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A CLOSE-UP LOOK AT IO FROM GALILEO’S NEAR-INFRARED SPECTROMETER
RosalyLopes-Gautier’, S. DoutC, W.D. Smythe, L.W.Kamp, R.W. Carlson, A.G. Davies, F.E. Leader, A.S. McEwen, P.E. Geissler, S.W. Kieffer, L. Keszthelyi, E. Barbinis, R. Mehlman, M. Segura, J. Shirley, L.A. Soderblom
Rosaly Lopes-Gautier, E. Barbinis, R.W. Carlson, A.G. Davies, L.W. Kamp,M. Segura, J. Shirley, W.D. Smythe, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 9 1 109. S . DoutC, F. Leader, R. Mehlman, IGPP, University of California, Los Angeles, CA 90095. S. W. Kieffer, Kieffer & Woo, Inc., PO Box 130, Palgrave, Canada LON 1PO. P. E. Geissler, A.S. McEwen, L. Keszthelyi, Lunar and PlanetaryLaboratory, University of Arizona, Tucson, A 2 8572 1. L. Soderblom, U.S.Geologica1 Survey, Flagstaff, Arizona 8600.
Infrared spectral images of Jupiter’s volcanic moon Io, acquired during October and November 1999 flybys of the Galileo spacecraft, were used to study the thermal structure and sulfur dioxide distribution of active volcanoes. Loki Patera, the solar system’s most powerful known volcano, exhibits large expanses of dark, cooling lava on its caldera floor. Prometheus, the site of long-lived plume activity, has two major areas of thermal emission, which support ideas of plume migration. Sulfur dioxide deposits were mapped at local scales and show a more complex relationship to of other sulfur surface colors than previously thought, indicating the presence compounds. A major objective of the Galileo mission was to investigate Io’svolcanoes and their surface modification processes using high spatial resolution spectral images. Observations were obtained during two flybys of Io in October 1999 (orbit 124) and November 1999 (orbit 125) using the near-infrared mappingspectrometer (NIMS). It was known previously that Io’s surface is dotted with active volcanoes (hot spots) ( I ) , and covered by SO? frost and other compounds (2,3,4). Some hot spots exhibit plumes that may inject gaseous SO? into the atmosphere (5), subsequently condensing as frost on the surface.
NIMS obtained 17 observations during I24 and 4 observations during 125, most with spatial resolutions from 0.5 to 25 km/NIMS pixel. These wereobtained at 14 fixed infrared (IR) wavelengths (in the range from 1.0 to 4.7 pm) instead of the planned 360, becausethe instrument’s spectral scanning capabilitymalfunctioned. This anomalous
operation provided greater sampling density (24 samples instead of 1 ) at each wavelength, increasing the signalhoke ratio. The reducednumber of wavelengths is suitable for temperature determination and SO? mapping, but our search for yet unknown surface compounds was compromised. The NIMS spectral range includes reflected sunlight and thermal emission components contain l a v a at (Supplementary Figure 1). A pixel showing volcanicactivitymay different temperatures (6), buthere we use a single-temperature Planck function to estimate the brightness temperature TBand the color temperature Tc, with a correction for reflected sunlight in daytime observations (7). TB is a measure ofthe average emitted thermal energy within a given wavelength interval and pixel area. Tc uses the shape and amplitude of the Planck function to determine a temperatureand its corresponding emitting area within the pixel. Tc estimates tend to be dominated by cooler materials that typically cover larger areas and thus emit greater power (in the wavelength range used) than the hotter materials having much smaller areas. For SO:! mapping, the reduced number of wavelengths precluded the full-spectrum modeling used previously (3). We used the 4.I-pm spectral channel, which lies within the strong SO:! V I + v3 absorption band (Supplementary Figure I), as a qualitative SO2 indicator. Specifically we form the relative band depth from theabsorption depth at 4.1 pm relative to measurements at a non-absorbing wavelength (3.0 pm) (8). The relative band depth depends on the SO2 abundance, the mean grainsize, and the presence of other materials and their mixing mode (spatially segregated or intimately mixed at the scale of photon path lengths). Here we assume spatially segregated mixing. SO:! absorbs strongly at 4.1 pm (absorption coefficient 11 cm”, Ref. 9) so non-zero reflectance at that wavelength indicates the presence of other, non-absorbing material. The assumption of spatially segregated mixing (10) with spectrally neutral material gives upper bounds to the SO2 abundance. The depth of sampling at 4.1 pm-wavelength is limited to about Imm, so we cannot distinguish between thick frost deposits and thin layers (that can be transparent in the visible and at non-absorbing IR wavelengths).
