Figure ih we have plotted the
average of the first six of the eight spectra obtained; the band appears to be
significantly deeper. if real, the variability could be due to nonuniformities
in the plumes. The Voyager images show such nonuniformities, anc it time of
flight for the particles is a few tens of minutes.
Additional evidence for the reality of the feature comes from the
post-eclipse spectrum shown in Figure 2. Out-of-eclipse spectra should show
sunlight reflected from the surface, with hot spot emission superposed on it.
The details of the pure reflection spectrum are uncertain, since they depend
upon the radiative transfer in the frost structure. However, the simplest
interpretation of the data would attribute a smooth, broad band from 4.0 to
4.1 µm due to reflection, with a dip at 4.07 µm due to rrlume absorption of
PER
K
6
1 9	 the hot spot component. This dip appears to be roughly equal in magnitude to
that seen in eclipse.
We can draw three immediate conclusions from the prer;ence and the depth of
the 4.07-µm band: (1) Plumes are still erupting 4 years after they were
observed by Voyager 2; (2) these plumes contain significant amounts of solid
50 2 ; and (3) the thermal radiation must come from at most a few hot spots,
which are associated with plumes. If it came from many small hot spots
distributed over the surface, then most of these would not lie under plumes,
and the absorption band would be much weaker.
ABUNDANCE and IMPLICATIONS
The equivalent widths of the absorption bands in Figures is and lb are
(4.4 t 2.0) x i0 -3 and (6.0 ± 2.0) x 10 -3
 µm, respectively. These are
derived by assuming that the average or the outer four points of the spectra
represented the continuum, and the middle t ►iree points, the band.
When compa^ed with laboratory transmission spectra (9), these
values imply pathlengths through SO 2 of (3.2 t 1.8) x 10 -4 and
(4.8 ± 2.8) x W 4 cm. Assuming a density of 2 sm/cm 3 results in
abundances along the line of sight of 6.4 x i0
-4
 and 9.6 x 10
-4
 gm/cm2.
We adopt a value of 8 x i0 -4
 gm/cm2 as a best estimate of the abundance of
solid SO 2 , and believe the error to be less than 50%. This abundance is the
average for the hot spots, weighted by their infrared brightness. For example,
all of the hot spots could be associated with plumes that have a v i+ v 3
band depth of one half. On the other hand, a relatively thick plume could
exist over a single hot spot contributing half of the flux, while the rest of
the energy came from spots not covered by plumes. However, in this latter case
the single plume could not be too much thicker than our estimate, or the band
would saturate and broaden beyond that seen in our spectra.
There is some evidence that most of the thermal flux we saw came from a
single hot spot; if this is true, then the abundance we quote applies
primarily to the plume associated with it. Johnson et al. (2) have
recently measured Id's rotational light curve at 8 - 148 µm ► concluding that most
of the body's emission originates from Just two hot sputs--one at the Loki
volcanic center (ti 319 0
 14 longitude, 13 0
 K lattitude) and a weaker one in
the opposite hemisphere. The sub-earth longitude during our observations was
15 0 , so the Loki center would have been visible, 24 0 from the limb. Luring
both Voyager encounters the Loki complex had a large hot spot and an active
plume. Although the shorter wavelength radiation we observe may not come from
exactly the same surfaces as the longer wavelengths, we assume that they are
nea^by.
Our particle abundance falls in the middle of the range estimated from
Voyager images of the Loki plume as seen above the planet's limb. Although
such results are highly sensitive to assumed particle siz ia, Collins (8)
suggested that i6 -4 to 10-1 gm/cm ` of small particles were pr-esent.
Because the Voyager t IRIS experiment obtained a gas abundance for the
Loki region, we can compare the relative amounts of solid and gas in the
plume. The value of 9.2 cm atm of SO, gas which they measured is equivalent
to S x i9 -4 gm/cm 2 . The similarity to our value. of 8 x 19 -4 gm/cm2
indicates that roughly equal amounts of SO 2
 gas and solid are present.
The principal uncertainty in the solid-to-gas ratio results from the
differences in time and viewing geometry between the gas and solid
observations. The Voyager measurements were made four years before ours.
Although the plume apparently shows short-term variations, the roughly similar
8appearance in the two Voyager encounters and the constancy of the Earth-based
eclipse radiometry, su^gvst that the activity continues at the same average
level. This provides some reason f, r believing that the IRIS gas abundance is
still a reasonable estimate. The path length effects due to different viewing
geometry through the plume are difficult to evaluate, since this would require
knowledge of the plume density as a function of height and distance from the
thermal source. For the present we regard the solid-to -gas ratio as uncertain
by a factor of 10. During future eclipses it may be possible to observe both
the 4 .07-µm frost band and the 4.80-µm gas band, thereby allowing a much
better comparsion. The low absorption coefficient for the gas will, however,
make this observation difficult.
