Color and Morphology of Lava Flows on Io

Icarus 148, 407– 418 (2000) doi:10.1006/icar.2000.6501, available online at http://www.idealibrary.com on

Color and Morphology of Lava Flows on Io Jennifer L. Piatek, Sarah B. Z. McElfresh, Jeffrey M. Byrnes, Amy Snyder Hale,1 and David A. Crown Department of Geology and Planetary Science, 321 Engineering Hall, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 E-mail: [email protected] Received April 14, 2000; revised July 24, 2000

Analyses of color and morphologic changes in Voyager images of lava flows on Io were conducted to extend previous flow studies to additional volcanoes in preparation for comparison to Galileo data. Blue and orange filter images of Atar, Daedalus, and Ra Paterae were examined to identify systematic downflow decreases in blue/orange reflectivity suggested in earlier studies as diagnostic of color changes in cooled sulfur flows. Analyses of the color and morphology of 21 lava flows were conducted at these volcanoes, with additional morphologic analysis of lava flows at Agni, Masaaw, Mbali, Shoshu, and Talos Paterae. A total of 66 lava flows of up to 245 km in length were mapped to identify morphologic changes consistent with the rheologic changes expected to occur in sulfur flows. Although downflow color changes are observed, the trends are not consistent, even at the same edifice. Individual flows exhibit a statistically significant increase in blue/orange ratio, decrease in blue/orange ratio, or a lack of progressive downflow color variation. Color changes have similar magnitudes downflow and across flow, and the color ranges observed are similar from volcano to volcano, suggesting that similar processes are controlling color ratios at these edifices. In addition, using flow widening and branching as an indicator of the low viscosity exhibited by sulfur cooling from high temperatures, these flows do not exhibit morphologic changes consistent with the systematic behavior expected from the simple progressive cooling of sulfur. °c 2000 Academic Press Key Words: Io; volcanism.

INTRODUCTION

Observations by the Galileo spacecraft are providing new data regarding the surface of Io, including high-resolution images and information about albedo and color at ultraviolet, visible, and near-infrared wavelengths (Carr et al. 1995, Smythe et al. 1995, Lopes-Gautier et al. 1999, Lopes-Gautier et al. 2000, McEwen et al. 2000). This study is an investigation of lava flows in Voyager images intended to provide a more complete dataset for comparison to Galileo data by applying to additional volcanoes the methodology utilized previously (Pieri et al. 1984) to examine downflow color changes. Voyager images of eight volcanoes in the Ruwa Patera region of Io were analyzed to doc1 Present address: Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA.

ument variations in color as well as in albedo and flow morphology. Trends within flows and for different flows were examined in an attempt to identify systematic changes predicted for cooled sulfur flows (Sagan 1979, Theilig 1982). While the presence of sulfur and sulfur compounds on the surface of Io is not disputed, the amount of these compounds present and their precise role(s) in ionian volcanism remain fundamental questions regarding the geology of Io. BACKGROUND

Spectroscopic studies of Io indicated the presence of sulfur or sulfur compounds on its surface and a lack of silicate absorption features (i.e., Cruikshank et al. 1978, Fanale et al. 1979, Hapke 1979, Nash and Nelson 1979, Pearl et al. 1979, Smythe et al. 1979, Simonelli and Veverka 1986), although recent studies have shown the presence of an absorption band near 1 µm due to an iron-bearing mineral, possibly orthopyroxene (Carlson et al. 1997, Geissler et al. 1999). Laboratory spectra of terrestrial sulfur samples show strong effects due to contamination by elements such as Fe, and are partial matches to Earth-based spectra of Io (Kargel et al. 1999). Combinations of laboratory spectra of various sulfur compounds (Moses and Nash 1991) and sulfur compounds mantling basalt (Hapke 1989) have produced results similar to spectra of Io, although neither conclusively shows whether or not silicates are affecting those spectra. In laboratory studies, molten sulfur is observed to undergo a sequence of color changes from black (>500 K) to yellow (<400 K) while cooling (Tuller 1954, Meyer et al. 1971, Meyer 1976, Theilig 1982). In addition, sulfur exhibits a large decrease in viscosity from 930 poise at 460 K to 0.07 poise at 425 K; an increase in viscosity is observed on further cooling (Bacon and Fanelli 1943, Tuller 1954, International Critical Tables, as referenced in Sagan 1979, Theilig 1982). Studies of Voyager images concluded that long flow lengths (Smith et al. 1979a) and observed variations in color were consistent with cooled sulfur flows (i.e., Smith et al. 1979b, Soderblom et al. 1980, Clancy and Danielson 1981, Sill and Clark 1982, Pieri et al. 1984). In particular, Pieri et al. (1984) used the ratio of blue reflectivity to orange reflectivity in Voyager data to infer that lava flows at Ra Patera were indeed cooled sulfur. This conclusion was supported by their flow maps, which showed morphologies consistent with