Loki Patera: NIMS imagedLoki Patera on Io’s nightside, targeting a light-colored, island-like feature in the caldera (Fig. 1). This feature has not changed significantly in appearance (at scales of several kilometers) since 1979. It is cut by a dark-colored, linear feature about 3.5 km wide that may be a crack exposing dark lava, or a valley flooded by caldera floor, having lower spatial lava. Additional NIMS dataonandbeyondthe sampling density, wereobtainedas the spacecraft’s scanplatformmovedtothenext target. Temperature maps (Fig. 1) show that the island and the light terrain outside the caldera exhibit low thermal emission (no Tc fits to these data were attempted), while the dark floor of the caldera and the crack have higher thermal output. The dark caldera floor is quiteuniform in brightness temperature (273+6K), with 70% of the pixels having median TB’Sof 273+2 K. Median TB’Sforthecrackmaterial are lower (2151tlOK). However, median Tc’s for the material in the crack are higher (350255K) than those for floor (305&44K). Thisindicates that the area of the hot crack is smaller than the size of the NIMS pixel with most ofthe contribution for the TB map arising from the cooler material. The areas corresponding to Tc’s can be obtained. For a pixel on the crack at the
median Tc, the area is 0.0065 km’ (0.28% of the pixel area that could be represented as a linear feature 4.3 m wide by the length of the pixel, 1.5 km). For a pixel on the floor, the area is 0.655 km’ (28.5 9% of the pixel area). The caldera floor’s temperature distribution shows that it is uniform in terms of energy output (similar TB’s) but at a scale below the size of individual pixels (Tc’s) the lavas show a more complex structure. This is consistent with a field of overlapping, cooling lava flows, orwith the cooling, non-uniform surface of a lava lake. Observations by Galileo’s photopolarimeter radiometer (1 I), taken at longer wavelengths, also show little variation in temperature on the eastern caldera floor, but show higher temperatures near the southwestern edge of the caldera and extending to the west of the island, regions not observed by NIMS. This is near the site ofan eruption that begun in early September 1999 (1 1). The lavas observed by NIMS on the caldera floor were probably emplaced during an earlier eruption. If the lava were erupted at 1475 K (typical for basaltic lavas), and were at least 5 m thick, the time (under Io conditions) for the lava tocool down to the median Tc for the caldera floor (305 K) would be about 127 days (12). However, the lava would only need about 39 days to cool down to the median Tc for the crack (350 K), so it is possible thatthelavas in the crack were emplaced during the earlier phase ofthe September 1999 eruption.
Tvashtar Catena: This chain of calderas were observed by NIMS and SSI in 125, and almost simultaneously from the ground. The ground-based (13) and SSI observations (14) were interpreted as images of a fire fountain erupting hot lavas. The observation by NIMS covers the eastern part of the active caldera and the pixels at most wavelengths are saturated. The hottest pixel for which a temperature could be derived (which is cooler than the hottest pixel observed) yielded Tc=1060&60K (area=0.003 km2). This estimate can mask small areas having high temperatures and is a lower limit that, although well within the range of basaltic lava temperatures, does not exclude ultramafic compositions such as those observed at Pillan Patera (15)- Data just returned from the third Io flyby (127) show that activity had fallen to a lower level by February 2000. NIMS pixels from the same locations as in I25 do not show saturation and yield temperatures between 500 and 600 K. Prometheus: Prometheus isthe site of a persistent plume discovered byVoyager in 1979 ( 5 ) . Observations by SSI during Galileo’s first orbitin 1996 showed thatthe Prometheus plume site had moved about 80 km west since 1979 (16), but the size and appearance of the displaced plume had not changed. Observations by SSI in I24 showed a caldera just to the north of the Voyager plume site (14), and a long lava flow between the Voyagerand Galileo plume sites. TheNIMSobservation of the same region (Fig. 2) showed 2 main hot spots, though thermal emission can be detected along the flow. The eastern hot spot is near the location of the 1979 plume, while the western hot spot is at plume. The eastern hotspotis = 0.007 km’) thanthewesternhot spot for a pixel between the two hot This temperature distribution is consistent with that of a lava eastern hot spot, which cools and becomes crusted over as it moves away from the vent.
