Two classes of volcanic plumes on Io

ICARUS

55, 191-217 (1983)

Two Classes of Volcanic Plumes on lo ALFRED

S. McEWEN U. S. Geological

AND LAURENCE Survey,

Flagstaff,

Arizona

A. SODERBLOM 86001

Received December 12, 1982, and in revised form March 3, 1983 Comparison of Voyager 1 and Voyager 2 images of the south polar region of IO has revealed that a major volcanic eruption occurred there during the period between the two spacecraft encounters. An annular deposit -1400 km in diameter formed around the Aten Patera caldera (3ll”W, 48’S), the floor of which changed from orange to red-black. The characteristics of this eruption are remarkably similar to those described earlier for an eruption centered on Surt caldera (338”W, 45”N) that occurred during the same period, also at high latitude, but in the north. Both volcanic centers were evidently inactive during the Voyager 1 and 2 encounters but were active sometime between the two. The geometric and calorimetric characteristics, as well as scale of the two annular deposits, are virtually identical; both resemble the surface features formed by the eruption of Pele (ZSYW, 18”s). These three very large plume eruptions suggest a class of eruption distinct from that of six smaller plumes observed to be continously active by both Voyagers 1 and 2. The smaller plumes, of which Prometheus is the type example, are longer-lived, deposit bright, whitish material, erupt at velocities of -0.5 km set-I, and are concentrated at low latitudes in an equatorial belt around the satellite. The very large Pele-type plumes, on the other hand, are relatively shortlived, deposit darker red materials, erupt at -1.0 km set-i, and (rather than restricted to a latitudinal band) are restricted in longitude from 240” to 36O”W. Both direct thermal infrared temperature measurements and the implied color temperatures for quenched liquid sulfur suggest that hot spot temperatures of -650°K are associated with the large plumes and temperatures <400”K with the small plumes. The typical eruption duration of the small plumes is at least several years; that of the large plumes appears to be of the order of days to weeks. The two classes therefore differ by more than two orders of magnitude in duration of eruption. Based on uv, visible, and infrared spectra, the small plumes seem to contain and deposit SO2 in their annuli whereas the large plumes apparently do not. Two other plumes that occur at either end of the linear feature Loki may be intermediate or hybrid between the two classes, exhibiting attributes of both. Additionally, Loki occurs in the area of overlap in the regional distributions of the two plume classes. Two distinct volcanic systems involving different volatiles may be responsible for the two classes. We propose that the discrete temperatures associated with the two classes are a direct reflection of sulfur’s peculiar variation in viscosity with temperature. Over two temperature ranges (-400 to 430°K and >650”K), sulfur is a low-viscosity fluid (orange and black, respectively); at other temperatures it is either solid or has a high viscosity. As a result, there will be two zones in IO’S crust in which liquid sulfur will flow freely: a shallow zone of orange sulfur and a deeper zone of black sulfur. A low-temperature system driven by SO1 heated to 400 to 430°K by the orange sulfur zone seems the best model for the small plumes; a system driven by sulfur heated to >650”K by hot or even molten silicates in the black sulfur zone seems the best explanation for the large plume class. The large Pele-type plumes are apparently concentrated in a region of the satellite in which a thinner sulfur-rich crust overlies the tidally heated silicate lithosphere, so the black sulfur zone may be fairly shallow in this region. The Prometheus-type plumes are possibly confined to the equatorial belt by some process that concentrates SO2 fluid in the equatorial crust. INTRODUCTION

Active volcanism on 10 has been one of the most spectacular discoveries of the Voyager missions. The first active eruption discovered appeared as an enormous umbrella-shaped plume rising more than 300

km above the surface, issuing from a feature that has been named Pele (Morabito et al., 1979) (see Fig. 1 and Table I). Eight much smaller active plumes were also seen in Voyager 1 images, two of which were associated with a linear feature named Loki (Smith et al., 1979a). Four months later, 191 0019-1035/83 $3.00 Copyright 8 1983 by Academic Press, Inc. All rights of reproduction in any form reserved.

192

TWO CLASSES

OF VOLCANIC TABLE

PLUMES

193

ON IO

I

KNOWN PLUME ERUPTIONS ON IO Designation

Name

Active during Voyager 1 Encounter

Active during Voyager 2 Encounter

Class

Pl P2 P3 P4 P5 P6 P7 P8 P9 PI0 Pll

Pele Loki (West) Prometheus Vohmd Amirani Maui Marduk Masubi Loki (East) surt Aten

Yes Yes Yes Yes Yes Yes Yes Yes Yes No No

No Yes Yes Yes Yes Yes Yes Yes Yes No No

Pele Hybrid Prometheus Prometheus Prometheus Prometheus Prometheus Prometheus Hybrid Pele Pele

when Voyager 2 arrived, all eight were still active (Strom and Schneider, 1982). The huge eruption of Pele, however, had ceased (Smith et al., 1979~). In addition, a major change had occurred in the color patterns on the surface centered on the Surt caldera (335”W, 45”N) indicating that a major volcanic eruption had occurred. Comparison of the Voyager 1 and 2 images revealed that the caldera floor changed from light orange to dark red, nearly black, and had become surrounded by a large concentric ring which resembled the fallout pattern around Pele (Smith et al., 1979~; Strom et al., 1981). We describe here the discovery of yet another large eruption, from the caldera named Aten Patera, that also occurred between the two encounters, this time in the south polar region, and with surface changes almost identical to those at Surt. The recognition of the strong similarity of scale and appearance of the deposits from these three large plume eruptions led us to investigate the possible existence of discrete classes of volcanic plumes on 10. Evidently, Pele, Surt, and Aten Patera represent a class of plume eruptions that are larger and shorter lived than a second class of smaller plumes. Albedos and colors of source regions and plume deposits, along with evidence for sizes, durations, and temperatures, suggest that there are, in fact,

two distinct and very different types of volcanic plumes. We present here the results of that analysis and examine proposed models for IO’S volcanism in the context of the two classes. IMAGE

AND SPECTRAL ANALYSIS PROCEDURES

Evidence from numerous studies suggests that the components of IO’S surface include various allotropes of elemental sulfur, sulfur dioxide frost, and other sulfur compounds (Nelson and Hapke, 1978; Nelson et al., 1979; Pearl et al., 1979; Sagan, 1979; Fanale et al., 1980; Soderblom et al., 1980). For the case of pure elemental sulfur, the color changes continuously with the temperature of equilibration: yellow or white (Ss allotrope) below its melting point of 393”K, orange at -4OO”K, red (with abundant S3 and S4 allotropes) at -450”K, brown at -5OO”K, and black at -600 to 700°K (Meyer et al., 1971). If the liquid is rapidly quenched, the color of equilibrium is preserved. Ascent of the magmatic liquid from the reservoir to the surface of 10 has been approximated as adiabatic and quasiisentropic, and thus nearly isothermal (Smith et al., 1979b; Kieffer, 1982). Therefore, extrusion at 10’s surface temperature of - 130°K should rapidly quench molten

194

MC EWEN AND SODERBLOM

sulfur and preserve the color of its equilibrium temperature in the magma source region. The occurrence on IO’S surface of red and black calderas and fissures, surrounded by red, orange, yellow, and white flows, is consistent with this concept that temperature of the source regions can be inferred from the colors, as previously suggested (Sagan, 1979; Soderblom et al., 1980). In the current paper we examine the spectral reflectance of the deposits and source regions of IO’S plume eruptions, along with their comparison to laboratory spectra of SO2 and of white, orange, red, and brown sulfur. The images of 10 used in the analysis were processed at the U. S. Geological Survey Image Processing Facility in Flagstaff, Arizona. The images were corrected for temporal and spatial variations in dark current and sensitivity of the Voyager vidicons and reduced to absolute intensity using calibration factors given in Danielson et al. (1981). North is up in all images presented in the figures except for oblique views. Table II provides information on the images used for the figures in this report. All wideangle and narrow-angle color images were produced with the violet, blue, and orange filters, composited as blue, green, and red, respectively. The color images presented in this work were processed to maintain the original color balance between the three filters. There is, of course, some shift from the actual color as would be seen by the human as the spectral bandpasses of the Voyager violet, blue, and orange filters are not the same as those of the human eye. Additionally, the linearity of the contrast in the images has been preserved and presented roughly at normal dynamic range; the very bright materials have reflectances near lOO%, the very dark, a few percent. The spectra presented in this discussion were reduced to normal albedo by removing the satellite’s average limb darkening, using the Minnaert function: B = Bdcos i)k(“)(cos E)~(+‘,