407 0019-1035/00 $35.00 c 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.

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the observed viscosity decrease in cooling sulfur. Temperature observations of hotspots on Io were presented as further evidence for sulfur volcanism, as they yielded results around 200 K, much lower than those expected for silicate volcanism (Hanel et al. 1979, Sinton et al. 1980, Pearl and Sinton 1982, McEwen et al. 1985), although the range of the IRIS instrument is 4–55 µm (equivalent to blackbody temperatures of 725–50 K) and the signal/noise ratio decreases at the ends of this range (Hanel et al. 1979). The combination of morphology, spectroscopy, and color variations led to the development of models for volcanism on Io dominated by sulfur (i.e. Reynolds et al. 1980, Lunine and Stevenson 1985, McEwen and Lunine 1990). Other studies, however, pointed out that the series of colors observed in laboratory-cooled sulfur would be difficult to maintain in large-scale flows (Fink et al. 1983, Gradie and Moses 1983) and that the simple color progression observed in experiments would not be preserved long after cooling (Young 1984, Greeley et al. 1990). Morphologic studies showed that parameters such as length/width ratio and flow sinuosity are similar to those of basaltic lava flows on Earth and Mars, and that flows on Io do not exhibit the changes expected due to the viscosity decrease in cooling sulfur (Schaber 1980, 1982). Recent studies of ionian hot spots yielded temperatures of 600 to 2000 K using occultation (Spencer et al. 1994, Stansberry et al. 1997), eclipse photometry (Blaney et al. 1997, Spencer et al. 1997), and Galileo SSI measurements (McEwen et al. 1998a,b). These hot spots and the observed relief of mountains on Io have been used in the development of models of volcanism on Io dominated by silicates (Clow and Carr 1980, Matson et al. 1981, Carr 1986, Nash et al. 1986, Hapke 1989, Howell 1997, Keszthelyi and McEwen 1997a,b). Mixed silicate/sulfur models of the surface of Io predict albedo changes over time as plume materials are deposited and older deposits of sulfur compounds are remobilized. Carr et al. (1979) predicted that the surface should become more homogenous

with time, as light areas darken and dark areas lighten, whereas Hapke (1989) suggested local intermittent resurfacing. In the latter scenario, the youngest lava flows consist of mafic silicates and should have a low albedo. As a flow ages, it is covered by layers of mantling material which consist of sulfur (PSO, SO2 , and S2 O) consistent with observed spectral features. The effect of this mantling is to increase a dark, mafic flow’s albedo with time. Images of Io from Galileo show a variety of colors, including red deposits associated with plumes, yellow/yellow-green equatorial deposits, red-orange polar deposits (possibly yellow deposits altered by radiation), white-gray deposits (identified as SO2 ), and a variety of dark deposits whose spectra exhibit an absorption at 0.889 µm (Geissler et al. 1999). The question of silicate vs sulfur volcanism is central to the study of Io. In order to evaluate relevant information from ionian lava flows, this study uses the method of Pieri et al. (1984) to analyze color changes along lava flows, intending to reproduce results at Ra Patera and extend the color analysis to two other edifices, Atar and Daedalus Paterae, for which suitable Voyager data exist. In addition, flow maps were completed for eight paterae (Agni, Atar, Daedalus, Masaaw, Mbali, Ra, Shoshu, and Talos) in order to examine changes in morphology that would be consistent with the viscosity minimum associated with cooling sulfur. In particular, changes in flow width and flow branching are examined. Albedo maps for volcanoes displaying distinct stratigraphic relationships (Masaaw and Mbali Paterae) were also constructed to identify changes in albedo predicted by models suggesting a silicate ionian surface mantled by sulfur compounds. IMAGE PROCESSING

All of the images used in this study were acquired during the Voyager 1 encounter with Io; digital versions were accessed from the NASA “Voyagers to the Outer Planets” collection.