Hot lava can flow through tubes and break out at the distal edge of the flow, spreading out over the surface, with hot material spreading over a larger area than at the vent. The vent associated with the eastern hot spot did not move. Instead, the plume moved to the western hot spot, even though the eastern hot spot remained active. This could arise from an interaction between hot lava and theunderlying SO? snowfield (17). The deposition of material from the Prometheus plume was mapped from a NLMS observation at spatial resolution of 20-25 km/NIMS pixel (Fig. 3). Thermal emission and SO2 absorption can be seen at 4.1 pm (Fig. 3B). The SO? relative depth map (Fig. 3C) hasbeen translated into a SO2 distribution map (Fig. 3D), using the linear correlation between the relative depth and the SO2 frost coverage (16). A striking feature (Fig. 3D) is the SO2 deposition ring that circles the vent. When compared with the low-phase visible image (Fig. 3A), the SO2 deposition ring appears larger than its visible white counterpart. This suggests that Prometheus emits several different compounds (such as sf4in the gas phase, that condense with a deposition distance fromthevent inversely related to volatility. Before the flybys, it was thought that areas that were bright and white at low phase angles corresponded to regions rich in coarse-grained SO:! (2,4). Our observation suggests that some other white material may be present.
Faint hotspots and vents:Several hot spots other than Prometheus are visible in this image (Fig. 3B). Camaxtli and Culann Patera had been detected before (1,16) but, at this higher spatial resolution, their complex structures are revealed. CulannPatera appears to have two separate hot spots within the same volcanic complex, while the Camaxtli Patera hot spot has two neighboring, small hot spots that may be independent volcanic centers. Six other relatively small and faint hot spots, which could notbe seen in the global-scale observations, were detected ( 1 8). Brightness temperatures for the faint hot spots are -220 K, and >270K for the brighter hot spots that had been detected at lowspatial resolution. One of the faint, previously undetected hot spots coincides with the feature Chaac Patera, a distinct green region in visible images (4).This hot spot becameactive sometime between the two fly-bys. NIMS data from I24 showed no thermal emission from Chaac Patera, but an observation at comparable spatial resolution in I25 showed that it had become active and that a region to the northwest of this hot spot had become darker at the shorter NIMS wavelengths. Dark materials on Io correlate with volcanic activity, as noted in previous studies (19), including those from NIMS and SSI images at low spatial resolution (1,4). Comparison of Figures (3A) and (3B) illustrates this correlation well, as areas of low-albedo deposits in (3A) coincide with areas of enhanced thermal emission which appear bright in (3B).
Culann Patera and Tohil Patera:The hot spot Culann (Fig. 4) and the Tohil region (where no hot spot has so far been detected) show distinct red deposits. SO? relative depth maps superimposed on SSI images show locally enhancedconcentrations of SO? that coincide with deposits having a pinkish-white to red appearance at visible wavelengths. Red deposits may be short-chain sulfur compounds (20) and are generally associated with hot spots (1.3).This intimate association of compounds having different volatility requires a particular emission or deposition mechanism that prevents their spatial segregation. A higher S2/S02ratio than that at Prometheus, for example, can lead
to a dense atmospheric population of radiatively cooled S:! solid nuclei on which SO:! condenses. Alternatively, boiling liquid SO:!with short-chain sulfur or other coloring agents dissolved in it may reach thesurface from below and freeze before complete sublimation. The diffuse appearance of the red deposits may arise from the entrainment of liquid droplets with escaping gases.
FIGURE 1: NIMS observation over Loki Patera obtained during the I24 flyby, superimposed on a SSI image. Top: Brightness temperaturemap,
temperatures for the lavas on the caldera floor are fairly uniform and hotter (red) than those for materials onthe “island” andonthe
caldera. Bottom: Color
temperature map, showing higher temperatures (reds andyellows)inthe
valley that runs through the light-colored “island” in the center of the caldera. The small hot areas in the “crack” appear cool in brightness temperature because of their small size relative to the pixel. NIMS spatial resolution ranges from 1.3 to 2.1 kdpixel (21) and the SSI image is 162 km across.
FIGURE 2: Color temperature map (A) made from NIMS data obtained during the I24 flyby, at spatial resolution of 5.5 to 8.5 kdpixel(21). The SSI image (B), taken in July 1999, is 142 km across. Two major hot spots are seen in (A): the eastern hot spot, which
has higher temperatures, is located near the site of the Voyager-era plume. The western hot spot coincides with the location of the currently active plume.
FIGURE 3: Prometheus region imaged by SSI in July 1999 (A) and by NIMS at 4.1 pm during the124 flyby (B). TheNIMS
krdpixel (21). Hot spots appear red in (B), while SO? appears blue. Hot spots are labeled 1 (Prometheus), 5 (Camaxtli), 7 (Tupan) and 8 (Culann). Hot spots 2, 3 , 4 and 6 were not
previously known. The color-bar scale in (B) represents radiance in units of bidirectional
. reflectance (solar irradiance/pi). The SO2 deposition ring around Prometheus is clearly seen in the center of (B) and in (C), the relative band-depth map (8). A qualitative SO2 distribution map is shown in (D). The color-bar scale gives the fractional area covered by SO2 frost and varies from 0 (very dark blue) to 1 (yellow). The area shown in each panel
is 1300 km across.