(1)

where B is the measured brightness, B0 is a model normal albedo for vertical illumination and viewing, i is the incidence angle; E is the emission or viewing angle, and k is a coefficient that varies with phase angle, (Y. Clancy and Danielson (1981) analyzed the Voyager imaging data for 10 by grouping together data for each color type (e.g., all red materials) and fitting a least-squares solution for a single Minnaert function to the ensemble of all values in the group. Thus, data for many different locations over IO’S surface were combined in the individual solutions. They found values for the coefficient, k, in the range of 0.5 ? 0.3 at a! = lO.Y, and recognized that the large error was due to variable albedo in the classes. Preliminary work in our study showed that if the data were analyzed by fitting Minnaert functions to data for each location independently (with data from different images) the solutions yielded a narrow range of values for k. Our preliminary results suggests a model of k = 0.5 + 0.005 (Y, where (Y is in degrees, with an error of +0.05 for (Y ~30”. An expanded discussion of this work is in preparation. In the form presented here, the Minnaert function does not completely account for the brightening of the integral disk toward low phase. For a limited range of phase angle (for example 0 to 40”), the residual difference is only a few percent and has been neglected. Although wide-angle images were used for visual analysis of surface changes and are presented in some of the figures, the spectral reflectance curves presented in this discussion were derived only from narrowangle camera data. The combined spectral response of the optics, filters, and vidicon of the wide-angle camera data is broader band than that of the narrow angle, making intercomparison of wide-angle and narrowangle spectral data difficult. For example, the position of the steep absorption edge in IO’S spectrum between 0.4 to 0.5 pm causes the wide-angle violet filter values to be lo30% higher (depending on the steepness of individual spectra) than the narrow-angle

TWO CLASSES OF VOLCANIC TABLE VOYAGER Image

Central Coordinates Lat. Long.

Fig. 3a

170s

3 17”W

1

Fig. 3b

2”s

33O”W

2

Fig. 3c

2”s

218”W

1

Fig. 3d

2”s

215”W

2

Fig. 3e

0”

156”W

1 1 1

Fig. 6a Fig. 6b

3”s 155”W 20% 256”W L(LIMB) 2”s 218”W 156”W 0”

Fig. 7a

45”N

338”W

2

Fig. 7b

48’S

3ll”W

1

Fig. 7c

48%

3ll”W

2

Fig. 7d

3”s

155”W

1

Fig. 7e

16”N

305-w

1

Fig. 7f

18%

255”W

1

Fig. Fig. Fig. Fig.

3”s 16”N 18% 22”s

155”W 305”W 255”W 321”W

1 1 1 1

Fig. 5a Fig. 5b

8a 8b 8c 10

voyager encounter

1 1

IMAGES

EVIDENCE

FOR

TWO

CLASSES

II USED IN FIGURES

Color display

Frame FDS

Filter/ camera

Red Green Blue Red Green Blue Red Green Blue Red Green Blue Red Green Blue BIW B/W

16390.57 16390.47 16390.51 20642.01 20641.58 20641.55 16375.40 16375.38 16375.36 20621.24 20621.21 20621.18 16368.42 16368.38 16368.36 16375.38 16368.36

O/WA B/WA V/WA OiNA B/‘NA VINA OiNA B/NA ViNA OiNA BINA V&IA OINA BINA V/‘NA BINA V/NA

B/W B/W Red Green Blue Red Green Blue Red Green Blue Red Green Blue Red Green Blue Red Green Blue B/W BIW BIW

16375.44 16368.50 20642.01 20641.58 20641.55 16392.43 16392.39 16392.41 20642.01 20641.58 20641.55 16368.41 16368.38 16368.36 16382.37 16382.35 16382.33 16382.13 16382.11 16382.09 16368.50 16382.39 16382.15 16391.30

uv/NA uv/NA OiNA B/NA V/NA O/WA B/WA V/WA OiNA O/NA VMA OiNA B/NA VINA OiNA BiNA ViNA OINA BINA V/‘NA uv/NA uv/NA uv/NA Cl/NA

values. This steep absorption edge must also be considered in comparisons of spectral reflectance derived from Voyager images with ground-based observations or with laboratory spectra (Veverka et al., 1978; Gradie and Veverka, 1982). The narrow-angle camera spectral responses for the ultraviolet, violet, blue, green, and orange bandpasses are shown in Fig. 2. OF PLUMES

Distinct Appearances

Our analysis of the Voyager images of 10

195

PLUMES ON IO

Resolution km/pixel

Geometric projection

Solar phase angle

Limb darkening correction

6.5 7.0 6.8 12.7 12.7 12.7 4.5 4.5 4.5 22.3 22.3 22.3 7.3 7.4 6.5 4.5 6.5

19.0 17.9 17.7 20.4 20.3 20.1 2.6 2.7 2.6 4.3 4.3 4.2 14.7 14.7 14.7 2.7 14.7

No No No No No No No No No No No No No No No No No

Point Point Point Point Point Point Point Point Point Point Point Point Point Point Point Point Point

4.5 7.3 12.7 12.7 12.7 2.1 2.2 2.1 12.7 12.7 12.7 7.3 7.4 6.5 3.4 3.4 3.4 3.4 3.4 3.4 7.2 3.4 3.4 0.7

2.7 14.5 20.4 20.3 20.1 46.9 43.5 45.2 20.4 20.3 20.1 14.7 14.7 14.7 10.5 10.5 10.5 10.5 10.5 10.5 14.5 10.5 10.5 22.2

No No Yes Yes Yes Yes Yes Yes Yes Yes Yes No No No NO No NO No No No No No No No

Point Point Orthographic Orthographic Orthographic Orthographic Orthographic Orthographic Orthographic Orthographic Orthographic Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Simple cylindrical Point

has revealed that, in addition to the changes reported by Smith et al. (1979b) that occurred in and around the Surt caldera in the north (338”W, 45”N), another major change in IO’S surface appearance also occurred between the two Voyager encounters-in this case, centered on the Aten Patera caldera in the south polar region (31 l”W, 48”s) (Figs. 3a and b). (Figure 4 indexes the regions on Fig. 3.) The Voyager 2 image reveals that the floor of the Aten Patera caldera had darkened and that a large concentric ring -1400 km in diameter had developed, changes which are almost iden-

196

MC EWEN

violet

AND

blue

8 !T I F f d (0 I 0.3

I 0.4

0.5 WAVELENGTH

0.6

0.7

Cgrn)

FIG. 2. Solar-weighted spectral responses for the Voyager narrow-angle camera bandpasses.

tical to those at the Surt caldera in the north. A third major change in IO’S surface appearance that occurred between encounters was in the shape of the fallout pattern around the large Pele plume (Figs. 3c and d). Notice the great similarity in the scale and color of the fallout patterns around all three of these volcanic centers. These surface changes suggest that two additional major plumes, comparable in size, character, and variability to Pele, erupted and ceased activity sometime during the four months between encounters. Arcuate surface markings may be the remnants of other large plume eruption deposits (Figs. 4a and b). The best examples of these are around the Loki fissure (18”N, 305”W) and around a center -1500 km to the east (12”N, 34o”W). These diffuse rings are about the same diameter and have similar color and albedo to the Surt, Aten Patera, and Pele eruption deposits, and occur in the same region of the satellite. In contrast to the deposits from the three large plumes, most of the remaining active plume deposits have a very different and characteristic appearance. Perhaps the best type example of these smaller plumes is Prometheus, the deposit of which appears as a bright white ring (Fig. 3e). Along with other smaller plumes (Volund, Amirani, Maui, Marduk, Masubi, and two at each

SODERBLOM

end of the Loki fissure, see Fig. l), Prometheus was seen to be continuously active during both encounters. A number of other deposits from the small active plumes, less pronounced than Prometheus, are visible and annotated in Fig. 4. The small plume deposits are typically 200 to 300 km in diameter. Although while spaceborne, the material in the Prometheus plume is dark against IO’S surface (Fig. 5a), the ring deposits are very bright, most notably in the ultraviolet (Figs. 6b and Sa). By contrast, deposits associated with Pele have an extremely low uv albedo (Figs. 6b and 8~). The deposits of the Surt and Aten Patera eruptions are also very dark in the uv. The deposits of Prometheus, the Loki plumes, and Pele are shown at the same scale in Figs. 7d-f and Fig. 8. Prometheus and Pele are type examples of the small and large plumes, respectively, while the two Loki plumes exhibit characteristics of both types and may be a complex case involving more than one volcanic mechanism. As noted by Strom et al. (1981), the Loki deposits include an outer orange ring -850 km in diameter with low uv reflectance, and two bright inner zones which correspond to the active plumes during Voyager 1. While the outer ring approaches the diameter of the Pele-type plumes, the inner plumes are about the same size as the Prometheustype, and like them are very bright in the uv (Fig. 8b). As such, Loki is treated as a special hybrid case in the discussions that follow. Bimodal Plume Sizes and Ejection Velocities Direct height measurements were possible for those plumes imaged near the bright limb (cf. Smith et al., 1979a; Strom et al., 1981), (see Fig. 5). Pele, with a height of -300 km, was much higher than any of the smaller plumes seen from Voyager 1 (Fig. 9a). The six plumes we include in the small class (Prometheus, Maui, Amirani, Marduk, Volund, and Masubi) all had measured heights of 100 2 40 km (Strom et al., 1981).