TABLE I Voyager Images Used in This Study Caldera center Patera

Latitude

Longitude

Agni Atar

40.34S 30.54N

333.09W 278.55W

Daedalus

19.23N

274.3W

Masaaw Mbali Ra

39.72S 31.58Sb 8.06S

339.07W 3.70Wb 354.951W

Shoshu Talos

19.12S 25.9Sb

323.72W 354.951Wb

Image

Filter

Resolution (km/pixel)

Emission angle

Incidence angle

1639142a 1638950a 1638952 1638950a 1638952 1639242a 1639227a 1639006a 1639008 1639259a 1639036a

Clear Blue Orange Blue Orange Clear Clear Blue Orange Green Clear

0.52 1.11 1.10 1.11 1.10 0.24 0.31 1.02 1.01 1.37 0.86

16.55◦ 58.19◦ 61.03◦ 58.19◦ 61.03◦ 7.28◦ 30.26◦ 15.74◦ 7.28◦ 34.58◦ 29.1◦

38.78◦ 53.55◦ 57.30◦ 53.55◦ 57.30◦ 47.83◦ 46.26◦ 12.47◦ 12.23◦ 32.58◦ 35.39◦

Note. Image information was retrieved from headers via ISIS. Image used for flow mapping. b Caldera not discernible in image; center is inferred.

a

COLOR AND MORPHOLOGY OF LAVA FLOWS ON IO

High-resolution (∼1 km/pixel) blue (410–600 nm) filter and orange (520–650 nm) filter images are necessary for analysis of color changes along flows using the methodology of Pieri et al. (1984); the lack of this type of coverage for most of Io’s surface limited the investigation to Atar, Daedalus, and Ra Paterae. Images used are listed in Table I. Flow maps were produced using clear filter (290–650 nm) images for Agni, Masaaw, Mbali, and Talos Paterae, a green filter (480–650 nm) image for Shoshu Patera, and blue filter images for Atar, Daedalus, and Ra Paterae. Flow maps, along with images used to create them, appear in Figs. 1 and 2.

FIG. 1. Single filter images and corresponding flow maps for Agni, Masaaw, Mbali, Shoshu, and Talos Paterae. Scale bars are 50 km long and north is to the top of each image. Image information is listed in Table I.

409

Calibrated images were produced from the raw dataset using the Integrated Software for Imagers and Spectrometers (ISIS) image processing system (version 4.2.1, U.S. Geological Survey, Flagstaff, AZ). Raw data were uncompressed and Reseau marks removed prior to data calibration using the equations and camera characteristics published by Danielson et al. (1981). Images were not reprojected unless otherwise noted. Images used for flow mapping were contrast enhanced to analyze the flow margins. Maps were compiled using these images, paying particular attention to flow margins and stratigraphic relationships. Blue filter and orange filter images used to analyze color changes required further processing. Since these images were not taken concurrently, they do not cover the same exact area on Io’s surface or have the same resolution. In order to directly compare the blue filter and orange filter images of a specific area, the higher resolution image of the pair was registered to the lower resolution image and reprojected; the blue filter and orange filter images were then mosaicked to produce a two-band image. The blue band was then divided by the orange band to produce an image that allowed for direct analysis of the color ratio at any point along a flow. Using the blue/orange ratio images, profiles were constructed both down and across flows at Atar, Daedalus, and Ra Paterae. Longitudinal (downflow) profiles were constructed as a series of line segments approximating the center of the flow. Transverse (across flow) profiles were taken roughly perpendicular to the downflow profiles. In total, 48 longitudinal profiles were acquired, and 85 transverse profiles were constructed for 15 of these flows. Profiles were constructed using a version of the Interactive Display Language (IDL, version 5.1) function “profile” modified for use in this study, which records the value of each point along a particular line. The modified version allows the user to directly input the line and sample of the desired endpoints, rather than choosing the endpoints on the image. This allows for direct correlation of profile segments with distance measurements made in ISIS. The number of data points along a profile segment was then divided by the measured length of the segment to determine the average distance between data points. This average distance was used in plotting the profile ratio values versus distance from the profile start. Longitudinal segments were joined to produce a profile showing the change in blue/orange ratio with distance downflow and a linear trend line was fit to the data. Transverse profiles were taken as single segments. Ratio images with the locations of profiles displayed as lines on the flow maps are shown in Fig. 2. In order to examine relationships between albedo and stratigraphy and evaluate models relating the two, albedo maps were created by conducting a supervised classification in ERMapper (version 5.0) using the single band images used for compiling flow maps. The supervised classification technique uses statistical parameters to assign each pixel to a class based on userdefined training areas. The albedo units were then compared with

