Io in the Near Infrared: Near-Infrared Mapping Spectrometer (NIMS) Results from the Galileo Tlybys in 1999 and 2000 Rosaly M

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. El2, PAGES 33,053-33,078, DECEMBER 25, 2001

Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo tlybys in 1999 and 2000 Rosaly M. C. Lopes,• L. W. Kamp,• S. Dout6,2 W. D. Smythe,• R. W. Carlson,• A. S. McEwen, 3 P. E. Geissler,3 S. W. Kieffer, 4 F. E. Leader, s A. G. Davies, • E. Barbinis,• R. Mehlman,s M. Segura,• J. Shirley,• and L. A. Soderblom6

Abstract. Galileo'sNear-Infrared Mapping Spectrometer(NIMS) observedIo duringthe spacecraft'sthree flybysin October 1999, November 1999, and February 2000. The observations,which are summarizedhere, were used to map the detailed thermal structure of activevolcanic regions and the surfacedistribution of SO2 and to investigatethe origin of a yet unidentified compoundshowing an absorptionfeature at ---1 •m. We present a summaryof the observationsand results,focusing on the distributionof thermal emission and of SO2 deposits.We find high eruption temperatures,consistent with ultramafic volcanism,at Pele. Such temperaturesmay be present at other hot spots,but the hottest areas may be too small for those temperaturesto be detected at the spatial resolutionof our observations.Loki is the site of frequent eruptions,and the low thermal emissionmay representlavas cooling on the caldera'ssurface or the coolingcrust of a lava lake. High- resolutionspectral observations of Emakong caldera showthermal emissionand SO2 within the same pixels,implying that patchesof SO2 frost and patchesof coolinglavas or sulfur flows are presentwithin a few kilometersfrom one another. Thermal maps of Prometheusand Amirani showthat these two hot spotsare characterizedby long lava flows.The thermal profilesof flows at both locationsare consistentwith insulatedflows, with the Amirani flow field havingmore breakoutsof fresh lava along its length. Prometheusand Amirani each show a white ring at visiblewavelengths, while SO2 distributionmaps showthat the highestconcentration of SO2 in both ring depositslies outsidethe white portion. Visible measurementsat high phaseangles show that the white depositaround Prometheusextends into the SO2 ring. This suggeststhat the depositsare thin and that compositionalor grain size variationsmay occur in the radial direction.SO2 mappingof the Chaac region showsthat the interior of a caldera adjacentto Chaac has almostpure SO2. The depositappears to be topographicallycontrolled, suggesting a possibleorigin by liquid flow.

1. Introduction with activehot spots[Lopes-Gautier et at., 1997;McEwen et at., 1997,1998a; Lopes-Gautier et al., 1999;Marchis et al., 2000] and Io is the only place outside the Earth where large-scale coveredby SO2 frost and other compounds[Carlson et al., active volcanismhas been observed.The dynamicnature of 1997;Geissler et al., 1999;Dout• et al., 2001a].It waspostulated Io's volcanismmakes temporal studies particularly important, that volcanicplumes, which are present at some hot spots, and suchstudies have been made usingboth spacecraftdata inject gaseousSO 2 into the atmosphere.The gaseousSO 2 and ground-basedobservations. One of the major objectivesof diffuses and condenses as frost on the surface. The frost de- the Galileo missionwas to acquirehigh spatialresolution cov- posits serve as tracers that are useful for studyingvolcanic erageof Io in the visibleand infrared.These observations were control over surfaceevolution. The hot spots,many of which obtainedduring three flybysof Io in October1999 (orbit 124), are persistently active [Lopes-Gautieret al., 1999; Lopes- November 1999 (orbit 125), and February 2000 (orbit 127). Gautier,1999], are surfaceexpressions of the tidal heatingthat Prior to these flybys,it was known that Io's surfaceis dotted providesthe energyfor Io's activevolcanism. Io's volcanicactivity and SO2 distributionhave been studied 1jetPropulsion Laboratory, California Institute of Technology,Pas- usingdata acquiredby the near-infraredmapping spectrome- adena, California. ter (NIMS) sinceJune 1996. Observationsprior to the flybys 2Laboratoirede Planetologiede Grenoble,CNRS, Grenoble, were obtainedwith spatial resolutionsranging from 65 km/ France. 3Lunarand PlanetaryLaboratory, University of Arizona,Tucson, NIMS pixel (for one observationin orbit C21) to over 500 Arizona. km/NIMS pixel. These distantobservations (111 in total) al- 4S.W. KiefferScience Consulting, Inc., Bolton, Ontario, Canada. lowed us to make global studiesof the distributionand tem- 5Institutefor Geophysicsand Planetary Physics, University of Cali- poral variationsof volcanicactivity [Lopes-Gautier et al., 1997, fornia, Los Angeles, California. 1999] and to map the SO2 frost and obtain insightsabout its 6U.S.Geological Survey, Flagstaff, Arizona. cycle [Carlsonet al., 1997; Dout• et al., 2001a]. Joint studies Copyright2001 by the American GeophysicalUnion. with Galileo's solid-stateimaging system (SSI) were used to Paper number 2000JE001463. detecthigh-temperature volcanism [McEwen et al., 1998b;Da- 0148-0227/01/2000JE001463509.00 vieset al., this issue]and to studythe correlationof thermal

33,053 33,054 LOPES ET AL.' IO IN THE NEAR INFRARED emissiondetected by NIMS with surfacecolors and plumes [Lopes-Gautieret al., 1999;Geissler et al., 1999].However, the relativelylow spatialresolution of the distantobservations did not permit local-scale,detailed studiesof particularvolcanic 0.6 regionsto be made. i Galileo's three flybysof Io provided an unprecedentedop- portunityto studyspecific volcanic regions at high spatialresolution, thus examiningthe role of volcanismat local rather than global scales.Preliminary results were reported by Lopes-

Gautier et al. [2000a]. This paper summarizesthe NIMS ob- 0.2 ,, ,!,, servationsand the resultsof our further analyses,focusing on 1 1.5 2 2.5 3 3.5 4 4.5 5 thermal mappingand new data from the 127 orbit. Det. 5 6 7 9 10 11 12 13 14 15 16 17