TWO

CLASSES

OF VOLCANIC

Some of the variation in the height may well be due to errors in the measurements themselves. The two plumes associated with the Loki fissure showed a high degree of variability. According to Strom et al. (1981), images acquired a few days prior to the Voyager 2 encounter suggest that one or the other, or both plumes had a height in excess of 300 km, but by the time of the encounter the height had dropped to about the same seen in Voyager 1 images (-165 km). Plumes were not visible above either Surt or Aten Patera during either encounter, so direct height measurements are not possible. Table III lists those images that would have shown plume eruptions from Surt and Aten Patera had they exceeded certain heights. From these direct observations we know that an eruption from Aten Patera could not have exceeded a few tens of kilometers during either encounter. We can place similar limits on a Surt eruption of a few tens of kilometers for Voyager 1 and -100 kilometers for Voyager 2. Aten and Surt deposited ejecta blankets of - 1400 km diameter, compared with a diameter of -1200 to 1500 km for Pele’s ejecta blanket. (The measurable plume of Pele was - 1200 km in width while surface deposits were -1300 by 1500 km.) We therefore use the Voyager 1 measured height of Pele, -300 km, as a model for the Surt and Aten Patera plumes, resulting in a clear bimodal distribution in plume heights. The heights of the Loki plumes oscillated between the two modes. The diameters of the outer boundaries of each plume deposit also show a strongly bimodal character, with the small and large plumes clearly separated (Fig. 9b). As already mentioned, the intermediate Loki plumes show two zones of mantling and form an exception. Most of the deposits are very diffuse; the diameters shown are therefore rough approximations. The greater plume heights and larger diameters of mantled areas of the large plumes imply higher ejection velocities than for the small plumes. Smith et al.

PLUMES

ON IO

197

(1979a) and Strom and Schneider (1982) estimate velocities of -1 km see-i for Pele and -0.5 km see-* for the small Prometheus-type plumes. Since the areas mantled by the Surt and Aten Patera plumes are about the same diameter as deposits from Pele, similar ejection velocities are likely. Ejection from the western plume of Loki was estimated from observations taken at two separate times to have been -0.47 and -0.75 km set-i (Strom and Schneider, 1982), consistent with its hypothesized hybrid character. Bimodal Eruption

Durations

During both encounters, separated by 4 months, all six Prometheus-type plumes were continuously active at roughly the same level and the two Loki plumes were continuously active at fluctuating levels. We can place a rough lower limit on their probable lifetimes. First, it is highly probable that all eight were continuously active during the period between the two encounters. If they were sporadically turning on and off between the encounters, the probability of their all being active during all observations is roughly 1 in 2” where IZ is the number of observations, so long as they were not turning on and off together, a rather unlikely situation. From Strom et al. (1981), 12 - 20, so the probability of their being short-lived compared to 4 months is less than 1 in 106. For all eight to remain active for both encounters, their probable mean lifetime must be greater than about 4 years; we cannot supply an upper bound on their lifetime. The evidence for estimating the mean lifetime of the Pele, Surt, and Aten Patera eruptions is sketchy but we certainly know it to be much shorter than that of the small plumes. The evidence for the duration of each of the three large plume eruptions can be summarized as follows: Pele. Although active during the Voyager 1 encounter, Pele ceased to erupt sometime before Voyager 2 arrived. Ground-based observations of the 10 plasma torus suggest

198

MC EWEN

AND

SODERBLOM

TWO

CLASSES

OF VOLCANIC

PLUMES VOYAGER

2

urt a f-“ ,-t-‘, t ,‘,-;-t?‘PlO

P2

I--

-..

:>’

I- 7-s \)I--

,.I-

-’

r:”

b

I

199

ON IO

l I d I 1 I ‘-’

\&Loki

.

\-

‘I Qx

P9

Pll)

\ -2

I Aten

‘4;

Patera

b.

d.

0 I

000 I

1600 I

km

FIG. 4. Outline of plume deposits for the images in Fig. 3. Solid lines outline the deposits from the known plumes, while the dashed lines show surface deposits of similar appearance. Solid dots locate the source regions (black calderas) for the large plumes. Larger plume outlines all occur in the region from 240 to 36O”W longitudes. Smaller plume deposits occur at all longitudes, but are concentrated at low latitudes.

FIG. 3. Comparison of hemispheres of 10 as seen by Voyager 1 and 2 (see Table II for supporting data). The left-hand column consists of Voyager 1 images; the right Voyager 2. The “mirror-image” eruptions from Surt and Aten Patera can be seen by comparing changes near the top and bottom of images a and b (see also Fig. 4). Bright areas in the wide-angle image (a) were saturated in the raw data from over exposure. Image c was reduced in scale and low-pass filtered to give it a resolution comparable to that of image d. Since these Voyager 1 and 2 views are within 3” of the same subspacecraft and subsolar positions, the photometric behavior is nearly identical, so changes seen in the plume deposits from Pele are real. Image e is a region that is dominated by the small plumes.

200

MC EWEN AND SODERBLOM

0

I

200 I

km

200

km

FIG. 5. Active plumes of Prometheus (a) and Pele (b), type examples of the small and large plume classes. Pele has a bright outer rim which Strom and Schneider (1982) interpret as a shock front.

that a major injection of material into the magnetosphere occurred within 6 days prior to the Voyager 1 closest approach (Eviator et al., 1981). Based on a long timeseries of optical spectroscopic observations, Mekler and Eviator (1980) hypothesize that the random-type variability of the cold inner portion of the torus is associated with stochastic volcanic activity on 10.

They believe that substantial enhancements of plasma density are caused by the violent injection of volcanic material. Pele, by far the largest of the Voyager 1 plumes, is the likely source of the enhancement that occurred within 6 days of the first encounter, which may represent the initiation of this eruption. Therefore, this plume may have been active for less than a week.

TWO

CLASSES

OF VOLCANIC

PLUMES

ON IO

201

MC EWEN AND SODERBLOM

TWO

CLASSES

OF VOLCANIC TABLE

Spacecraft

PLUMES

203

ON IO

III

Frame i.d.‘s (FDS)

Filters

Arc distance to limb

Maximum height (km)

(degrees)

Surt observations:

Aten Patera observations:

Voyager 1

16354.34 to 16354.42

Voyager 2

20671.30

to 20671.51

Voyager 1

16375.36

to 16375.44

Voyager 2

20671.30 to 20671.51

uv, clear, violet, blue, orange uv, clear, violet, blue, orange uv, violet, blue, orange uv, clear, violet, blue, orange

0.9 to

1.4

36.2

to 36.4

17.6

to 18.1

111.5 to 117.2

3.0

to 3.8

11-6 to 13.2

1.0 to 1.6

22.8

to 23.1

Note. The following frames were examined to search for active plumes above the Surt and Aten Patera volcanic centers that were erupting during the Voyager encounters. No plumes at these sites were detected so the images can be used to place upper limits of the plume heights at the times of the observations. These maximum heights are listed above.