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FIG. 2. Single filter images, flow maps, and blue/orange ratio images for Atar, Daedalus, and Ra Paterae. Lines on maps indicate locations of longitudinal and transverse profiles examined in this study; numbered boldfaced lines indicate the locations of corresponding numbered profiles shown in Fig. 3. Scale bars are 50 km long and north is to the top of each image. Image information is listed in Table I.

stratigraphic relationships observed in the course of compiling flow maps. RESULTS

Color Analysis Representative blue/orange profiles for lava flows at Atar, Daedalus, and Ra Paterae are shown in Fig. 3. Linear fits to longitudinal profiles exhibit strong positive and negative trends as well as a lack of prominent downflow trends. The significance of a calculated trend line is determined by the linear correlation coefficient (R), which is based on the distance of any datapoint from the trend line and the total number of data points, and the P level, which is defined as the probability that an observed “trend” is the result of a random distribution—the lower the P level, the more likely it is that the trend is real (Freund and Wilson 1997, p. 130). Statistical analysis of the linear fits indicates that there is little correlation between the length of the profile and the P level of the fit, suggesting that the number of

datapoints along a profile is not controlling the observed result. A graph of the relationship between R and P level appears in Fig. 4. Values of the linear correlation coefficient range from −0.9 to 0.8, showing a range in both slope and degree of linearity. For the linear correlation coefficients, values closest to ±1 are the most statistically significant, whereas values that are closer to zero are less significant. This result suggests that the linear trends observed in some profiles represent actual downflow variations, but these variations can show opposite trends, as indicated by the sign of the linear correlation coefficient. Statistically, highly significant linear trends are seen both in increasing and decreasing blue/orange ratio; however, nonlinear trends are observed as well. The lack of systematic downflow color changes is inconsistent with the predictions for the simple, progressive cooling of sulfur flows. The systematic color changes reported by Pieri et al. (1984) were not observed, even for the specific Ra Patera flows that they analyzed. In addition, longitudinal and transverse profiles have similar maximum and minimum values at the three volcanoes (see Fig. 5). The fact that the range of values seen along a flow’s length

411

COLOR AND MORPHOLOGY OF LAVA FLOWS ON IO

a

0.51

0.51

Atar Profile 2

Atar Profile 1 0.50

0.50

0.49 0.49

0.48

0.48

0.47 0.46

0.47

0.45

R = -0.6670 0

10

20

0.49

Blue/Orange Ratio

R = -0.8104

0.44

0.46 30

40

50

60

0

70

10

20

30

40

50

60

70

50

60

70

0.49

Atar Profile 6

Atar Profile 7

0.48

0.48

0.47

0.47

0.46

0.46

0.45

0.45 R = -0.0449

R = 0.1985 0.44

0.44 0

20

0.50

40

60

0

80

0.49

Atar Transverse Profile 1a

0.49

10

20

30

40

Atar Transverse Profile 6a

0.48

0.48

0.47 0.47

0.46

0.46 0.45

0.45 0

5

10

15

20

25

30

35

0

10

20

30

40

50

Distance (km) FIG. 3. Representative profiles for Atar, Daedalus, and Ra Paterae. Profile numbers correspond to numbered profile locations on maps shown in Fig. 2. Linear fits and their corresponding linear correlation coefficients are given for longitudinal profiles. (a) Selected profiles at Atar Patera. (b) Selected profiles at Daedalus Patera. (c) Selected profiles at Ra Patera. Profile 1 is along the same flow studied by Pieri et al. (1984).