le-02 2. Observations =. 8e-03 The NIMS instrumenthas been describedby Carlsonet al. : • i • ! i . i , [1992] and Smytheet al. [1995].NIMS includesa spectrometer 7 6e-03 with a scanninggrating and spansthe wavelengthrange 0.7 to 5.2/•m, therefore measuringboth reflected sunlightand ther- i =i. 4e-03 i i i i i i/i i mal emission.NIMS forms spectrawith 17 detectorsin com- ,' • i i i i . i i binationwith the movinggrating. The 17 wavelengths(spaced ' [ [ i : ! i i 2e-03 i • '. t , ! ! acrossthe wavelengthrange) obtainedfor each gratingposi- ...... i , i• ! i i i tion are acquired simultaneously.During the Galileo main • 0e+00 ",•,,,,,. ,",....."•,',•,i,,i.., missionand the Galileo Europa mission(GEM), two of the .5 3 3.5 4 4.5 5 Wavelength(pro) NIMS detectors(3 and 8) stoppedworking, and the sensitivity of the first two detectorswas considerably reduced. Prior to the Figure 1. NIMS spectrashowing wavelengths obtained dur- first NIMS observationin I24, gratingmotion ceased,probably ing the flybyobservations, compared to NIMS spectrashowing from radiation damage to the electronics.The grating is wavelengthsavailable prior to the gratingbecoming stuck. The stoppedat the short-wavelengthend of the scan.Recovery has horizontalscale gives wavelength and the correspondingNIMS not been successful so far. detectors.(top) NIMS spectrumof Io in reflectedsunlight (solid line) obtainedfrom a globalobservation taken in Sep- Becauseof the stoppedgrating, the observationsduring the tember 1996 (orbit G2). The spectrum(a regionalaverage) Io flybysobtained 13 fixed infrared (IR) wavelengths(in the showsSO2 absorptions and is describedby Carlsonet al. [1997]. rangefrom 1.0 to 4.7/•m) insteadof the planned360 (Figure Vertical lines showthe wavelengthsobtained during 124 and 1). However,this anomalousoperation provided greater sam- 125, after the instrument'sscanning grating malfunctioned. pling frequency(24 samplesinstead of 1) at eachwavelength, Stars showa typical spectrumobtained during 124 with this increasingthe signal/noiseratio substantiallyand mitigating reducednumber of wavelengths(the differencebetween the the problemof radiationspikes in the data. The reducednum- radiancefactors for thesespectra is due to different illumina- ber of wavelengthsis suitablefor temperaturedetermination tion angles).Detector 7 hasnot givena reliablesignal since the and band ratio mapping(for SO2 and the absorptionband in gratingmalfunctioned and henceis not shownin thisspectrum. SO2 mappingwas done usinga ratio of detector12 (3.28/•m) the 1-/•m region), but our search for yet unknown surface to detector15 (4.13/•m) radiances.(bottom) NIMS spectrum compoundswas compromised.SO2 band ratio mappingfor over the Isum hot spot,with the reflectedsunlight contribution specificregions was reportedby Lopes-Gautieret al. [2000a]. removed(method explained by Soderblomet al. [1999]).This Modeling to relate the band ratio mapping from the flyby spectrumwas obtainedfrom the same global observation(in observationsto results obtained from the full NIMS spectral orbit G2) as the spectrumshown above. The reducednumber range is being done by Dout• et al. [2000]. of wavelengthsin 124 (shownby the dotted vertical lines) is The NIMS observations are listed in Table 1. NIMS ob- sufficientfor obtaininga single-temperaturethermal fit to the tained 17 observationsduring I24, 4 observationsduring I25, data. and 10 observationsduring I27. Additional observations,at reduced spatial sampling,were obtainedwhile SSI acquired data. These are listed in Table 1 as "ride-alongobservations." neously.The SSI observationsare describedby Keszthelyiet al. Spatial resolutionranged from 0.5 to 25 km/NIMS pixel for [thisissue]. NIMS data were navigatedonto SSI imageswhen- mostobservations. The gain state(GS) wasset at nominal(GS ever possible,so that correlationscould be made between 2) for daysideobservations, twice the nominalgain (GS 3) for features and colors seen in the visible images and thermal observationsnear the terminator, and 4 timesthe nominal gain output and spectralratio mapsobtained from NIMS data. The (GS 4) for nightsideobservations. Because the nightsideob- NIMS data have not all been analyzedin detail. Here we add servationsof activevolcanic regions during I24 and I25 showed to the preliminary results reported by Lopes-Gautieret al. saturation, GS 2 was used for nightsideobservations during [2000a] and focuson the analysisof data from 127. 127. In the case of Pele, considerable saturation was still present,particularly at wavelengthsin the middleof the NIMS range. 3. SOz Mapping The observationstargeted volcanic regions of diversetypes. For SO2 band ratio mapping,we used the 4.1-/•m spectral Nearly every observationwas collaborativewith SSI, so that channel,which lies within the strong SO2 t,• + t,3 absorption both instrumentsobserved the same target almost simulta- band (Figure 1). We use the radiancevalues at 4.13/•m rela- LOPES ET AL.: IO IN THE NEAR INFRARED 33,055

Table 1. NIMS ObservationsFrom the Galileo Flybysin Orbits 124, 125, and I27

Resolution, Gain km/NIMS Observation Target State pixel Size, km Detections/Comments

NIMS Observations in 124 24INLOKIRA01 Loki, nightside GS 4 1.3-2.1 45 x 18 a Loki "island" and caldera floor/temperaturesmaps showcolor temperatures<350 K 24INHSPELE01 Pele, nightside GS 4 1.0 15 x 1.3 hot spot detectedat (-18.5 ø, 255.6øW) 24INPILLAN01* Pillan, near GS 4 O.5 data obtainedas "ride-along"with SSI, terminator meaningfew samplescollected 24INPILLAN01 Pillan, near GS4 0.5 7x 1 hot spot detectedat (-10.5 ø, 241.8øW) terminator 24INCOLCHS01 ColchisRegio GS 2 0.4 22 x 61b bright white region, no hot region, centeredat spot/problemwith target motion +3.6 ø, 214.1øW, compensation(TMC) led to partial dayside coverage 24INZAMAMA01 Zamama hot spot, GS 2 1.0 88 x 104b missedhot spot at + 18ø, 173øW,but dayside detected thermal emission at +17.9 ø, 170.4øW)/problemwith target motion compensation(TMC) led to partial coverage 24INPROMTH01* Prometheus flow, GS2 data obtainedas "ride-along"with SSI, dayside meaningfew samplescollected/data showthermal emissionalong length of flow 24INPROMTH01 Prometheus,dayside GS 2 1.8 70 x 14 hot spot at -1.7 ø, 155øW T24INCOLCHS02 ColchisRegio GS 2 4.5 92 x 14 no hot spot, little apparentspectral region, near variation Mulungu, dayside 24INNTOHIL01 Tohil (no hot spot GS 2 5.0 112 x 13 red depositsshow enhanced SO2 but red areas) concentration 24INPROMTHS02 Prometheus,dayside GS 2 5.5-8.5 186 x 80 two major hot spotsdetected, thermal emissiondetected along length of lava flow 24INZAMAMA02* Zamama, dayside GS2 data obtainedas "ride-along"with SSI, meaningfew samplescollected 24INZAMAMA02 Zamama, dayside GS 2 9.5 234 x 46 hot spot (flow) detected 24INDORIAN01 golf course,dayside GS 2 11.0 253 x 28 missedarea of bright green deposits due to targetinguncertainty 24INAMSKGI01* Amirani flow, GS2 data obtained as "ride-along"with SSI, dayside meaningfew samplescollected 24INAMSKGI01 Amirani flow, GS2 12.0 306 x 85 three major hot spotsdetected along dayside flow/SO2analysis shows positive correlationwith red deposits 24INTERMAP01 region near GS 3 14.0 319 x 28 a few pixelsshow weak thermal terminator emission 24INREGION01 Prometheusregion GS 2 22.0-25.0 972 x 168 Prometheusfallout ring from SO2 plume detected/thermalemission detectedfrom eight main locations: Prometheus(two adjacentareas), Culann (two adjacentareas), Tupan, Camaxtli,(+14 ø, 150øW),(+22 ø, 146øW),(+15 ø, 139øW),and (+12 ø, 134øW) 24INPPLUME01 Pillan plume, dayside GS 4 32.0 surface and off limb plume not detected/Pillanhot spot detected on limb 24INPELEPM01 Pele plume, dayside GS 4 52.0 surface and off limb plume not detected/thermalemission from Marduk and Reiden detected (first detectionof Reiden sinceG1) 24INREGION02 nearly global GS 2 105 nearly global thermal emission detected from 13 main locations:Culann, Tupan, Malik, Prometheus(two adjacent areas),Zamama, Itzamna, Isum (two adjacentareas), Marduk, Pillan, Pele, Gabija, Ot, and (+ 22ø, 146øW)/Prometheusfallout ring from SO2 plume detected 24INGLOCOL01* global mosaic, GS2 data obtained as "ride-along"with SSI, dayside meaningfew samplescollected 33,056 LOPES ET AL.' IO IN THE NEAR INFRARED

Table 1. (continued)