Surt. This plume was not active during the Voyager 1 encounter, and most probably not during Voyager 2. If it was still active during the second encounter, it must have been less than -100 km high (Table III). The size of Surt’s ejecta blanket suggests that the plume reached a height comparable to that of Pele (-300 km), so its activity was at least greatly diminished by the time Voyager 2 arrived. The rapid decay of a 5-pm outburst located at the longitude of Surt (Sinton, 1980), suggests that an event associated with the plume eruption was short-lived, on the order of a few hours to a day or 2. Aten Patera. This plume center clearly had no significant active plume eruption

during either Voyager encounter (Table III), so its duration must have been less than 4 months. No ground-based observation of a thermal outburst has been reported correlated with the longitude of this feature. This evidence suggests that the class of large plume eruptions has a probable lifetime much less than a few months, and is most likely between a few days and a few weeks. Comparing the two classes, it is likely that they differ by at least 100: 1 in the typical duration of their eruptions. BIMODAL

TEMPERATURES

Several lines of evidence suggest that the Prometheus-type eruptions are associated

FIG. 7. Images a, b, and c are orthographic projections, centered on Surt-Voyager 2, Aten Patera-Voyager 1, and Aten Patera-Voyager 2, respectively. Both the Surt and Aten Patera calderas are about 50 km in diameter. The plume deposits are not optically opaque, so they must be very thin, with an average thickness of less than a few millimeters. Image b is a high-resolution wide-angle image, and, from shadow lengths, Aten Patera can be estimated as 500-600 m deep (David Arthur, 1981, personal communication). Emanating from the caldera is a -7OO-km-long lava flow to the southwest, which covers an area of over 100,000 km2, and was probably a very low-viscosity flow of -400°K orange sulfur magma. Images d-f show surface deposits surrounding the eruptions of Prometheus, type example of a small plume; Loki, with two hybrid plumes; and Pele, type example of a large plume, respectively. Each are simple cylindrical projections to the same scale. Typical deposits are white from the small plumes and orange or brown from the large plumes, while Loki deposits both orange and white materials.

TWO

CLASSES

OF VOLCANIC

PLUMES

205

ON IO

EXPLANATION [PI=

Pete-type

L = Loki

plumes

plumes

p = Prometheus-type

Height

distribution

plumes

in kilometers

-;;;i-;;.;;;‘:l

Diameters

of

plume

deposits

in

100

kilometers

-1

0

5

10

15

FIG. 9. Distributions of plume heights and diameters of surface ejecta. Except for the hybrid Loki plumes, the distributions are strongly bimodal, which imply bimodal ejection velocities.

with hot spot temperatures of <400”K, while the Pele-type is associated with temperatures of 600 to 700°K. The evidence includes measurements by the Voyager 1 infrared spectrometer (IRIS), ground-based observations, and inferred color temperatures for quenched liquid sulfur. While there is significant uncertainty in each individual line of evidence, their combined weight is more convincing. Infrared

Observations

The most direct evidence is from IRIS, which detected hot spots over three active plume areas (Pearl and Sinton, 1982): (1) Pele has been modeled as consisting of two regions at different temperatures: a circular spot of 6-km radius at 654°K superimposed on an area with an 80-km radius equivalent at 175°K; (2) a thermal anomaly over the small plumes Amirani and Maui was modeled with an area of 13-km radius at 395°K and of 49-km radius at 200°K; (3) the hot spot over Loki, which we consider a hybrid of the two classes, was fit by an area of 21km radius at 450°K and of 121-km radius at 245°K.

Ground-based observations from 2.2 to 50 pm, made by a number of observers over the last decade, have been modeled by two regimes of hot spots at -300°K and -600°K (Sinton, 1981). When combined with the IRIS data, this bimodality is preserved, with hot spots clustering at ~400°K and 500 to 700°K (see Table 19.16, Pearl and Sinton, 1982, p. 748). The low-temperature flux has been fairly constant while the higher temperatures have been variable (Morrison and Telesco, 1980), including several shortlived outbursts (Witteborn et al., 1979; Sinton, 1980; Pearl and Sinton, 1982). This time variability suggests that the long-lived Prometheus-type eruptions are associated with the steady low-temperature emission, while the short-lived Pele-type is associated with the variable and transient high-temperature events. Reflection Spectra and Implied Temperatures

The two classes of plumes differ both in the reflectance spectra of their source regions (calderas or fissures) and from their surface plume deposits, which suggests that

206

MC EWEN

AND

SODERBLOM

sulfur is erupted at higher temperatures from the Pele-type events than from the Prometheus-type (Fig. 10). The suspected source regions of the plumes are much darker for the large class of plumes than for SMALL

a. Source

PLUMES

DIrJ

“II.

LARQE

the small class (Fig. lOa). Many calderas and fissures on 10 are complex, with two or three different color units (cf. Fig. 7b). On Earth, calderas are typically much larger than any included vent or vents; this ap-

PLUMES

=

LABORATORY

SPECTA

IeQions

0.8

r---l 0.8

0.4

0.2

CALOERA

FLOORS.

VOYAGER

n

1

=

“8.

I

I

0.41

0.48

VOYAGER

2 m

n

J 0.35

WAVELENGTH

0.59

0.35

0.41

0.48

0.59

IN MICROMETERS

FIG. 10. Reflectance spectra of vent regions (a); plume deposits (b); caldera floors (d-f); compared with laboratory spectra of sulfur and SO2 (c). Hatchered areas indicate error limits and real variability of the materials. The spectra show a match between the source regions of the large plumes with black or brown sulfur, while the small plume sources are brighter (a). Surface deposits of plume ejecta (b) appears to match red or orange sulfur for the large plumes, while small plume deposits may be a mixture of white sulfur and SO*. The floor of Pele changed little by the second encounter (d), though the eruption had ceased, so it may consist of quenched black sulfur. The floors of Surt and Aten Patera darkened between encounters (e and f), possibly due to eruption of higher temperature allotropes of sulfur.

TWO CLASSES OF VOLCANIC pears to be true on 10 as well. None of the Voyager images have sufficient resolution to clearly identify the source vent of any of the active plumes. The highest resolution image (0.7 km/pixel) of an active vent, that of Pele, is shown in Fig. 11. The source region appears to be the very dark area - 10 x 24 km in dimension at the southern end

PLUMES ON IO

207

of the -24 x 30-km caldera where it is intersected by an east-west trending fissure (Strom et al., 1981). Source regions of all the other plumes also appear to be within the darkest spots in each region, so this is where the spectra were collected. If the spectrum of these source regions is dominated by liquid sulfur, which becomes con-

FIG. 11. Voyager 1 high-resolution image of Pele. Very low brightness values were enhanced to show the plume above the limb. The source region is the elongate black feature which is the center of radial surface deposits. No erupting column of dark material can be seen, indicating that the erupting material is of low particle density. (Material can be seen erupting from Prometheus, Fig. 5a, and Loki, Fig. 7e.)

208

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AND

tinually darker as the temperature rises, then the Pele-type plume vent areas reach higher temperatures than vents of the Prometheus-type. There is strong evidence for the presence of elemental sulfur on 10 and temperatures sufficient to form the brown or black forms have been detected; hence, sulfur quenched in these forms may well be present. Most of the dark features on 10 are not as dark, nor as “red” (the spectrum is not as steep), as is brown sulfur (Fig. lOc), which may be due to the presence of SO2. The darkest spots within the source regions of Pele, Surt, and Aten Patera (during Voyager 2) are among the few areas on IO which do have spectra matching that of laboratory brown or black sulfur. This implies equilibrium temperatures of at least 500°K; IRIS detected a temperature of -650°K at Pele (Pearl and Sinton, 1982). The spectral reflectance of the inactive source region of Pele, observed by Voyager 2, is nearly equal to the Voyager 1 spectra (when at 650”K), which supports the concept of preservation of the colors of equilibrium temperatures by rapid quenching (see Fig. 10d). Voyager 1 and 2 average spectra for the entire floors of the Surt and Aten Patera calderas (each -50 km diameter) are compared in Fig. 10e and f. The spectral reflectance of both changed from that of orange and white sulfur to more like that of dark red sulfur. Since Surt and Aten Patera erupted after the Voyager 1 encounter, the changes in caldera floor albedos imply caldera floor temperatures increased to the order of at least 500°K. Furthermore, an Earth-based infrared observation of a 5-pm excess at the longitude of Surt, during the time between encounters, has been modeled as a 50-km-diameter spot at 600”K, based on radiative cooling rates for liquid sulfur (Sinton, 1980). Source regions of the small plumes are brighter in blue and orange than those of the large plumes (Fig. 1Oa). In addition, they are far more reflective in violet and ultravi-