is similar to those observed across the same flow is also inconsistent with changes attributed to the cooling of sulfur flows. While most of the transverse profiles show concave shapes, with more orange material at the lateral margins, this trend is not consistent for all flows. Even if a distinct concave shape is present, the lowest point is not necessarily at the center, suggesting more complicated controls on flow color. Maximum and minimum values are also similar among all three volcanoes, suggesting that there is no discernible difference between the processes controlling color ratios at the three edifices. The lack of consistent progressive color changes suggests that if these ionian flows are sulfur, then they are not behaving as

simple flows observed in laboratory studies (Bacon and Fanelli 1943, Sagan 1979, Theilig 1982). The observed range of color changes may be the result of changes in the vesicularity of the flow surface, changes in the nature of a glassy surface crust, or perhaps differential mantling of the flow surface. Fink et al. (1983) suggest that the upper surface of a sulfur flow could be influenced by numerous complex processes that would cause any systematic color patterns to be disrupted. Emplacement of compound lobes, as suggested by Galileo images (McEwen et al. 2000), and typical of terrestrial sulfur flows (Theilig 1982) also cannot be ruled out. Rothery et al. (1996) note the presence of thin, light-colored coatings, often containing sulfur compounds,

412

PIATEK ET AL.

b

0.52

Daedalus Profile 12 0.58

0.50

0.53

0.48

0.48

0.46

0.43

Daedalus Profile 16

0.44 R = -0.5701

R = 0.5172 0.42

0.38 0

10

20

0.58

40

50

60

70

0

20

0.58

Daedalus Profile 20

0.56

Blue/Orange Ratio

30

40

60

80

100

120

140

Daedalus Profile 29

0.56

0.54

0.54

0.52

0.52

0.50

0.50

0.48

0.48

0.46

0.46

0.44 0.42

0.44

R = -0.1587

R = 0.7679

0.42

0.40 0

10

20

30

40

50

60

0

70 0.61

0.65

Daedalus Transverse Profile 5g

20

40

60

80

100

120

Daedalus Transverse Profile 29l

0.59

0.60

0.57 0.55

0.55

0.53 0.50

0.51 0.49

0.45

0.47 0.45

0.40 0

5

10

15

25

20

0

10

20

30

40

Distance (km) FIG. 3—Continued

on terrestrial basalt flows. Such coatings, if present on ionian flows, would be spectrally dominant, potentially masking the silicate nature of these flows. In particular, SO2 frost is nearly everywhere on the surface (Carlson et al. 1997), and could easily be altering the color and spectra of these lava flows. Morphologic Analysis Geologic mapping investigations of the surface of Io have resulted in the identification of individual eruptive centers, their areal extents, and the margins of associated flow fields (e.g., Moore 1987, Greeley et al. 1988, Crown et al. 1992). Other studies have focused specifically on the morphologic properties of individual lava flow lobes as a means of constraining the composition of ionian lavas, their rheologic behavior, and eruption

conditions (Fink et al. 1983, Greeley and Fink 1984, Pieri et al. 1984). These works discuss the expected morphologic properties of lava flows resulting from the unusual rheologic behavior of sulfur as it cools from high temperatures and include comparisons to several individual ionian volcanoes. Fink et al. (1983) used thermal modeling to show that a cooling sulfur flow will develop a series of interior zones of low and high viscosity that result in a flow surface that is highly disrupted and can exhibit ridges, fractures, and resurfaced zones due to crustal instability and upwelling from the interior. Greeley and Fink (1984), on the basis of laboratory experiments with molten sulfur, suggest that Atar, Daedalus, and Kibero Paterae are sulfur volcanoes characterized by a high-viscosity central mass with a quenched crust from which emanate breakout flows of cooler, low-viscosity lava. The study by Pieri et al. (1984) suggested