Gain Resolution, Observation Target State km/NIMS pixel Detections/Comments

NIMS Observations in 125 INGIANTS01 Tvashtar Catena GS 3 9.2 very bright hot spot, fire fountain eruption detectedby SSI and from ground/minimum temperaturefrom NIMS of 1120 K INCULANN0i Culann hot spot, dayside GS 2 1i positivecorrelation between SO2 and red deposits ININTERM01 Gish Bar GS 3 13 thermalemission detected from Gish Bar (two adjacentareas) INREGION01 Prometheusregion, GS 2 i4-25 Prometheusregion; PrometheusSO2 fallout dayside ring detected;thermal emissiondetected at 5 main locations:Prometheus (two adjacent areas),Chaac, (-5 ø, 132øW),(-2 ø, 144øW), and (+9.5 ø, 132øW)/firstdetection of a hot spot at Chaac NIMS Observations in 127 27INHRPELE01 Pele, nightside GS 2 1-1.9 mosaicof Pele hot spot 27INPELE_01* Pele, nightside GS 2 0.85-0.9 data obtained as "ride-along" with SSI, meaning few samplescollected 27INPROMTH01*B Prometheus,dayside GS 2 0.6-0.7 data obtained as "ride-along"with SSI, meaning few samplescollected 27INICHAAC01 Chaac, dayside GS 2 0.9-1.4 short mosaicof Chaac hot spot 27INMOSAIC01 Chaac region, dayside GS 2 1.7-5.3 Chaac,part of the caldera(not hot spot) and nearbycaldera to the eastwhere SO2 abundanceis very high 27INPROMTH0i Prometheus,dayside GS2 6-7.5 mosaicof Prometheusflow and inner SO2 ring 27INTOHIL_01* Tohil, dayside GS 2 8 data obtained as "ride-along"with SSI, meaning few samplescollected 27INPROMTH02* Prometheus,dayside GS2 8.5 data obtained as "ride-along"with SSI, meaning few samplescollected 27INCAMAXT01 Camaxtli, dayside GS2 9-10 NIMS mosaicof Camaxtli hot spot plus SSI ride-alongdata west of Camaxtli, including Chaac; thermal emission detected from Camaxtli(two adjacentareas), Chaac, (+ 14ø, 150øW),(+ 15ø, 139øW),and (+ 12ø, 134øW) 27INAMIRANI01 Amirani flow, dayside GS2 10.5-15 mosaicof Amirani flow and SO2 plume deposit; SO2 fallout ring from plume detected; thermal emissiondetected all along main flow (orientedN-S) but not from westernflow; thermal emission detected from dark caldera southwest of the flow 27INTVASHT01 * B Tvashtar,dayside GS2 15.5 data obtained as "ride-along"with SSI, meaning few samplescollected 27INZALTRM01 * Zal, dayside GS2 16.5 data obtained as "ride-along" with SSI, meaning few samplescollected; thermal emission detectedfrom Zal hot spot;thermal emission also detectedat edge of observationat (+ 36.5ø, 88øW)/positionof this previously unidentifiedhot spot could not be determined accuratelyas not enougharea was coveredby NIMS 27INTVASHT01B Tvashtar GS 2 18.5 NIMS mosaicof easternpart of Tvashtar 27INREGION01 Prometheusregion GS 2 20-29.5 Prometheusregion; Prometheus SO2 fallout ring detected;thermal emissiondetected from 15 main locations:Prometheus (two adjacent areas),Tupan, Camaxtli, Emakong, (+22 ø, 152øW),(+22 ø, 146øW),(+14 ø, 150øW),(0, 164øW),(-5 ø, 132øW),(+9.5 ø, 132øW), (+12 ø, 134øW),(+15 ø, 139øW),(+20 ø, 13løW), and (+ 16.5ø, 124øW) 27INGLOBAL01 part global coverage GS 2 80.5-87.5 strongthermal emissiondetected from Tvashtar;nine other hot spotsdetected but significant"wobble" in observationmakes identificationof hot spotsdifficult, as accuratepositions could not yet be obtained

•Plus data as platform scannedto Pele. bpartialcoverage. LOPES ET AL.: IO IN THE NEAR INFRARED 33,057 tive to the radiancesat a nonabsorbingwavelength (3.28 rim) Figure 4]. However, in the caseof an area east of Prometheus, as a measureof the amount of SO2 absorption.The spectral red depositsare found within and beyondthe SO2-richring, ratio is definedas the ratio [A(3.28)/A(4.13)], whereA (X) is but there is no local enhancementof SO2 in the red-colored the measuredalbedo (bidirectional reflectance) at wavelength deposits.Red depositson Io are thought to be short-chain ,k rim. Pixelsin which significantthermal emissionis present sulfur allotropesSx (x = 2, 3, 4, 6) [Spenceret al., 1997]. (TB > 200 K, see section4) do not showthe absorptionat Their occasionalassociation with SO2 at small scalessuggests 4.13 rim (exceptin the caseof Emakong,discussed in section a depositionor emissionprocess that preventsthe spatialseg- 4.3); hencefor thesepixels the ratio is closeto zero.We did not regation of S and SO2, even though these compoundshave take photometricvariations into account,as the phase angle different volatilities. Two possibilities were suggestedby does not change significantlyduring the acquisitionof the Lopes-Gautieret al. [2000a]. In one case, a relatively high high-resolutionobservations used in the mapping, and com- S2/SO2 ratio can lead to a dense atmosphericpopulation of parisonsare not made betweenobservations at different phase radiatively cooled S2 solid nuclei on which SO2 could con- angles. dense.Another possibilityis that boilingliquid SO2with short- This ratio is a qualitative SO2 indicator, as discussedby chain S or other coloringagents dissolved in it may reach the Lopes-Gautieret al. [2000a],because the spectralratio depends surfacefrom below and freeze before complete sublimation. on the SO2 abundance,the mean grain size,and the presence Radiolysisof SO2 and sulfur may also produce short chain of other materialsand their mixingmode (spatiallysegregated allotropes.The presenceof both SO2 and S2 in a singleplume or intimatelymixed at the scaleof photonpath lengths).Work hasbeen detected by Spenceret al. [2000]at Pele. Spenceret al. is in progressto actually invert both a macroscopicand an suggestedthat simple cocondensationof the two gasescould intimate mixing model using two spectral ratios [A(3.28)/ result in an intimate mixture in which the less abundant but A(4.13)] and [A(3.28)/A(3.56)]. Such analysisleads to darker S2 could dominatethe visiblespectrum. quantitativevalues for SO2 abundanceand mean grain size with reasonableaccuracy [Dout• et al., 2000]. In this paper,we 3.2. Current Analysis assumea spatiallysegregated mixing with spectrallyflat mate- Our current analysisof SO2 distribution uses observations rial, which gives upper bounds to the SO2 abundance.The from I27 of the Amirani and Chaac regions.We alsouse a SSI depth of samplingat 4.13 rim is limited to ---1 mm, so we high-resolutionimage of Culann from I25 to test the correla- cannotdistinguish between thick frost depositsand thin layers tion presentedby Lopes-Gautieret al. [2000a], and we present (that can be transparentin the visibleand at nonabsorbingIR the SO2 distribution map of Tohil from I24 which was not wavelengths). shownin our previouswork. 3.2.1. Amirani. The SO2 ratio map for the Amirani re- 3.1. Background gion (from 27INAMIRANI01) showsthe distributionof SO2 Preliminary resultson the distributionof SO2 at local scales aroundthe flow and caldera(Plate 1). The map showsseveral applyingthis method to data from I24 and I25 were presented notable features.The first is the presenceof an anticorrelation by Lopes-Gautieret al. [2000a]. This earlier analysisused a betweenSO2 spectralratio and the low-albedoflow to the west different wavelengthas the nonabsorbingwavelength (3.00 of the main Amirani flow, from which no significantthermal rim). In the current work, we used 3.28 rim becausewe have emissioncan be detected by NIMS (see section4.2). Low- found it to be less susceptibleto random noise, though the albedo areas on Io are often hot (for example,the Amirani overall resultsdo not changesignificantly. Our earlier maps caldera and the main north-southlava flow) and therefore [Lopes-Gautieret al., 2000a]were obtainedfrom observations often must be excludedfrom our SO2 band ratio maps, as of the Prometheusregion and Culann Patera. The SO2 ratio explainedabove. The low-albedowestern Amirani flow is one map of the Prometheusarea showeda strikingSO2 deposition of the few low-albedoareas imaged by NIMS at high spatial ring circling the present plume vent. When comparedwith resolutionthat does not show significantthermal emission.It low-phasevisible images, this ring appeared larger than its appearsto be an older, cooledlava flow. The flow leadsto the visiblewhite counterpart.However, Geissleret al. [this issue] locationof the Maul plume imagedby Voyager (this location have shownthat visible imagesobtained at low phase angles was not coveredby the NIMS observation).Galileo has not are insensitiveto thin, nonabsorbingfrost deposits.Geissler et detecteda plume at Maui and the plumemay have been driven al. proposedthat areasof Io that appearunexpectedly bright at by the lava flow [McEwen et al., 1998a], perhaps in a way high phaseangles are coveredby fine-grainedSO 2 froststhat similarto the Prometheusplume. The cessationof flow activity are too thin to be seen in visible wavelengthsunder normal may therefore have led to that of the plume. illumination.These frostscorrespond to the areasrich in SO2 Another feature shownby the map is a relatively low SO2 frosts identified from earlier high-resolutionmapping from band ratio over the bright white area that appearsto form a NIMS data [Dout• et al., 2000; Lopes-Gautieret al., 2000a]. ring around the southernpart of the Amirani main (north- Visible imagesof Prometheusobtained at a high phaseangle south) lava flow. This lack of a positive correlation between give shapesand dimensionsof the ring that are consistentwith bright white depositsand SO2 concentrationagrees with our the SO2-richring seen in NIMS data. other resultsfor Prometheusdescribed by Lopes-Gautieret al. NIMS SO2 determinationsand high-phaseSSI measure- [2000a].The white ring-like depositon Amirani is interpreted ments correlate well at local scales and, to a lesser extent, at from the visibleimages as a plume deposit[Keszthelyi et al., this globalscales [Geissler et al., thisissue], but comparisonsof SO2 issue],but it is not clear from the imageswhere the plume vent mapswith SSI-derivedcolor mapsshow only somesmall-scale is. The SO2 ratio map showsthat thiswhite plume depositdoes correlations.For example, the SO2 ratio maps of Culann Pa- not correspondto the main area of SO2 depositionbut shows tera and Tohil Patera showedlocally enhancedconcentrations that SO2 is more abundantin areasfarther out from the plume of SO2 that coincidedwith depositshaving a pink to red ap- vent and may form a secondring. The NIMS observationdoes pearance at visible wavelengths[Lopes-Gautier et al., 2000a, not cover a sufficientlylarge area to be able to confirm the 33 058 LOPES ET AL.: IO IN THE NEAR INFRARED