Approximately equal amounts of gas and solid are predicted by models in
which the plumes are driven by SO,, ( 1 0). Although the alternate model, in
which the plumes are driven by elemental sulfur, might have equal amounts of
gaseous and solid sulfur, we expect that the higher temperatures would not
allow condensation of a significant fraction of any SO 2 that might be
present. Since detailed thermodynamic models of the plumes have only been
generated for single gas systems, this last assertion needs to be checked.
Kevs: rtheless, at the time of our observation, the plume appears to have been
so  driven.
We intend to obtain observations at a variety of sub -earth longitudes
during the next year, The longitude at eclipse varies from 340 0
 to 200
during an apparition of Jupiter. At 340 0 , Loki would be much closer to the
center of the disk, and measurements there could help check both the effect of
viewing geometry upon abundance, and the identification of Loki as the primary
source. The other hemisphere of Io is never seen in eclipse, but because there
may be a change in plume character,
 with longitude (!:), observations of it
Sare also important. If, as mentioned above, the dip at 4.07 µm in the
out-of-eclipse spectra is due to plume absorption of the :.ded hot spot
emission, then this feature proves the presence of S0 2 in plumes at these.
longitudes. A detection of rar:d variability would confirm this.
In conclusion, the discovery of an absorption band of soli.l SO2 in
spectra of Io's hot spots provides important clues regarding the nature of the
volcanic processes. The spectra we present demonstrate that at least one plume
is still active some +our years after the Voyager 2 fly-by, and confirm the
existence of significant amounts of solid S0 2 in it. They show that the hot
spot emission comes from at most a few sources (perhaps gust one) which must be
located near plumes. The suggested solid-to-gas ratio in the Loki plume
implies that it is S0 2
 driven. Future observations may provide more accurate
solid-to-gas ratios for this plume and perhaps for those in the opposite
hemisphere. Eclipse observations provide a way to monitor the long-term
stability of the plume activity in the Loki hemisphere. This work was
supported by NASA grant NGL 12-W-057.
, 0
10
to	 References
1. W. M. Sinton, A. T. Tokunaga, E. E. Becklin, I. Gatley, T. J. Lee, and
C. J. Lansdale, Science 218, 1915 (1980).
2. T. V. Johnson, D. Morrison, D. L. Matson, G. J. Veeder, R. H. Brown, and
R. M. Nelson, $mince, submitted.
3. R. WadR ► UKIRT Cooled Grating Spectrometer Users' and Operators' Manual.
United Kingdom Infrared Telescope, Hilo. (1983).
4. A. Barba ► A. Delahaigue ► and P. Jouve, ,Spectrochimica Act} 27A ► 1439
(1971).
S. R. R. Howell, D. P. Cruikshank, and F. P. Fanale, Icarus, in press.
A. J. Pearl, R. Hanel, V. Kunde, W. Maguire, K, Fox, S. Gupta,
C. Ponnamperuma, and F. Raulin, Nature 289, 755 (1979).
7. L. S. Slobodkin, I.F. Buyakov, H.S. Triput, J. Caldwell, and R.D. Coss,
J. Quant. Soectrosc. Radiat. Transfer 26 ► 33 (1981).
8. S. A. Collins, JOR 86A, 8621 (1981).
9. G. T. Sill, private communication (1984).
7"'r'
,
-t ...
11	 `^
11. S. W. Kieffer, in Satellites of Juciler, D. Morrison, Ed. (Univ. of
Arisona Press, Tucson, 1992), p. 647.
11 A. S. McEwen and L .A. Soderblom, Icarus SS, 191 (1983).
r", V
4
12
FIGURE CAPTIONS
FIG. i a) The average of 8 spectra takfin while lo was in eclipse. The
error bars are the standard devia-lion of mean, computed after the individus.
spectra have been normalized to the same total flux . The solid line is the
band expected for 6.4 x i0 -4 gm/cm 2 of solid SO 2 . b) The same as !a .
exccpt that only the first 6 spectra have been averaged. The solid line is the
band expected for 9.6 x it -4 gm/cm 2 of SO 2' Variations seen in the
individual spectra suggest that the feature is variable on a time scale of a
few minutes.
FIG. 2 The lower squares are the eclipse spectrum from figure ib ► while
the upper ones are spectrum obtained immediatel, after the satellite emerged
from Jupiter's shadow. The line is a spectrum obtained approxirt-ately one month
earlier ► viewing the other hemisphere. The presence of the dip at 4.07 µm in
the reflection spectra may result from the superposition of a sharp ban,) to due
plume absorption of hot spot emission on top of the broad, sm- nth band in the
light reflected from surface. The sharp bands at 4.11 and 4.14 µm are the
34S and 18 0 isotopic bands due to the surface SO 2 frost.
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The plumes of IO: A detection of solid sulfur dioxide particles

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