413

COLOR AND MORPHOLOGY OF LAVA FLOWS ON IO

c

0.65

0.65

Ra Profile 4

Ra Profile 1 0.60

0.60

0.55 0.55 0.50 0.50

0.45 R = -0.3415

R = -0.5246

0.40

0.45 0

50

100

150

200

250

0

100

150

0.55

0.65

Blue/Orange Ratio

50

Ra Profile 13

Ra Profile 19

0.60

0.53

0.55

0.51

0.50

0.49 0.47

0.45 R = 0.1355

R = 0.7246 0.45

0.40 0

20

40

60

0

80

0.62

10

20

30

40

0.70

Ra Transverse Profile 1j

Ra Transverse Profile 1n 0.65

0.57 0.60 0.52

0.55 0.50

0.47 0.45 0.42

0.40 0

5

10

15

20

25

0

5

10

15

20

Distance (km) FIG. 3—Continued

that thin sheets of low-viscosity sulfur would emanate from sinuous, high-viscosity flows. Their characterization of three flows at Ra Patera showed morphologic variations that they believed were consistent with cooling molten sulfur. These flows were narrow and highly sinuous upstream, widened and branched further downstream, and exhibited prominent widening in more distal regions. Lava flows on Io have been noted to have gross morphologic similarities to terrestrial silicate flows (Carr 1986). However, interpretations of flow morphologies are hindered by the lack of constraints of temporal and spatial variations in factors related to eruption conditions (e.g., effusion rate) and flow emplacement (e.g., local topography, rheology, and the development of channels, levees, and lava tubes). In the current analysis, flow maps were created that show planform morphologies of discrete lobes at each of the eight

volcanoes studied (Figs. 1 and 2). Each edifice exhibits, or is inferred to have, a central vent from which one or more flows (up to 245 km in length) extend. Calderas range from 10 to 80 km in diameter. Summit heights and flank slopes at ionian volcanoes are largely unknown, although Voyager 1 stereo images have recently been used to estimate the relief (∼1 km) and flank slopes (0.1–0.3◦ ) of Ra Patera (Schenk et al. 1997). All edifices are characterized by an apron of volcanic deposits around the central vent, from which discrete flows become discernible downflow. Maps were constructed by drawing contacts along discernible flow margins at each edifice. Flows are not in all cases traceable back to the vent. Note that the “orange pond” mapped previously by Pieri et al. (1984) is not interpreted herein to be part of the discrete flow lobes mapped at Ra Patera. The number of individual discernible flows at a given volcano ranges

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PIATEK ET AL.

R vs. p-level

Ra Atar Daed Daedalus

Not Statistically Significant 0.05

Borderline Significant 0.01

Significant

0.001

Highly Significant

0.0

-1.0000

-0.8000

-0.6000

-0.4000

-0.2000

0.0000

0.2000

0.4000

0.6000

0.8000

1.0000

R

FIG. 4. Plot of linear correlation coefficient (R) vs P level for linear trend lines fit to longitudinal profiles taken at Atar, Daedalus, and Ra Paterae. The y-axis scale is logarithmic: points falling on the x-axis have a P level of 0. Ranges of P level values corresponding to various degrees of statistical significance are noted.

from 1 (at Shoshu) to 12 (at Agni and Masaaw); 66 total flows were mapped. Lava flow lobes mapped in this study display a wide range of planform shapes. For example, Ra and Talos Paterae display long (up to 240 km), narrow (typically <20 km) flows; Agni Patera flows are similar but shorter (<50 km). Atar, Daedalus, Masaaw, Mbali, and Shoshu Paterae display flows that have higher width to length ratios. Individual flow lobes at Atar, Daedalus, and Mbali Paterae commonly show an overall downflow widening that is not typical of flows at the five other volcanoes. Flows at Masaaw and Mbali Paterae tend to display more small scale variation (relative to resolution along flow margins) than flows at other edifices examined. Flow lobes were analyzed for features expected from the four orders of magnitude viscosity decrease in sulfur cooling over ∼35 K from ∼460 to 425 K. Such features are not expected for silicate flows, as a silicate melt does not exhibit a viscosity minimum but rather progressively increases in viscosity during cooling. In this study, the widening and branching behaviors of lava flows on Io were examined to assess whether the morphologies attributed to cooling sulfur in previous studies are evident at other volcanoes. For flows that could be mapped as discrete emplacement units, flow widening and branching were examined as a function of downflow distance. Prominent increases in flow width over relatively short downflow distances were used as an indicator of a potential viscosity decrease based on previous studies (Fink et al. 1983, Greeley and Fink 1984, Pieri et al. 1984). Occurrences of