0.0

-4.5

Plate 1. Amirani lava flow imagedby NIMS during I24 and I27. (left) A NIMS band ratio map showing relativeSO2 concentration, made from the I27 data.Higher values of the ratio (up to 4.5 on the sidebarscale) representenhanced concentration of SO2. (right) Image obtainedby SSI during orbit C21, with the area coveredby NIMS delineated.The NIMS band ratio map showsthat deeperband depthsof SO2 (bright) do not coincidewith the visiblewhite ring in the SSI image. The dark pixels correspondingto the area of the activeflow (trendingnorth-south) contain significant thermal emission; thus the valueof the bandratio is close to zero and thesepixels appear dark. SO2 is depleted(dark) over the flow that runstoward the west,but this flow is not activeat present.A generalgradient for the band ratio can be seenfrom southto north, with SO2 being enhanced toward the greenish terrain to the north. This may indicate possiblegas transport from Amirani to higher latitudes.SO2 appearsless abundant on the yellow terrain southand east of the flow and on a patch to the northwestof the flow. It is also possiblethat the yellow terrain may representa deposit containingnon-SO 2 compoundsthat is overlyingthe greenishmaterials. presenceof a second,outward ring that is richer in SO2, aswas as discussedin the thermal analysissection below and by Kesz- observedfor Prometheusfrom 124 data [Lopes-Gautieret al., thelyiet al. [this issue]and Davieset al. [2000a].Furthermore, 2000a].The visiblewhite ring at Amirani may containanother Prometheusand Amirani are sitesof long-livedplumes that may volatile such as sulfur or, possibly,anhydrous salts such as be causedby the interactionof hot lavawith SO2 frost [Kiefferet suggestedby Fanale et al. [1974]. However, if sulfur is present al., 2000;Milazzo et al., this issue].The NIMS data indicatethat in the white-coloreddeposits, the sulfur allotropesare likely to plumesat both sitesmay be producingcompounds other than be longer chain (possiblyS8) than those responsiblefor the SO2,such as sulfurmolecules, that condensecloser to the plume orange/reddeposits such as present around Pele [Spenceret al., vent than SO2 and form the white deposits.However, the plume 2000]. depositat Prometheusis considerablymore sharplydefined in The eruptivestyles at Amirani and Prometheusmay be similar, both the visibleand infraredwavelengths. LOPES ET AL.: IO IN THE NEAR INFRARED 33 059

Plate 2. (left) Culannhot spotas observedby NIMS and SSI duringI25 and (right) Tohil Patera,which is not an activehot spot,observed by NIMS during I24. In both observations,NIMS obtaineda narrow strip of data from which a band ratio map showingrelative SO2 concentrationwas obtained.The backgroundimages are from SSI, showingvisible colors, with the area coveredby the NIMS data delineated.In the caseof Tohil the SSI imageis from orbit C21. For Culann,high-resolution imaging is availablefrom orbit I25 (the northern part is a lower-resolutionimage from orbit C21). The NIMS band ratio mapsare shownto the right of each image.Enhanced concentrations of SO2(bright) are seenover areasthat are pink to red in visiblewavelengths for bothTohil and Culann.The relativeconcentration of SO2is low (dark) near the Culanncaldera, which has a greenishcolor. The spatialresolution of NIMS is 11 km/pixelfor Culann and 5 km/pixelfor Tohil.

Another significantdifference between the SO2 distribution vent as the lava flow, are causedby sublimationof an SO2 around Prometheusand Amirani is that the Amirani region is snowfieldas proposedfor Prometheus,or are a combinationof richer in SO2.Using a linear-mixingmodel, Dout• et al. [200lb, both mechanisms.A north-southgradient for the spectralratio also Dynamicsand evolutionof SO2 gas condensationaround can alsobe noticed,indicating possible SO2 gastransport from Prometheus-likeplumes on Io as seen by the Near-Infrared Amirani to higher latitudes.These trendsare compatiblewith Mapping Spectrometer,submitted to Journal of Geophysical the conclusionsof Dout• et al. [2001a]. Research,2001] find areasaround Amirani havingsurface cov- 3.2.2. Culann. This current analysisof the I25 Culann erage of 80-100%, whereasmaximum values of 80-90% are data usesthe high-resolutionimage of Culann taken by SSI in observedaround Prometheus for a lessextensive area (south- that orbit [McEwenet al., 2000]. This higher-resolutionimage east of the ring). In addition, in the areas surroundingthe allowsus to correlatein greater detail the SO2 ratio map with Amirani dark lava flows,the meanvalue of the spectralratio is surfacecolors than was done previouslyby Lopes-Gautieret al. higher than the level generally observedover the equatorial [2000a]. For comparison,we showthe Tohil Patera SO2 map plains of Io as reported by Dout• et al. [2001a]. This implies discussedby Lopes-Gautieret al. [2000], which showsan en- that Amirani is an important sourceof SO2 throughdegassing hancementof SO2 coincidingwith reddishareas. The SSI Tohil or sublimation.This activityis probablylinked with the exis- image used is from the C21 orbit. Registration between the tenceof one or more volcanicplumes that erupt from the same NIMS and SSI imageswas done by resamplingthe SSI image 33,060 LOPES ET AL.' IO IN THE NEAR INFRARED

B A

C D

lOO lO

135 . 0

Plate 3. Chaacregion as observed by SSI andNIMS duringthe 127flyby. (a) SSI high-resolutionimage from I27. (b) SSI lower-resolution,color image from the C21 orbit,showing dark greenmaterials inside the Chaaccaldera. (c) NIMS band ratio map (1.31/1.03/xm)showing the anticorrelationbetween the depthof the 1-/xmabsorption and the greendeposit in the interiorof the caldera.Note that thisratio map is not correlatedto albedo,as the ratio showslittle variation outside the Chaaccaldera. (d) NIMS bandratio mapshowing the relativeabundance of SO2. The brightwhite material inside the smallcaldera just to theeast of Chaac(lower right in thecamera image) is filled by SO2 that is purer than at anyother locationso far observedby NIMS on Io. It may representa frozenlayer of SO2 ice on the floor of the caldera.The width of the NIMS imageis ---100km.