SODERBLOM

olet wavelengths and cannot be matched with the spectral reflectance of pure sulfur allotropes. This is probably due to the presence of SO2 (Soderblom et al., 1980). It is therefore difficult to infer eruption temperatures from comparison with spectra of sulfur allotropes. However, the spectral reflectance of the source regions of the two classes are clearly distinct, and must represent some difference, such as SO2 content, particle size or density of vented material, or sulfur allotropes. The spectral reflectance for the plume deposits are plotted and compared in Fig. lob. Whereas the large plume deposits are darker and redder, matching orange or red sulfur, the small plumes deposit bright white material, possibly a mixture of white sulfur with SOZ (Soderblom et al., 1980). If these spectra are primarily due to quenched liquid sulfur, then this again implies higher eruption temperatures for the large plumes. If the deposits are largely condensed sulfur vapor, then the temperature-color relations are more difficult to interpret (Sill and Clark, 1982). Loki again appears to be an exception, a complex combination of the two classes. To recap, three lines of evidence suggest a bimodal distribution of hot spot temperatures associated with the two classes of plumes. (1) The inferred color temperatures of the plume source regions and plume deposits suggest that eruptions of liquid sulfur associated with the large plumes are of higher temperatures than eruptions associated with the small plumes. (2) The Voyager 1 infrared spectrometer (IRIS) detected a 650°K area at the source of Pele and a 395°K emission over the small plumes Amirani and Maui (Pearl et al., 1979; Pearl and Sinton, 1982). (3) Ground-based observations cluster at -300 and -600°K (Sinton, 1981; Pearl and Sinton, 1982). These studies indicate that while the high-temperature surges have occurred, the total heat flow at lower temperatures has been fairly constant; this is con-

TWO

CLASSES

OF VOLCANIC

209

ON IO

large, red plume signatures are seen in the longitude belt between 0 and 24O”W. Ground-based observations since 1926 have consistently shown that the longitude zone near 3OO”W is much darker and redder than the central and eastern longitudes (Morrison et al., 1979). The variation in IO’S reflectance spectrum with rotational phase angle (equivalent to central longitude) is shown in Fig. 13. The plume deposits around Pele, Surt, Aten Patera, and Loki have spectra typical of the region at 3Oo”W, which suggests that this region is darker and redder due to a concentration of the large eruptive plumes, and that this concentration has persisted for at least 55 years. From analysis of spectra obtained by the International Ultraviolet Explorer Spacecraft, a longitudinal variation in the distribution of sulfur dioxide frost has also been reported; the frost is most abundant at orbital longitudes from 72 to 137” and least abundant at longitudes from 250 to 323” (Nelson et al., 1980). These authors concluded that the white material on 10 is asso-

&tent with our model of large, short-lived plumes from hotter source regions and small, long-lived plumes from cooler sources. GLOBAL

PLUMES

DISTRIBUTION OF THE TWO CLASSES OF PLUMES

The sites of all four of the Pele-type eruptions (including Loki three days prior to the Voyager 2 encounter) are restricted to longitudes 3OO”W +-60” and latitudes ?50 (Fig, 12). By contrast, the small plume eruptions (again including the Loki plumes) are distributed uniformly with respect to longitude but are concentrated in an equatorial band +30” north and south. Additional arcuate surface markings which are quite similar in appearance to the deposits of the four known Pele-type plume eruptions are indicated in Fig. 4. All occur in the same region as the known large plumes. These markings occur around Babbar Patera at 273”W, 40”s; Singen Patera at 306”W, 32%; an unnamed fissure at 338”W, 15”N; and as a broad arcuate marking at 275-3OO”W, 30”s (see Fig. 1). No such

EXPLANATION [P]

Pele-type Loki

P

Prometheus-type

Longitude LPI

M

3600

plumes plumes

distribution

LPI

LL

P

I

plumes

L

I

I

3000

P

I

I

240’

I

P

I

1800

Latitude

I

I 00

f10’

I

1200

I

I

600

I

00

distribution

CPI

LL PP

P

PP

I

Iplcpl PP

P

I

I

I

*zoo

*30*

400

I f50’

I

III

f6OO f 700

FIG. 12. Spatial distribution of the plumes. While the large plumes occur from 240 to 36O”W longitude, the small plumes (including the two hybrid Loki plumes) are uniformly distributed with respect to longitude. Latitude distributions, scaled proportionally to the areas represented by each latitude band, show a concentration of the small plumes at low latitudes.

210

MC EWEN

0.60

.> . ..a .r

5

.

*...

b-y

.* ..q

070

C-

. 0.

7

AND

. ..

.**

SODERBLOM

sitions within the region from 0 to 9O”W longitudes (Pearl and Sinton, 1982; Witteborn ef al., 1982) which we have not considered part of the Pele-type region. The IUE and visual spectra (Fig. 13) show this area to be broadly transitional to the Pele-type region, so perhaps it is of mixed or transitional volcanic character. IMPLICATIONS FOR VOLCANIC HEAT TRANSFER AND VOLATILES

260

280 I1

I1

90

ROTATIONAL

,,,,,I

160

PHASE

270

ANGLE

(9)

FIG. 13. Ground-based spectra of IO by rotational phase angle (equivalent to central longitude). The hemisphere centered on 3oo”W has been consistently darker and redder during over 50 years of observations (after Morrison et al., 1974).

ciated with a greater proportion of SOZ frost, and the red material with a larger proportion of other sulfurous materials. This is consistent with the concentration of the Pele-type plumes in the 3OO”W region and with their suspected lower abundance of soz. Hot spot observations have been strongly biased toward the region of 10 dominated by Pele-type eruptions, due to the flyby trajectory of Voyager 1 and to the eclipse observations of the Jupiter-facing hemisphere. Disk-averaged heat flux calculations of 1.5 to 2.0 W rnm2, made by several workers (Morrison and Telesco, 1980; Matson et al., 1981; Sinton, 1981; Pearl and Sinton, 1982), may not apply to the anti-Jupiter facing hemisphere. However, several outbursts have estimated po-

The hot spots seem to be due to surface materials rather than the plumes. IRIS coverage included the areas of the active small plumes Marduk and Volund, at about the same resolution as Amirani/Maui and Loki, yet no thermal anomalies were detected. In addition, no anomaly comparable in size to Pele’s plume (500- to 700-km radius) was detected in this region. Therefore, the actual plumes would not seem to be the source of hot spots, which is consistent with adiabatic cooling in which the entrained pyroclastics are in thermal equilibrium with the gas. Solids and liquids have a much higher thermal capacity than gas and account for most of the heat content of an erupting plume. The time required for a thermal wave to pass from the center to the edge of large particles is slow compared to eruption rates, so only ejecta less than about 5 mm in diameter will reach equilibrium with the gas during a high-velocity eruption (Sparks and Wilson, 1976). Therefore, a large proportion of large, hot pyroelastics must not have been present in these plumes. The source of the hot spots must be surface materials such as lava flows or lava lakes. The bimodal hot spot temperatures correlate with the bimodal viscosity of liquid sulfur. From its melting point, -393”K, to -430”K, it has a viscosity of the order of 0.1 poise. With a temperature increase of only a few degrees, the viscosity increases dramatically due to polymerization, reaching over 600 poise at 455°K. Viscosity then gradually falls with increasing temperature, dropping to about I poise at the boiling

TWO CLASSES OF VOLCANIC

point, 718°K at 1 atm (Sill and Clark, 1982). Low-viscosity fluids are more mobile and can more easily extrude onto the surface, so the correlation with detected hot spot temperatures is probably not a coincidence. This is also consistent with the short-lived 600°K signals: low-viscosity sulfur would rapidly spread out over the surface, causing a rapid rise of the signal, and since radiative transfer is proportional to the fourth power of the absolute temperature, material at 600°K will cool much more rapidly than material at 3OO”K, as discussed by Sinton (1980). Furthermore, the hot spot sizes are typically from 10 to 100 km in diameter, which is too large for volcanic vents and too small for the plumes, but about right for lava flows or flooded calderas. Therefore, the observed hot spot temperatures, sizes, and time variability are exactly what is expected due to extrusions or exposure of low viscosity liquid sulfur. Furthermore, many caldera floors on 10 are orange and black, indicating equilibrium at the two low-viscosity temperatures. The association of -600°K spots with Pele-type eruptions and up to 400°K spots with the Prometheus-type provide minimum temperature estimates of the magma source regions. Since sulfur magma as a coherent liquid can rise nearly isothermally due to its low heat of fusion, the initial temperature of erupted liquid sulfur is a good estimate for the source region (Kieffer, 1982). For the high-temperature spots, radiative heat loss is so rapid that 600°K could be reached in the top few centimeters in just a few minutes after an initial temperature as high as 13WK, based on the estimate of thermal inertia made by Sinton (1980). The boiling point of sulfur at 1 atm is 718”K, but is a strong function of pressure, so higher temperatures are required for volatilization in the reservoirs whereas lower temperatures are needed at the surface of 10. Therefore, the source regions of the Pele-type plumes could easily be hot enough to volatilize sulfur if the foci are sufficiently shallow so that the pressures