sudden widening were identified qualitatively as locations where there is significant widening over a small downflow distance. Instances where a flow suddenly becomes narrow and then widens again are not considered because this planform morphology is not consistent with predicted viscosity effects of cooling sulfur and more likely results from local topographic control. Measurements of sudden widening include: flow width before and after widening, flow length over which the widening occurs (1x), distance from the vent to the point where widening is initiated, and total flow length from the vent to the flow terminus. Widths were measured roughly perpendicular to the inferred flow direction. In addition, occurrences of flow branching were identified at locations where flows diverge without reconverging downflow (i.e., anabranching is not considered). Measurements of branching include distance to branching location from the central vent, number of branches at the branching location, and total flow length from the vent to the flow terminus. Ten instances of sudden widening were identified for 10 flows at five edifices (Table II); 21 instances of branching on 10 flows were identified at five edifices (Table III). Widening occurs between 40 and 220 km from the vent, at distances between 28 and 96% down the total flow length. Half of the occurrences of widening occur at Mbali Patera, where none of the discrete lobes are distinguishable for at least the first 50% of the total flow length. If this population is not considered, sudden widening occurs at distances between 28 and 84% of the total flow length. Of the 10 occurrences of sudden widening, only half fit

415

COLOR AND MORPHOLOGY OF LAVA FLOWS ON IO

TABLE II Flow Widening Flow width (km)

Patera

Distance to wideninga (km)

Before

After

1x

Total flow length (km)

% Downflow

Locationb

Daedalus Masaaw Masaaw Mbali Mbali Mbali Mbali Mbali Ra Shoshu

97 40 51 151 165 174 183 195 183 29

14 6 1 4 2 6 5 5 5 5

33 12 5 10 8 10 16 17 24 29

7 1 3 2 6 8 3 8 19 9

164 65 69 162 193 200 192 244 219 102

59 62 74 93 85 87 95 80 84 28

E (29) SE SW NE SE SE SE SE SW (1) NW

a b

Flow widening from an initial position (distance to widening) over an interval 1x. Location of sudden widening is identified by figure quadrant and profile number (where applicable).

Atar Transverse Profiles

Atar Longitudinal Profiles 0.73

0.73

0.68

0.68

0.63

0.63

0.58

0.58

0.53

0.53

0.48

0.48

0.43

0.43

0.38

0.38 1

2

3

4

5

6

7

5a

5b

Blue/Orange Ratio

Daedalus Longitudinal Profiles

5c

1a

1b

1c

6a

6b

6c

6d

Daedalus Transverse Profiles

0.73

0.73

0.68

0.68

0.63

0.63

0.58

0.58

0.53

0.53

0.48

0.48

0.43

0.43

0.38

0.38 1

3

5

7

1a

9 11 13 15 17 19 21 23 25 27 29

5c

5h 12b 15d 15i 15n 24b 26b 29d 29i 29n

Ra Longitudinal Profiles 0.74

0.78

0.7

0.73

0.66

0.68

0.62

0.63

0.58

Ra Transverse Profiles

0.58

0.54

0.53

0.5 0.46

0.48

0.42

0.43

0.38

0.38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

1a

1c

1e

1g

1i

1k

1m

1o

1q

Profile # FIG. 5. Bar graphs of ranges of blue/orange ratio value for all profiles. Longitudinal profiles are numbered. Transverse profiles are given a number corresponding to the longitudinal profile number of the flow and a letter; letters are given alphabetically downflow with (a) being closest to the source.

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PIATEK ET AL.