down to NIMS resolution and comparing albedo profiles to used the SSI filter image at 0.664/xm, and for Tohil we used obtain the best match between the two data sets. This was done the SSI image at 0.756/xm, as imagesin the filter at 0.968/xm, usingthe shortestavailable NIMS wavelength(1.03 /xm) and normallyused in the coregistering,were not availablefor ei- the SSI filter image closestto this wavelength.For Culann we ther case.The coregisteredimages are shownin Plate 2. The LOPES ET AL.: IO IN THE NEAR INFRARED 33,061

NIMS data show enhancedconcentrations of SO2 in the red- wavelengthsif the frost is thin (as discussedabove and by colored depositsto the south and north of Culann, as found Geissleret al. [this issue]). previouslyby comparisonwith the SSI data from C21 [Lopes- The frequency of occurrencefor depositssimilar to this Gautieret al., 2000a]. Our new result also showsthat the SO2 cannot be estimated from the current Io data set, because the ratio is at a local minimum in the regionsthat have a green sizeof thefeature, •--100 km 2, is below the spatial resolution of coloration in the SSI images.These greenishmaterials were the availableregional spectralratio maps of Io. The observa- initially definedby their negativenear-infrared spectral slopes tion from which this feature was detected(27INMOSAIC01) in SSI data [Geissleret al., 1999]. Their reflectancereaches a has a spatialresolution ranging from 1.7 to 5.3 km/pixel. This maximumin the SSI greenor red filter and then monotonically feature is present but cannot be detected as a bright SO2 decreaseswith wavelengthto 1/•m, unlike other Ionian mate- depositin mapsmade from NIMS observationsat spatialres- rials that haveneutral or positivenear-infrared slopes over this olutionsof 9-10 km/pixel (27INCAMAXTLI) and 20-30 km/ wavelengthinterval. Green materialsare also observedin the pixel (25INREGION01 and 27INREGION01). interior of the Chaac caldera and are discussed further in The anticorrelationbetween green depositsand local SO2 section 3.2.3. enhancement observed at Culann is also observed at Chaac 3.2.3. Chaac. This active caldera has a distinctivelygreen (Plate 3). The local SO2 minima in both areascould be due to depositcovering its floor [Geissleret al., 1999]. Chaacwas de- proximity to warm, active regions or to differencesin origin tectedas a hot spotby NIMS in 125 (25INREGION01). Both between the red (possiblymixed pyroclasticsfrom the gas NIMS and SSI observedChaac at high spatialresolution during phase) and the green (possiblyemplaced lavas) materials. 127 (Plate 3). The NIMS observation(27INMOSAIC01) only High-resolutionimages from SSI [McEwenet al., 2000] show coveredpart of the caldera. Neither NIMS nor SSI observa- that the greenishcoloration is restrictedto calderasand lava tions covered the hot spot that, accordingto the NIMS 125 flows, suggestingthat they may be formed through alteration data, is located in the southwesternpart of the caldera.Kesz- or contaminationof sulfur depositsby the hot lavas. thelyiet al. [this issue]analyzed the SSI data for Chaac and It is noticeablethat the broad 1-/•m absorptionidentified in suggestedthat low areasof the calderahave been floodedby a NIMS data by Carlsonet al. [1997] is anticorrelatedwith the bright fluid, similar to what they proposedhappened at Tohil greenmaterial inside Chaac (Plate 3) [alsoLopes-Gautier et al., 2000b]. This suggeststhat the areas having a negative near- Patera. They suggestedthat nonsilicatefluids are an integral infrared slope (from red to 1 /•m [Geissleret al., 1999]) also part of Chaac Patera. have a negativeslope from 1 to 1.5/•m and hencedo not show The NIMS observationdid not cover the southwesternpart the broad absorptioncentered at around 1 /•m. This is not of the Chaac caldera that is bright in visiblewavelengths, dis- characteristicof pure sulfur or SO2 compounds,but one pos- cussedby Keszthelyiet al. [this issue]. However, the NIMS sible compositionalcandidate is sulfur contaminatedby iron, observationcovers a patch of bright material inside a smaller suchas pyrite, as suggestedby Kargelet al. [1999]. Other pos- calderato the east of Chaac (Plate 3), which in visiblewave- sibilitiesinclude sulfur radiolysisproducts and sulfur contam- lengthsappears similar to the bright material on the edge of inated by trace elements [Carlsonet al., 2001; Dout• et al., Chaac describedby Keszthelyiet al. The NIMS data alsocover 2001b]. the low-albedo interior of the Chaac caldera and the surround- ing plains. The SO2 band ratio map showsthat little SO2 is present inside the Chaac caldera. This may be due to the 4. Thermal Mapping currentvolcanic activity just to the southof the area observed NIMS data from the flybys allow us to map the thermal sublimatingany SO2 deposits,or it could be a further indica- structurewithin hot spotsfor the first time. This couldnot be tion of the relatively low SO2 concentrationwithin green- done from any previouslyacquired data becausethe sizesof colored areas, as observed in Culann. As with the Amirani the pixels were so large that they contained a mix of many nonactiveflow describedabove, recent activity in this area (or different typesof terrain of varyingalbedos and temperatures nearby)could have causedSO2 depositsto sublimate. and also severaldifferent hot spots.Therefore previouswork The band ratio map's most noteworthy result is the ex- focusedon thermal analysisof a limited set of observations tremely high concentrationof SO2 (a surfaceproportion of obtainedduring nighttime in whichthere wasno reflectedlight 100% if a macroscopicmixing mode is retained, a grain pro- component [e.g., Lopes-Gautieret al., 1997; Davies et al., portion higher than 90% in the case of an intimate mixing) 2000a]. The large sizesof the NIMS pixels(at least 120 km x inside the small caldera to the east of Chaac. The high SO2 120 km) of the distant observationsmade it impossibleto concentrationin the interior of this small caldera implies a observethe albedoassociated with individualhot spots.During mechanismof depositionin which SO2 was topographically the closeflybys, the muchsmaller pixel sizesthat fall within the confinedby the calderawalls. This is the purest SO2 deposit hot spotsallowed us to model the albedo and obtain temper- that NIMS has detectedon Io and the only one that appearsto atures of hot spots seen under illuminated conditions.The be topographicallyconfined. Smythe et al. [2000] suggested albedois estimatedas a functionof the shorterwavelengths in emplacementby liquid flow of SO2 rising from lower layers. the NIMS range where the radiancefrom thermal emissionis While liquid would normally boil off when exposedto Io's negligiblecompared with the radiancefrom reflectedsunlight. tenuous atmosphere,preliminary estimatesby Smytheet al. At wavelengthsbetween 3 and 5 /•m the contribution from [2000] indicatethat givensufficiently large quantities,some of reflectedsunlight is no greaterthan 20% of the thermal signal, the liquid couldfreeze to form a layer of sulfurdioxide ice (at so slight errors in albedo estimateshave small effect on the least a few millimetersthick) insidethe caldera.If this mech- derivation of temperature. anismhas taken place,there may be a halo around the caldera A NIMS pixel showingvolcanic activity most likely contains where SO2 that boiled off has recondensed.Neither NIMS nor lavas at a range of different temperatures.Techniques for SSI data showa halo, thoughit couldbe transparentin visible derivingtemperatures from NIMS observations,and some of 33,062 LOPES ET AL.: IO IN THE NEAR INFRARED the associatedconstraints, were discussedpreviously [Lopes- shorterwavelengths in the NIMS range (1-2 tzm) and applied Gautier et al., 1997; Davies et al., 1997]. In the presenceof as a correctionto the longerwavelengths (beyond 2.4 tzm). sunlightit is not possibleto use two-temperaturefits to the Pixels where Tn < 200 K are omitted, as otherwisethe Planck function in order to derive the "hot component"be- signal/noisein the thermal componentis too low to yield a causethe signal at the shorter wavelengthsis dominatedby reliablesolution to (2) and alsothe strongSO2 absorptionat reflectedsunlight. Therefore, for the daytimeobservations ob- 4.1 tzm affects the shape of the spectrum.The strongSO2 tained during the flybys,fitting more than one temperatureis absorptiondisappears in hot regions(except in the caseof not feasible.A singletemperature can thus be regardedas a Emakong, describedin section4.3). Correct albedo values lower estimateof the temperature range presentin the pixel. shouldyield I(X) - 0 for the cold areasof Io. Smallresidual In the caseof the Zamama hot spot observedduring Galileo's uncertaintiesin the NIMS calibrationcan sometimesyield a first orbit, a single-temperaturefit gavea resultthat waswithin spuriousTn --• 180 K for theseareas, but thesepixels were not 70 K of the coolercomponent of the two-temperaturefit [Da- used in the analysis. vieset al., 1997].It mustbe noted that the physicalenvironment 4.1. Hot Spots in the Prometheus Region of a hot spot is more complex than can be describedby a two-componentmodel, althoughsuch a model hasbeen shown Hot spotsand plumeshave been detectedand monitoredby to give a fairly good fit to the spectraof someterrestrial lavas NIMS and SSI from distantobservations prior to the Io flybys. [Crispand Baloga, 1990;Flynn and Mouginis-Mark,1992]. Table 2 (updatedfrom Lopes-Gautieret al. [1999]) lists the We present our resultsin terms of two types of tempera- activevolcanic regions known on Io from Galileo, Voyager, tures, the brightnesstemperature T B and the color tempera- ground-based,and Hubble SpaceTelescope (HST) observa- ture T c. The brightnesstemperature is just a convenientrep- tions.We now know of 97 major activevolcanic regions on Io resentationof the emitted thermal radiation measuredby the and 27 locationsthat are possiblyactive volcanic centers but instrument in a form that is more intuitive than when ex- await confirmation(Table 3). The flybyobservations revealed 16 additionalhot spotsnot previouslyknown. These couldbe pressedin radiation units. The brightnesstemperature can be sitesof new volcanicactivity but giventhat most are faint hot used to distinguishbetween measurementsof sky, Io's back- spots,it is more likely that theywere not previouslydetected in ground, and hot spotson the surface,but it providesan accu- lower spatialresolution observations. It is noteworthythat 14 rate temperature measurementonly when the pixel is com- of thesenew detectionscame from the NIMS regionobserva- pletely filled by this singletemperature blackbody. The color tions(Table 1 and Plate4) that togethercover an area around temperature(To), however,represents the physicaltempera- Prometheusequivalent to ---5% of Io's surface,at spatialres- ture of a blackbodythat is assumedto model one component olutions---10 times higher than the distantglobal observations. of a hot spot. The defining equationsare Prior to the flyby observations, only four hot spots B(X, TB): I(X) (1) (Prometheus,Camaxtli, Tupan, and Culann) had been re- solved in this area. 8(x, = (2) It is possibleto use the distributionof hot spotsin this area to estimatethe total numberof hot spotson Io by inferringthe where B is the Planck function, X is the wavelength,I is the overallglobal distribution of volcaniccenters. Carr et al. [1998] correctedradiance, and f is the fractionalpixel area, all for a mapped active and inactive volcanic centers and concluded given spatial pixel. Note that TB is a function of wavelength, that the distribution is uniform at mid and low latitudes, but but T c and f are, for a given pixel, constantover the whole there may be fewer hot spots at high latitudes. The same spectrum.We assumesurface emissivity to be 1. If emissivityis conclusionwas reachedby Lopes-Gautieret al. [1999] on the <1, then our brightnesstemperatures will be underestimates. basisof the distributionof known active hot spots.If we as- For daytime observationsthe correctedradiance is defined sumethe global distributionof hot spotsto be uniform (in- as I(X) = l{l(X) - AF(X), where Io(X ) is the observed cludingthe polar regions),we can expectIo's surfaceto have radiance,F(X) is incidentsolar flux at that wavelengthand A around 300 hot spotsof brightnesssimilar to thosethat were is an albedo(bidirectional reflectance) estimate derived from detectedfrom the NIMS regionobservations. However, there the 1-2 •m region. For nighttime observations,I - I o. are suggestionsof a concentrationof active and inactivevol- Equation(1) is a straightforwardcalculation, but (2) mustbe caniccenters near the sub-Jovianand the anti-Jovianregions solvedas a fit to a set of points. For the latter, we use as cost [e.g.,Schenck and Hargitai,1998; Radebaugh et al., this issue]. function the sum of the squaresof the differencesbetween Prometheusis locatednear the anti-Jovianpoint. The paucity computed and observedspectra divided by the estimateder- of high-resolutiondata on the sub-Jovianhemisphere, plus rors ("chi-square").The fit is done usingthe method of "sim- uncertaintyin the distributionof hot spotsin the polar regions, ulated anncaling"described by Metropoliset al. [1953], a good doesnot allowus to quantifyhow muchricher in hot spotsthe descriptionof whichis givenby Presset al. [1986].This is not a Prometheusregion is relativeto the rest of Io's surface.How- deterministicalgorithm but allowsstochastic variations within ever, we can use the work of Radebaughet al. [this issue]to a graduallydecreasing range and is particularlywell-suited for make an estimate that the Prometheusregion has, at most, highly nonlinear problemswith many local minima. This was twice the concentration of active volcanic centers as other applied to cach pixcl for which at least four wavelengthswere regionson Io. In thiscase, we can expecta minimumof 150hot observedin the range 2.5-5 •m (which containsup to eight spotson Io. Therefore we now know the locationsof at least NIMS wavelengths).In this range,thermal emissiondominates one third, and possiblyup to two thirds, of the expectedhot reflectedsunlight for temperaturesgreater than --•300K. spotson Io. In daysideobservations T B and, in particular, T c are sensi- NIMS observationsfrom the flybys(at resolutionsfrom 1 to tive to the albedo used to correct the observed radiances, both 20 km/pixel) show that some individual hot spots,such as in amplitude and slope. The albedo is computed from the Prometheusand Amirani, have more complexthermal struc- LOPES ET AL.: IO IN THE NEAR INFRARED 33,063