211

PLUMES ON IO

are not too great. Temperatures sufficient to volatilize sulfur are less likely for the Prometheus-type reservoirs. Radiative cooling from 1300 to 400°K requires - 10 hr and convection would have time to maintain higher temperatures for longer periods, yet no hot spot above 400°K has been detected near this plume type. If source regions are <700”K, sulfur cannot be the driving volatile of the plumes (Kieffer, 1982). These temperature indications, along with spectral evidence for SOz-rich Prometheus-type deposits and sulfur-rich Peletype deposits, suggest that SO2 and sulfur are the driving volatiles for the Prometheus- and Pele-type plumes, respectively. As additional evidence, gaseous SOZ absorption features were identified over Loki by IRIS (Pearl et al., 1979), which Collins (1981) believes is SOZ from the volcanic plumes rather than from an equilibrium atmosphere. (The Loki plumes had characteristics more like the Prometheustype during the Voyager 1 encounter.) DISCUSSION

OF MODELS ERUPTIONS

FOR

IO’S

PLUME

The models that have been proposed for mechanisms which drive IO’S plumes can be examined in the context of the two discrete classes of plume eruptions proposed in this work. Basically two models have been proposed: (1) a sulfur-SO2 model developed by Smith et al. (1979) and Kieffer (1982) and (2) silicate-sulfur models discussed by various workers (Cat-r et al., 1979; Consolmagno, 1979; Hapke, 1979; Sinton, 1980; and Reynolds et al., 1981). In the sulfur-SO2 model, heat from the silicate lithosphere melts sulfur that in turn heats and volatilizes SO* to drive the plumes. This model requires that sulfur be heated to at least 393”K, the liquidus temperature. The silicate-sulfur models require reservoir temperatures for the silicates in the general range of 700 to 1500°K. In these models, hot, perhaps molten, silicates volatilize sulfur which in turn drives the

212

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AND

plumes. Both types of models generally rely on heat that may be tidally generated in IO’S solid silicate lithosphere, as proposed by Peale et al. (1979). Various criticisms have been raised for both models, as discussed by Kieffer (1982). In this section we examine these models and conclude that the criticisms that have been raised can be largely resolved if the silicate-sulfur model is applied to the larger Pele-type plumes and the sulfur-SO2 model is used to explain the smaller Prometheus-type plumes. One of the difficulties with the lowertemperature (393°K) sulfur-SO2 model has been in attaining the necessary ejection velocities for the large plumes, while the hightemperature silicate-sulfur models can easily produce the required velocities to explain the eruptions of Surt, Pele, and Aten Patera. By assuming conditions of thermodynamic equilibrium in the sulfurSO2 model, ejection velocities as high as 1 km set-l (calculated for the plume of Pele) are just barely met at 393°K (Smith et al., 1979). However, the formation of shock waves and frictional losses would tend to reduce this velocity (Reynolds ef al., 1980). Therefore, this model is energetically marginal to explain the Pele, Surt, and Aten Patera eruptions, but easily meets the estimated ejection velocity of -0.5 km set-’ of the small plumes. At higher temperatures, expansion of SO;? could eject material at 1 km set-i or higher velocities (Kieffer, 1982), but the low-temperature model is more consistent. with likely reservoir temperatures of the Prometheus-type plumes. Spectral reflectance evidence from the International Ultraviolet Explorer (IUE), from ground-based telescopic observations, and from Voyager imaging observations indicate that in the large plume deposits, and in the hemisphere in which large plumes are found, there is little or no evidence of SO2 (Nelson et al., 1980). By contrast, SO2 is evidently abundant in the deposits and equatorial band in which the small plumes are found, which is consistent with the premise that sulfur is the driving volatile of

SODERBLOM

the large plumes and SOZ is the driving volatile for the small plumes. Additionally, SO* was observed over Loki by IRIS. Although sulfur was not detected over Pele, the absorption bands are difficult to detect, and are probably below the resolution of IRIS (Pearl and Sinton, 1982). The premise that two different driving volatiles are responsible for the two classes of plumes has important implications for the supply of material to the plumes and for their durations. According to work by Smith et al. (1979) and Kieffer (1982), for a reasonable set of thermal gradients in IO’S upper crust SO2 will be stable as a liquid over a considerable depth, a zone of the order of a kilometer thick. This implies that SO* in the form of a very low-viscosity liquid (-0.005 poise, about half that of water) can laterally feed the small plumes over this great depth thus permitting them to erupt continuously for several years. By contrast, sulfur generally has a much higher viscosity than liquid S02. From its melting point -393°K to -430”K, it has its lowest viscosity, -0.1 poise, then greatly increases to ~600 poise at -4WK, and then gradually falls to -1 poise at 718°K. As a result, sulfur will be mobile over a range of depth corresponding to -400 to 430°K and in a second zone at greater depth where the temperature exceeds -650°K. The temperature range for the upper zone (400” to 430”K), with a thermal gradient of -200°K km-‘, will occur at a depth somewhere between 1 and 2 km and will be only -150 m thick; the second zone will occur below 2 to 3 km, depending on the actual thermal gradient. Since most of the heat flow of 10 may be carried through volcanic materials, a simple thermal gradient based on conduction is not possible (Matson et al., 1980; O’Reilly and Davies, 1981). Nevertheless, temperatures will generally increase with depth, and the concept of thermal gradient remains useful for our discussion. The density of sulfur decreases with increasing temperature, so these zones may be unstable, and the lateral supply of sulfur to the large

TWO CLASSES

OF VOLCANIC

plumes through these zones may also be unstable. Minor fluctuation in flow rates, introduction of materials at other temperatures, or temperature changes from the latent heat of transformation could easily cause temperatures to increase or decrease and the viscosity of the sulfur to increase, choking off the supply. We propose this effect may be the cause of the short-lived intermittent nature of the large plumes. This model supposes a more diverse crustal composition than the sulfur ocean proposed by Smith et al. (1979a) and Sagan (1979). If liquid sulfur is the chief component, convection will prevent the development of temperature zones. Instead, we propose that the relatively pure liquid sulfur at depth exists within a solid matrix of some other composition, perhaps silicates, impure sulfur, or sulfur compounds. The peculiar behavior of the viscosity of sulfur with temperature may also naturally explain why the two classes appear to be associated with discretely different surface temperatures. As developed above, mobile molten sulfur will exist in 10’s crust in two separate temperature regimes: 400-430°K and >650”K. We suggest that the smaller Prometheus-type plumes simply arise in the 400-430°K zone and the large Pele-type originate in the high-temperature zone. However, these zones are not necessarily at the same depth across 10, as we will discuss in the next section. Two different driving volatiles (sulfur and SO*) is not the only possible explanation for the two classes of plumes. A single volatile heated in these two thermal regimes may be the explanation. According to Kieffer (1982), the calculated pressure, temperature, and density distributions of SO2 gas with a solid/vapor mass ratio of 1 : 1 can be nearly identical to that of pure sulfur (SZ) vapor. Perhaps SOZ drives both plume types, arising in two separate temperature zones and with different pyroclastics. However, spectral evidence suggests that SO* is present in the surface deposits of the small plumes, but not the large plumes; we there-

PLUMES

ON IO

213

fore favor a model involving both separate temperatures and separate volatiles. Perhaps Loki is an example of SO2 volcanism driven by the high temperature sulfur zone. A POSSIBLE

GLOBAL

ASYMMETRY CRUST

IN IO’S

The next major question is what controls the regional distributions of the two classes. As described above, the small Prometheus-like plumes occur dominantly in the equatorial band ?30” latitude. Of the eight Prometheus-like plumes (including the two Loki plumes which behaved dominantly like this type), seven are within _t30 of the equator. This belt constitutes half the surface area of the planet, so the equatorial concentration seems statistically significant. Of the seven in the 530“ belt, six are in two narrow belts 25” +5” north and south which may or may not be statistically significant (cf. Fig. 12). These narrow belts generally coincide with the edge of the broad equatorial swath of bright material thought to contain substantial SO2 frost. It appears likely to us that the distribution of the small plumes must be related to some set of latitudinally controlled conditions and processes that enhance the supply of SO2 to the equatorial belt. A variety of processes might introduce lateral pressure gradients that cause SO* to flow toward the equator. As noted by Smith et al. (1979b), cold trapping SO2 at high latitudes, burial, and migration toward the equator, might produce such a flow pattern. If IO’S equatorial bulge is in fact less than that expected for a planet rotating in hydrostatic equilibrium, as suggested by some preliminary work by Merton E. Davies, Rand Corporation, Santa Monica, California, then subsurface SO2 must flow toward the equator. In contrast, the large Pele-type plumes (including Loki as a transitory example of this type) are concentrated in a region centered near 3Oo”W ranging from -~50” north and south. Further, this region does not cover a full hemisphere but is only about 120” wide in longitude. The region is also