TABLE III Flow Branching

Patera

Distance to branching (km)

Total flow length (km)

% Downflow

Locationa

Agni Masaaw Masaaw Masaaw Masaaw Ra Ra Ra Ra Ra Ra Ra Ra Ra Shoshu Talos Talos Talos Talos Talos Talos

18 33 42 51 62 38 41 41 50 67 75 79 125 209 181 11 34 46 46 60 72

26 100 100 67 85 79 219 108 240 108 173 240 240 240 244 30 84 84 83 84 83

69 33 42 76 73 48 19 38 21 62 43 33 52 87 74 37 40 55 55 71 87

SE NE NE NE SW NW NW SW SE NE NE NE NE SW NE SE SE SE NW SE SE

a

Location of branching is identified by figure quadrant.

the criteria for sudden widening established by Peitersen and Crown (1999), who consider sudden widening to have occurred if 1W/1x ≥ 1.5, where 1W is the increase in flow width over a distance 1x. Branching occurs between 15 and 220 km from the vent, at distances between 18 and 77% of the total flow length. The number of branches ranges from 2 to 4, with 66% of all branching resulting in only two branches. The lava flows examined on Io do not show any consistent widening or branching relationships. It would be expected that the predicted viscosity-induced morphologies should occur at the same absolute distance from the source if flows are volume limited, and that the occurrences should be at the same downflow proportion of the total flow length if flows are cooling limited (Wilson et al. 1993). Flows commonly display both increases and decreases in downflow width, and in many cases, an individual flow exhibits branching at different locations. Both of these observations are inconsistent with morphologies expected of sulfur flows and may be attributed to factors other than viscosity, such as topography or variation in effusion rate. No flows were identified with morphologic characteristics indicative of the viscosity minimum expected of proposed ionian sulfur flows. Albedo Analysis Albedo analysis was conducted for Masaaw and Mbali Paterae because distinct stratigraphic relationships were observed at these locations during flow mapping. At Mbali Patera, there is a clear progression from dark to light as the flows age; the only exception is a very bright, stratigraphically young unit, which

could represent remobilization of PSO mantling material or SO2 frost. The observed albedo changes at Mbali Patera are consistent with the model proposed by Hapke (1989), suggesting that the surface of Io is mostly composed of a mafic silicate with a thin mantling of sulfur compounds. No correlation between albedo and stratigraphy was observed at Masaaw Patera. CONCLUSIONS

Analysis of the color, morphology, and albedo of lava flows on Io using Voyager 1 images indicates that these flows are not analogous to the simple, progressive cooling of sulfur observed in laboratory studies (Bacon and Fanelli 1943, Tuller 1954, Meyer et al. 1971, Meyer 1976, Sagan 1979, Theilig 1982). The change in blue/orange reflectivity ratios suggested by Pieri et al. (1984) for flows at Ra Patera is not observed. A wide range of downflow color change trends, including both progressive decreases and increases in blue/orange ratio, as well as a lack of any systematic changes, are evident. Flow morphology studies show that while there are instances of flow widening and branching, these are fairly minor and do not occur at consistent downflow distances. In addition, neither branching nor widening are observed at all flows, though other factors expected to affect flow morphology, most notably topography, are poorly constrained for Io. The results presented here are not consistent with a simple sulfur cooling model, but are consistent with models for silicate volcanism (i.e., Clow and Carr 1980, Matson et al. 1981, Carr 1986, Nash et al. 1986, Hapke 1989, Howell 1997, Keszthelyi and McEwen 1997a,b) or possibly more complex behavior of sulfur flows. The resulting color of a lava flow is affected not only by composition, but also by such effects as vesicularity, surface roughness, and mantling of the flow surface. Eruptive parameters such as effusion rate and flow composition, as well as underlying topography and changes in composition along the flow, affect the resulting flow morphology, and it is difficult to separate these effects from those of a possible viscosity decrease. No observations made during flow mapping suggest a significant viscosity decrease during flow emplacement. Albedo changes consistent with proposed models of a silicate Io with a mantling of sulfur compounds were observed at Mbali Patera. Galileo observations can be compared with these results to further examine changes in color and morphology along lava flows on Io. In particular, additional volcanoes can be examined for color changes using higher spectral resolution. Highresolution images can be used to look for small-scale sinuosity changes at flow margins and to more precisely constrain stratigraphic relationships at a given volcano. Topographic data can be used to assess potential topographic controls on flow morphologies. These new data should contribute to interpretation of the composition of lava flows on Io. REFERENCES Bacon, R. F., and J. Fanelli 1943. The viscosity of sulfur. J. Am. Chem. Soc. 65, 639–648.

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