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Table 3. Identificationsof PossiblyActive Volcanic Centersa

Location of Volcanic Candidate Ground-Observed Surface Center SurfaceFeature SSI Hot Spot? Hot Spot? Plume? Change? Notes

Unnamed 15.3øN, 4.7øW 9606D? no no faint bright spot in SSI G8 eclipseimage Unnamed 2øS, 13øW no no faint bright spot in SSI G8 eclipseimage Unnamed 11.5øN, 13.3øW no no Unnamed 5.2øN, 24.1øW no no faint bright spot in SSI G8 eclipseimage Unnamed 6øS, 19øW no no faint bright spot in SSI G8 eclipseimage Unnamed IøN, 21øW no no faint bright spot in SSI G8 eclipseimage Unnamed løS, 23øW no no faint bright spot in SSI G8 eclipseimage Unnamed 9øS, 27øW no no faint bright spot in SSI G8 eclipseimage Unnamed 13.1øS,22.8øW 9606C? no no faint bright spot in SSI G8 eclipseimage Unnamed 16.5øS,27.9øW 9606C? no no faint bright spot in SSI G8 eclipseimage Lei-zib 10øN,45øW no no yes Unnamed no NICMOS 14 no no faint NICMOS spot at 54.4ø _+5øN, 60.1øW (NICMOS) (error in latitude up to -18 ø) "Poliahu" 19.4øS,81.8øW no yes no no reported at 22ø + 5øS,79 ø _+5øW by Goguenet al. [1988] Ekhib 26øS,88øW no no no Shangob 32øN,100øW no no faint bright spot in SSI--eclipse image Catha Patera 53øS, 100øW no no yes Unnamed 42øN, 134øW no no yes reddish materials Unnamed 31øN, 146øW no no yes large region of red deposits,coordinates are of center Unnamed 22øS, 168øW 22øS, 168øW no no SSI faint spot in Ell Namarrkunb 11øN,175øW no no no Donarb 24øN,185.5øW no no no Unnamed 28.5øN, 189.4øW no no Unnamed 38øS, 291øW no no no low albedoand bright red materials Huo Shen 15øS,330øW no no no HST changes[Spencer et al., 1997] not Patera confirmedby SSI Unnamed 17.1øN, 332.9øW no no Unnamed 2øS, 352øW no no faint bright spot in SSI G8 eclipseimage; possiblysame as hot spot detectedby C. Dumas on June 3, 1999 Unnamed 4.8øN, 356.1øW no no

aUpdatedfrom Lopes-Gautieret al. [1999]. bMajorchanges from Lopes-Gautier et al. [1999].