214

MC EWEN

AND

not centered on the trailing hemisphere but is shifted substantially to the west or towards the Jupiter-facing hemisphere (cf. Fig. 1). As noted above, the location of the dark-red region containing the large plumes fits precisely the dark-red, low uv longitude in the ground-based rotational spectral data (Fig. 13). It seems unlikely that this distribution is related to a simple trailing hemisphere effect, due for example to ion bombardment. In fact the distribution of the Pele-type plumes does not show a logical relation to latitude or to the leading, trailing, Jupiter-facing or anti-Jupiter facing hemispheres. We suggest that this distribution is related to an assymmetry in the nature of IO’S crust and the tidally heated, hot silicate lithosphere is simply nearer the surface in this region. Our hypothesis is based primarily on the existence of major topographic landforms in the region of the large plumes. The Voyager 1 close-encounter high-resolution images were confined to this region, primarily to the southern hemisphere. Mountains over 9-km high and steep scarps more than 2-km high have been mapped in the south polar region and along the terminator from -240 to 27O”W (Masursky, et al., 1979; Schaber, 1980, 1982). The mountains protrude above the otherwise generally smooth and flat surface of the planet. There is general agreement that these mountainous landforms must be silicate; if they were primarily composed of sulfur they could not sustain their own weight and would collapse (Clow and Can-, 1980). Further, such relief occurs near the Pele vent (Fig. 11) that had a temperature of -650°K and should have melted the surrounding crust if it was chiefly composed of sulfur (Pearl and Sinton, 1982). Many of these silicate landforms appear to be volcanic. Schaber (1980) recognized a mountain ring east of Pele that may be the rim of a large caldera complex in the silicate lithosphere protruding above the surface. The evidence to demonstrate that such silicate landforms are more abundant in the

SODERBLOM

region of the Pele-like plumes is skimpy, but, oddly enough, the best information on the global distribution of topography comes from examination of the low resolution photography. Some of the global images, like those shown in Figs. 3a, b, and e were acquired at phase angles around 20”, hence, the terminator region is in view near the limb. In these regions, even at low resolution, mountainous topography in the Peletype plume area can be seen casting enormous shadows across the otherwise flat, featureless surface. For example, in Fig. 3b, a low-resolution Voyager 2 frame (resolution 12.7 km/pixel, phase angle 20”), at least three large topographic protrusions can be plainly seen along the terminator. By contrast, Fig. 3e, which is actually higher resolution than 3b and similar phase angle (resolution 7 km/pixel and phase angle 14.7”), shows no large relief in the longitudes east of Prometheus. By a systematic survey of data of this type, we conclude that there is a reasonable argument for the existence of a concentration of large topographic landforms in the region of the large Pele-like plumes. We are proposing, then, that the concentration of large plumes may be related to an asymmetry in IO’S silicate subcrust such that the sulfur-rich debris mantle is simply thinner so that the heat source, the hot silicate lithosphere, is closer to the surface. The two low-viscosity sulfur zones, then, would be compressed in this region. High temperatures generated in the silicate subcrust could heat the sulfur-rich mantle at relatively shallow depths, where pressures are sufficiently low to allow volatilization of sulfur. By contrast, the silicate-sulfur interface may be at greater depths away from this region, where higher pressures do not allow sulfur volatilization. SUMMARY

AND

CONCLUSIONS

We have identified two discrete classes of volcanic plume eruptions on IO that we refer to as Prometheus-type and Pele-type.

TWO

CLASSES

OF VOLCANIC

A newly discovered eruption that occurred between the Voyager encounters in the south polar region is of the Pele-type. The characteristics of the larger, higher reservoir temperature and higher velocity Peletype are summarized as follows. (1) The eruptions last somewhere between a few days and a few weeks. (2) The plumes reach altitudes of about 300 km, producing concentric deposits about 1400 kilometers in diameter. (3) The temperature of associated sulfur lava exposed near the vent is in the neighborhood of 650”K, which is also the temperature at which sulfur is a low-viscosity black fluid. (4) The distribution of the large plumes corresponds to a historically dark red region centered near 3OO”W. Voyager and IUE evidence indicate a lower abundance of SO2 frost in this region relative to other longitudes. (5) The deposits and source regions are very dark in the Voyager uv spectra suggesting a low abundance of SO*. The spectra of the source regions match black sulfur. The deposits are made up of relatively dark-orange, red, and brown concentric rings that also match the spectra of various allotropes of sulfur. (6) The large plumes are concentrated in a region in which large silicate landforms protrude above the otherwise flat, smooth surface. (7) Numerous arcuate patterns similar to the deposits of the known plumes are evidence for other similar eruptions in this region. The smaller, cooler reservoir temperature, lower velocity eruptions of Prometheus-type plumes are characterized as follows. (1) The duration of the Prometheus-type plumes is estimated to be in excess of several years. (2) The eruptions reach heights typically between 60 and 100 km, forming ringed deposits typically 250 km in diameter. (3) They occur dominantly in the equatorial belt, mostly along the margin of a band of bright white material occurring generally in the +30” latitude belt. Voyager data, ground-based spectra, and IUE observations suggest this belt is rich in SOZ. (4) The high uv reflectance of the de-

PLUMES

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posits and source regions suggest the small plume eruptions are rich in SO*. Based on the evidence for and nature of the two classes, we present the following hypotheses. (1) We propose the Loki eruptions to be a hybrid case intermediate between the two classes, as the Loki deposits show a supf:rposition of both types of deposits, the plumes oscillated in size between the two types, and temperatures and ejection velocities were intermediate. (2) The strange viscosity-temperature behavior of sulfur is responsible for the discrete temperatures of erupted sulfur liquid associated with the two classes. Sulfur is a low-viscosity fluid between 400 to 430°K and again above 650”K, so there should be two crustal zones in which sulfur is mobile. We propose that the bimodal hot spot temperatures are simply a reflection of these two zones. (3) We further propose that the driving volatile for the Prometheus-type plumes is SOZ heated to 400 to 430°K in the upper liquid sulfur zone and the driving volatile for the large Pele-type plumes is sulfur heated by hot silicates in the lower zone. (4) We ascribe the differences in the durations of the two classes to the differences in temperature viscosity behavior of SO2 and sulfur. The eruptions of the large plumes will be intermittent due to the fact that if the sulfur in the supply conduits, magma chamber, or exit conduit changes temperature, the eruption can be choked off as the viscosity of sulfur changes so radically. By contrast, SOZ has extremely low viscosity (- half that of water) and will be stable over a great range of depth in IO’S crust and can thereby easily and continuously feed the small Prometheus-like plumes. (5) We further speculate that the large plumes are concentrated in a region of IO’S crust in which the sulfur-rich debris mantle overlying the hot silicate lithosphere is thinner so that high temperature volcanic reservoirs and the two low-viscosity sulfur

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zones are closer to the surface. (6) Finally, we suggest that the distribution of the Prometheus class is in some way controlled by a process which causes subsurface SO2 to move toward the equator, collecting in a ponded supply. ACKNOWLEDGMENTS This work was supported by funding from the NASA Voyager Project, the NASA Planetary Geophysics and Geochemistry Program, and the NASA Planetary Geology Program. We thank Susan Kieffer, Gerald Schaber, Henry Moore II, Peter Thomas, Gene Shoemaker, and Michael Carr for careful reviews. We would also like to thank W. Scott, E. Sanchez, E. Eliason, and K. Edwards of the U. S. Geological Survey, Flagstaff, Arizona for numerous technical contributions and assistance in the computerized image analysis. REFERENCES BENESH, M., AND P. JEPSEN (1978). Voyager imaging science subsystem calibration report. In Jet Propulsion Lab. Internal Document, pp. 3-34 to 3-35, Pasadena, Cahf. CARR, M. H., H. MASURSKY, R. G. STROM, AND R. J. TERRILE (1979). Volcanic features on 10. Nature 280,729-733.

205, 397-398.