FIGURE 4: NIMS obtained data over Culann Patera during the I25 flyby. An SO2 relative band depth map (8) is shown over a SSI image acquired in July 1999. NIMS data shows enhanced concentrations of SO2 coinciding with deposits that are pink to red in visible wavelengths. Spatial resolution is 11 kdpixel(21) and the SSI image is 340 km across. The color scale is the same as in Fig. 3C.
NOTES AND REFERENCES:
I . R. Lopes-Gautier et ai., Zcarus, 140,243 (1999). 2. R.W. Carlson et al.,Geophys. Res. Lett. 24, 2479-82 (1997). 3. S; Dout6 et al., submitted to Zcarus (1999). 4. P.E. Geissler et al., Zcarus 140, 265 (1999).
5. R.G. Strom and N.M. Schneider, in Satellites of Jupiter, D. Morrison, Ed. (University of Arizona Press, Tucson, AZ, 1982), pp. 598-633. 6. A.G. Davies et al. 1999, Lunar Planet. Sci. XXX (1999) [available on CD-ROM]. 7. The brightness temperature, TB,is defined as: B (X, TB)= I (X), where B is the Planck function, h is the wavelength, and Z is the corrected radiance. The corrected radiance is defined as: Z (h) = I,, (h) - A * F (A), where lo (h) is the observed radiance, F (A) is incident solar flux at that wavelength and A isan albedo estimate derived from the 1 to 2 micron region. For nightside observations, Z=Z,,. The color temperature, Tc, is defined as: €3 (hi, Tc) / B (h2,Tc) = I (hl)/l (A21 We used two pairs of (hl, h2): (4.1, 4.4 pm) and (4.4, 4.7 pm). The reported Tc is the average between the two values. In dayside observations, TB and, in particular, Tc, are sensitive to the albedo used to correct the observed radiances, both in absolute value and its spectral slope (due to the SO2 absorption at 4.1 pm). Therefore, calculations are made value obtained atshort using a range of albedos, typically 60% to 80% ofthe wavelengths. The results are then averaged. Results displaying a high sensitivity to this variation (typically when Ti< 200K) are omitted, as are those in which the error between the two estimates of Tc exceeds 20% of the mean. Tc is only calculated for pixels in which TB exceeds a threshold (usually 210 to 250 K, depending on the noise level), to avoid errors due tothe SO2 absorption, since this absorption disappears at hot spots. Correct albedo values should yield I(h)=O for the cold areas of Io. Small residual uncertainties in the NIMS calibration can sometimes yield a spurious TB-180K for these areas, but pixels where T~<200Kwere not used in this analysis.
8. The relative band depth is defined as the ratio [A(3.O)/A(4.l)], where A(h) is the measured albedo (reflectance) at wavelength h. This ratio was not obtained for pixels in which thermal emission could be detected. 9. B. Schmitt et. al., Zcarus 111, 79 (1994), and B. Schmitt et al, in Solar System Ices, B. Schmitt, C. De Bergh, and M. Festou, Eds. (Kluwer Academic Publ., Dordrecht, 1998), pp. 199-240. 10. We have checked and quantitatively calibrated this simple approach (segregated mixing and linear combination of computed radiances) by comparing the relative band
depth map of a global observation taken in I24 with the SO? distribution maps of S. Doute' et al. (3), which modeled 408-wavelength NIMS spectra. 1 I. J. R. Spencer et al. (2000), this volume. 12. Times calculated using the modelof A.G. Davies, Icarus 124, 45 (1996). These temperatures and cooling rates assume that the surface heat loss is buffered by the release of latent heat from a still-molten flow interior. 13. R. Howell et al., Lunar Planet. Sci. XXXI (2000)[available on CD-ROM]. 14. A.S. McEwen et al. (2000), this volume. 15. A.S. McEwen et al., Science 281, 87 (1998). 16. A.S. McEwen et al., Icurus 135, 181 (1998). 17. S.W. Kieffer et al., (2000), this volume. 18. Previously undetected hot spots detected by NIMS during I24 and I25 arelocated at: (+21, 146W, near Surya), (+14, 150 W), (+20, 149W), (+12, 158W, near Chaac), (-5, 132W, the dark feature Seth), and (+9, 133W) 19. J.C. Pearl and W.M. Sinton, in Satellites of Jupiter, D. Morrison, Ed. (University of Arizona Press, Tucson, AZ, 1982), pp. 724-755. 20. Spencer, J.R. et al. 1997, Icarus 127,221-37 21. The pixel size in NIMS.processed images is 0.5 x the instrument's spatial resolution. 22. Portions of this work were performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.