tures than could be detected at global scales.Thermal maps where S is the slant distance and FOV is the field of view of the from the flyby observations,discussed below, show different NIMS pixel (in the caseof a NIMS data cube) or the field of stylesof eruptionfor different hot spots.Analysis of the three view of the NIMS detector(in the caseof a NIMS data tube); region observationsalso showsthat significantvariations in and E is the emissionangle; cos (E) is appliedto correctto level or activityoccur at shorttimescales. Table 4 is a compar- vertical perspective. ison of hot spotsin all three region observationsand it states The backgroundis the medianof the radiancesfrom a 20 x whethereach hot spotwas observed and whetheror not it was 20 pixel area, excludingthe hot spot pixels themselves.We detected. Color temperatures (To) were obtained by the assumethat the albedo at 4.7 •m is constant between the method explained above for all the caseswhen there were backgroundand the hot spots.If the hot spotsare darker at 4.7 sufficientdata points (less than four wavelengths)and the •m than they are in the visible,then the powerreported will be error returnedby the Metropolisfitting algorithm(see above) an underestimate. However, the differences will be constant was <30%. In a few cases,an approximatecolor temperature betweenobservations for a givenhot spot. is givenbased on fewer data points (three wavelengths),and The calculationwas done using one of two methods,de- errors can be as high as 50%. We also report the power at 4.7 pendingon whether the hot spotwas consideredan extended •m. This wavelengthwas chosen for two reasons.First, it is the source(over severalNIMS pixels)or a subpixelsource, one longestwavelength available in the flyby data set and has the using resampleddata (NIMS "cubes") and the other using lowest signalfrom the solar flux in daytime observations.Sec- originalunresampled data (NIMS "tubes").For an extended ond, to facilitatecomparisons with ground-basedobservations source,NIMS data cubeswere used and (RA) was summed [e.g., Veederet al., 1994]. The power (emitted into a hemi- over all pixelsin the vicinityof the hot spot havingR over a sphere)was derivedusing threshold of twice the backgroundvalue. In the case of Prometheus,because of the largeextent of the hot area (Plate P(4.7 •m): (RA)/cos (E), 5), three valueswere obtained:west, middle, and east. whereP(4.7 •m) ispower at 4.7 •m (in unitsof W •m-•); R In the caseof a subpixelsource the NIMS projectionalgo- is observedradiance (W m-2 sr-• •m-•), correctedfor back- rithm used to make data cubes can introduce errors in the groundradiation; A is emittingarea (m 2) equalto S*FOV, integratedradiance because of the nonuniformityof the NIMS LOPES ET AL.. IO IN THE NEAR INFRARED 33 067

c D

Plate 4. Westernpart of BosphorusRegio and the Prometheusregion as observedby NIMS during(b) I24, (c) 125,and (d) 127.These region observations had spatial resolutions of approximately25 km/NIMS pixel.(a) A SSI imageof the regioncovered by NIMS. The NIMS observationsare shownhere as 4.8-p•mmaps, and the hot spotsare shownas yellow to red to white areas.See Table 1 for observationdetails and Table 4 for hot spot temperatureand power.

detectorresponse function [Carlson et al., 1992].Therefore, in The power at 4.7 p•mwas computedin order to searchfor thesecases, original NIMS data tubeswere used (an array of variationsin power output betweenhot spots.As can be seen unresampledoriginal NIMS radiances).Adjacent mirror steps from Table 4, the power at 4.7 p•m for Tupan, Culann, and in the tubes were co-added to take into account the NIMS Prometheusissignificantly larger (>45 x 108W m-2) thanthe responsefunction, and the backgroundvalue was adjusted valuesfor mostof the hot spotsthat were detectedfor the first accordingly.For each hot spot a histogramwas formed of all time from the high-resolutiondata (mostyielded power <10 x the tube pixelsshowing a contributionfrom the hot spot(usu- 108W m-2). ally 10-20 pixelsbecause of the slow slew rates during these Significantvariations in power at 4.7 •m between orbits observations).The peak of the histogramwas taken as the were observedfor hot spots Camaxtli "c" (-40% decrease radiance(R) of the hot spot and usedin the aboveequation. from 125to 127), Camaxtli"west" (-75% decreasefrom I24 to The data tube method was also used to double-check the 127), and Tupan (-30% increasefrom 124 to 127). We can power calculationsfor someextended sources. In thesecases, qualitativelysuggest that changesin power output may have the power calculatedusing data tubeswas in substantialagree- happened at hot spotsI25A, Chaac, and I27D, as these hot ment with that obtained from cubes. spotswere observedat similarspatial resolution in more than 33,068 LOPES ET AL.: IO IN THE NEAR INFRARED one orbit but not detectedin all orbits.We giveupper limits for The temperatureprofile of the Prometheusflow is consistent the power for these hot spotsbased on the detectableupper with that of an insulatedlava flow field. Hot material is erupt- limit of radiance in the NIMS tubes pixels. ing from a vent in the eastern end of the flow and may be movingthrough lava tubes,keeping the most of the surfaceat 4.2. Lava Flow Activity: Prometheus and Amirani fairly constant,lower temperaturesthan at the vent area. At Prometheusand Amirani are two persistenthot spotson Io the westernpart of the flow, closeto where the presentplume that were observedto be active by Voyager and repeatedlyby is erupting,the increasedtemperatures are consistentwith lava Galileo [Lopes-Gautieret al., 1999]. One differencein their beingexposed and spreadingout asit emergesfrom tubes.This historyis that the Prometheusflow has increasedin length by is not a unique interpretation, but it is consistentwith the some 80 km between the Voyager fiybysin 1979 and the first interpretation from SSI images, including those from I27 Galileo observationsin 1996 [McEwen et al., 1998a]. The [Keszthelyiet al., thisissue]. SSI analysisof flow imagessuggests Amirani-Maui flow field is -300 km in lengthand includesthe that other than a small activebreakout midway down the flow longestactive lava flows known in the solar system[Keszthelyi field, no hot lava is exposeduntil it reachesthe westernend of et al., this issue]. McEwen et al. [2000] interpreted the the flow field,where it breaksout. In termsof composition,our Prometheusflow as a long-lived,tube-fed, compoundpahoe- results(temperatures up to 910 _+40 K) only providea lower hoe flow field. Keszthelyiet al. [this issue]proposed that the limit for the eruptiontemperature, which is likely to be several Amirani-Maui region is a complex,compound flow field. Here hundred degreeshotter. The lower limit that we obtained is we use thermal data from NIMS to map these two flows and consistentwith a wide range of liquidustemperatures, includ- obtain cluesto their emplacementstyles. ing those in the ultramafic and basalticranges. 4.2.1. Prometheus. One of the most interesting results Comparisonof the resultsabove with thosefrom the NIMS from the high-resolutionobservations was the movement of regional observation(27INREGION01) discussedin section the Prometheusplume. Observationsby SSI during Galileo's 4.1 illustrate well how higher spatial resolutionobservations first orbit in 1996 showedthat the Prometheusplume site had are better capableof detectinghigher temperatures. Typically, moved -80 km west since1979 [McEwenet al., 1998a],but the the highesttemperatures in a flow field are found in very small size and appearanceof the displacedplume had not changed. areas relative to the total size of the flow field. NIMS data from Observationsby SSI in I24 showeda calderajust to the north the regionalobservation (at spatialresolution of-25 km pix- of the Voyagerplume site [McEwenet al., 2000] and a long lava el-x) couldnot resolve the highest temperatures (910 _+ 40 K) flow betweenthe Voyager and Galileo plume sites.The NIMS measuredfrom the higher resolutionobservation (at 7.5 km observationin I24 of the same region [Lopes-Gautieret al., pixel-•).The regional observation was taken -1 hourafter the 2000a] showedtwo main hot spots,one in the eastand another high-resolutionPrometheus observation and showedtop tem- in the westernpart of the flow, thoughthermal emissioncan be peratures(allowing for errors)being under 735 K. Prometheus detectedalong the whole flow. The easternhot spotis near the showedlittle changein temperature between high-resolution locationof the 1979plume, while the westernhot spotis at the observationstaken monthsapart in I24 and I27 (Plate 5), and location of the current plume. SSI and NIMS data analysis its activityappears to be at a steadylevel. The changein peak [McEwenet al., 2000; Lopes-Gautieret al., 2000a] showedthat temperaturemeasured between the high- and low-resolution the likely vent area for the lava flow is locatedjust southof the observationsis therefore causedby changein resolutionrather Prometheuscaldera, in the region where NIMS detects the than changesin lava temperatures. highesttemperatures. The conclusionbased on the NIMS and 4.2.2. Amirani. The Amirani-Maui region is character- SSI I24 data was that the vent associated with the eastern hot ized by a longlava flow that runsnorth-south and a secondflow spot did not move. Instead, the plume moved to the western that runs from the main flow west towardsMaui (Plate 6). hot spot, even though the eastern hot spot remained active. Voyager 1 data showedAmirani and Maui together to be a Kiefferet al. [2000] modeledthe plume'smovement in termsof prominent thermal anomaly [Pearl and Sinton, 1982]. Both the interactionbetween hot lava and the underlyingSO2 snow- NIMS and SSI have detected thermal emission from Amirani field. during the main Galileo mission, and NIMS detected a hot Prometheus was observed by NIMS again in I27 spot near the locationof Maui [McEwenet al., 1998a;Lopes- (27INPROMTH01) at spatial resolutionunder 7.5 km pix- Gautier et al., 1997, 1999]. SSI data from the main Galileo el-•. A temperaturemap was obtained from the NIMS data, mission showed surface changesin the dark flows between and this was matched to the SSI observation of the same Voyager and Galileo observations[McEwen et al., 1998a]. area (Plate 5). The temperature map showsthat the highest During I24 and I27, thermal emissionwas detected by NIMS temperature that could be reliably determined (based on alongthe north-southflow and from a calderato the southwest four wavelengths)is 910 _+40 K. This is at the eastern end of the flow that may be the sourceof the lava. NIMS 127 data of the flow, near the proposedvent area. Neighboring pixels showed thermal emission from a small caldera to the northwest in the same region may be hotter but have too many satu- of the flows (I27E, Table 2) that appearsto be unrelatedto rated wavelengthsfor a reliable temperature determination Amirani and had not previouslybeen detectedas a hot spot. to be made. A temperature profile along the flow (Plate 5) No thermal emission was detected from the flow that runs from shows lower temperatures away from the proposed vent the main Amirani flow to the west toward Maui, suggesting area. Temperatures along the middle of the flow are fairly that activityhas halted in that region.This flow appearsto be constant, ranging from 240 K to 460 K. Temperatures rise fed from the samevent as the main Amirani flow. Keszthelyiet again at the western end of the flow up to 560 _+70 K near al. [thisissue] suggests that Maui Patera (Plate 6) is unrelated the site of the present plume. A profile from I24 data (Plate to the flows.The NIMS hot spot Maui (Table 2) reportedby 5) showsthe same general pattern and temperatureswithin Lopes-Gautieret al. [1997, 1999] may be located at Maui Pa- the error bars of the I27 values,indicating that at least in the tera, however;the spatial resolutionof the distantNIMS ob- scale of months, the lava flow activity remained steady. servations does not allow us to locate the source of the thermal LOPES ET AL.' IO IN THE NEAR INFRARED 33,069