CONSOLMAGNO, G. J. (1981). 10: Thermal chemical evolution. Icarus 47, 36-45. DANIELSON, G. E., P. N. KUPFERMAN, SON, AND L. A. SODERBLOM (1981). performance of the Voyager cameras. Res.

models and T. V. JOHNRadiometric J. Geophys.

86, 8683-8690.

EVIATAR, A., Y. MEKLER, N. BROSCH, AND T. MAZAH (1981). Ground-based observations of the 10 torus during Voyager 1 encounter: Indications of enhanced plasma injection and transport. Geophys. Res.

Left.

204,972.

MORRISON, D., AND N. D. MORRISON(1974). Photometry of the Galilean satellites. In Planerary Sate/lifes (J. A. Bums, Ed.), pp. 363-378. Univ. of Arizona Press, Tucson. MORRISON, D., D. PIERI, J. VEVERKA, AND T. V. JOHNSON(1979). Photometric evidence on long-term stability of albedo and colour markings on 10. Nature 280,753-755.

MORRISON, D., AND C. TELESCO (1980). Observational constraints on the internal energy source of 10. 44, 226-233.

NELSON, R., AND B. HAPKE (1978). Spectral reflectivities of the Galilean satellites and Titan 0.32 to 0.86 micrometers. Icarus 36, 304-329. NELSON, R. M., A. L. LANE, D. L. MATSON, F. P. FANALE, D. B. NASH, AND T. V. JOHNSON (1980). 10: Longitudinal distribution of sulfur dioxide frost. Science

210,784-786.

O’REILLY, T. C., AND G. F. DAVIES (1981). Magma transport of heat on 10: A mechanism allowing a thick lithosphere. Geophys. Res. Letters 8,313-316. PEALE, S. J., P. CASSEN, AND R. T. REYNOLDS (1979). Melting of 10 by tidal dissipation. Science 203,892894.

PEARL, J. G., R. HANEL, V. KUNDE, W. MAGUIRE, K. Fox, S. GUPTA, C. PONNAMPERUMA, AND F. RAULIN (1979). Identification of gaseous SO2 and new upper limits for other gases on 10. Nafure 280, 755-758.

109-

PEARL, J. C., AND W. M. SINTON (1982). Hot spots of 10. In The Satellites of Jupiter (D. Morrison, Ed.), pp. 724-755. Univ. of Arizona Press, Tucson. REYNOLDS, R. T., S. J. PEALE, AND P. CASSEN (1979). 10: Energy constraints and plume volcanism. Icarus

mag-

SAGAN, C. (1979). Sulphur flows

8, 249-252.

GRADIE, J., AND J. VEVERKA (1982). When are tral reflectance curves comparable? Zcarus 49, 119. HAPKE, B. (1979). IO’S surface and environs: A matic-volatile model. Geophys. Res. Let?. 6, 802.

KIEFFER, S. W. (1982). Dynamics and thermodynamits of volcanic eruptions: Inferences for the plumes on 10. In The Satellites of Jupiter. (D. Morrison, Ed.), pp. 647-723. Univ. of Arizona Press, Tucson. MASURSKY, N., G. G. SCHABER, L. A. SODERBLOM, AND R. G. STROM (1979). Preliminary geologic mapping of 10. Nature 280, 725-729. MATSON. D. L.. G. A. RANSFORD. AND T. V. JOHNSON (1981). Heat flow from 10 (Jl). J. Geophys. Res. 86, 1664-1672. MEKLER, Y., AND A. EVIATAR (1980). Time analysis of volcanic activity on 10 by means of plasma observations. J. Geophys. Res. 85, 1307-1310. MEYER, B., OOMMEN, T. V., AND D. JENSEN (1971). The color of liquid sulfur. J. Phys. Chem. 75, 912917. MORABITO, L. A., S. P. SYNNOTT, P. M. KUPFERMAN, AND S. A. COLLINS (1979). Discovery of currently active extraterrestrial volcanism. Science

Icarus

CLANCY, R. T., AND G. E. DANIELSON (1981). High resolution albedo measurements on 10 from Voyager 1. J. Geophys. Res. 86, 8627-8634. CLOW, G. D. AND M. H. CARR (1980). Stability of sulfur slopes on 10. Icarus 44,268-279. COLLINS, S. A. (1981). Spatial color variations in the volcanic plume at Loki, on 10. J. Geophys. Res. 86, 8621-8626. CONSOLMAGNO, G. J. (1979). Sulfur volcanoes on 10. Science

SODERBLOM

spec-

44,234-239. 799-

on 10. Nature

280,

750-753.

SCHABER, G. G. (1980). The surface of 10: Geologic

TWO

CLASSES

OF VOLCANIC

units, morphology, and tectonics. Icarus 43, 302333. SCHABER, G. G. (1982). The Geology of IO. In The Satellites of Jupiter, (D. Morrison, Ed.), pp. 556597. Univ. of Arizona Press, Tucson. SILL, G. T., AND R. N. CLARK (1982). Composition of the surfaces of the Galilean satellites. In The Satellifes of Jupiter (D. Morrison, Ed.), pp. 174-212. Univ. of Arizona Press, Tucson. SINTON, W. M. (1980). IO: Are vapor explosions responsible for the 5 pm outbursts? Icarus 43, 56-64. SINTON, W. M. (1981). The thermal emission spectrum of IO and a determination of the heat flux from its hot spots. J. Geophys. Res., in press. SMITH, B. A., L. A. SODERBLOM, T. V. JOHNSON, A. P. INGERSOLL, S. A. COLLINS, E. M. SHOEMAKER, G. E. HUNT, H. MASURSKY, M. H. CARR, M. E. DAVIES, A. F. COOK, II, J. BOYCE, G. E. DANIELSON, T. OWEN, C. SAGAN, R. F. BEEBE, J. VEVERKA, R. G. STROM, .I. F. MCCAULEY, D. MORRISON, G. A. BRIGGS, AND V. E. SUOMI (1979a). The Jupiter system through the eyes of Voyager 1. Science 204, 951-972. SMITH, B. A., E. M. SHOEMAKER, S. W. KIEFFER, AND A. F. COOK, II (1979b). The role of SO2 in volcanism on IO. Nature 280, 738-743. SMITH, B. A., L. A. SODERBLOM, R. BEEBE, J. BOYCE, G. BRIGGS, M. CARR, S. A. COLLINS, A. F. COOK, II, G. E. DANIELSON, M. E. DAVIES, G. E. HUNT, A. INGERSOLL, T. V. JOHNSON, H. MASURSKY, J. MCCAULEY, D. MORRISON, T. OWEN, C.

PLUMES

ON IO

217

SAGAN, E. M. SHOEMAKER, R. G. STROM, V. E. SUOMI, AND J. VEVERKA (1979~). The Galilean satellites and Jupiter: Voyager 2 imaging science results. Science 206, 927-950. SODERBLOM, L. A., T. JOHNSON, D. MORRISON, E. DANIELSON, B. SMITH, J. VEVERKA, A. COOK, C. SAGAN, P. KUPFERMAN, D. PIERI, J. MOSHER, C. AVIS, J. GRADIE, AND T. CLANCY (1980). Spectrophotometry on IO: Preliminary Voyager 1 results, Geophys. Res. Lett. 7, 963-966. SPARKS, R. S. J., AND L. WILSON (1976). A model for the formation of ignimbrite by gravitational column collapse. J. Geol. Sot. 132, 441-452. STROM, R. G., N. M. SCHNEIDER, R. T. TERRILE, A. F. COOK, AND C. HANSEN (1981). Volcanic eruptions on IO. J. Geophys. Res. 86, 8593-8620. STROM, R. G., AND N. M. SCHNEIDER (1982). Volcanic eruption plumes on IO. In The Satellites of Jupiter (D. Morrison, Ed.), pp. 598-633. Univ. of Arizona Press, Tucson. THEILIG, E. (1982). A primer on sulfur for the planetary geologist. NASA Contract or Report 3594, 34 p. VEVERKA, J., J. GOGUEN, S. YANG, AND J. L. ELLIOT (1978). How to compare the surface of IO to laboratory samples. Icarus 34, 63-67. WITTEBORN, F. C., J. 0. BREGMAN, AND D. F. LESTER (1981). Spectrophotometry of the IO 5 micron brightening phenomenon. Submitted for publication. WITTEBORN, F. C., J. 0. BREGMAN, AND J. B. POLLACK (1979). IO: An intense brightening near 5 micrometers. Science 203, 643-645.