Table 4. Hot SpotsDetected in NIMS RegionObservations During I24, I25, andI27 Name Location 24INREGION01 25INREGION01 27INREGION01

Emakong 3ø ___løS, not observed detected detected 119ø _ løW Tc • 300 K T c • 300 K P(4.7 •m) = (0.2•0.1) P(4.7 rim): (0.4r10.2)x 108 x l0 s W rim-1 W •m -1 I27C 16.5ø +_iøN, not observed not observed detected 124 ø __+løW P(4.7 •m) = (2.3 _+0.2) X 108 W •m -1 I25B 5 _+løS, not observed detected detected "Emakong 132ø+løW Tc = 511 _ 24 K Tc = 514 _+ 20 K flow"(Seth?) P(4.7 rim) = (24.1+ 2.4) P(4.7 •m) = (28.9 _+3.0) x 108W •m -1 x 108W rim-• I27B 20ø _+iøN, not observed not observed detected 131 ø _+ iøW Tc = 531 ___33 K P(4.7 rim) = (9.8 -+ 1.0) x 108W •m- 1 Camaxtli"main" 14.5ø _+iøN, detected not observed detected 135ø _ løW P(4.7 •m) = P(4.7 •m) = (1.9 -+ 0.2) (2.3_ 0.2)x l0s W •m -1 X 108W •m -1 Camaxtli "c" 9.5ø +_iøN, not observed detected detected 132ø_løw Tc = 420_+ 31K Tc = 380-+ 114 K P(4.7 •m) - (4.9 + 0.5) P(4.7 •m) = (2.8 __0.3) x l0 s W rim-• X 108W •m -1 Camaxtli"east" 12ø _+iøN, detected not observed detected 134ø-+ løW Tc = 467 _+ 115 K P(4.7 •m) = (2.0 -+ 0.2) x l0 s W rim-• P(4.7 rim)= (2.3+_ 0.2) x 10SWrim-1 Camaxtli"west" 15ø + iøN, detected not observed detected 139ø__+ løW T c = 415 _+ 24K Tc • 500 K P(4.7 •m) = (1.5 _ 0.2) P(4.7 rim): (0.36 _+0.13) x 10SW •m -1 x l0 s W •m -1 Tupan 18ø _+ løS, detected notobserved detected 141ø-+ løW Tc = 548 + 80 K Tc = 600-+ 61 K P(4.7 rim) = (45.3 -+ 4.5) P(4.7 •m) = (64 -+ 6.4) x l0 s W rim-1 x 108W rim-1 I25A 2ø -+ løS, not detected detected not detected 144ø _+løW P(4.7 •m) < 0.3 P(4.7 •m) = (0.5 -+0.4) P(4.7 rim) < 0.3 x 10sw•m-1 x 10sw•m- • x 10s W rim-1 I24A (nearSurya) 22ø -+ IøN, detected(on edgeof notobserved detected observation) 146 ø -+ løW Tc = 378 -+ 23 K P(4.7 rim) = (53.0 -+ 5.0) x 10s W rim-1 I24B 14ø -+ iøN, detected not observed detected 150ø-+ løW Tc = 526 _+ 26 K Tc = 539 _+ 32 K P(4.7 rim) = (7.7 _+0.8) P(4.7 •m) = (9.6 __1.0) x 10s W rim-1 x 108W rim-1 I27A (Surya) 22ø + iøN, notobserved notobserved detected 152 ø +__løW Tc • 500 K x l0 s W rim-1 = (0.9 + 0.2) x l0 s W •m -1 Prometheus 2øS,154øW detected detected detected rwest-- 456 -+ 24 K Twest= 473 -+ 3 K Twest= 472 -+ 1 K Tmin = 473 _+27 K Tmid = 554 _+25 K Tmid = 532 __18 K Teast-- 596 _+42 K Teast= 684 -+ 81 K Teast= 652 __83 K P(4.7 rim) = (87.0_+ 9) P(4.7 rim) = (101.6_+ 10) P(4.7 •m)- x l0s W/.tm-1 X l0s W/.tm-1 (98.4__ 10) x l0s W •m -• Chaac 11øN,158øW not detected detected detectedonly in higher resolution observation P(4.7 rim) < 0.3 P(4.7 rim) = (0.18 -+ 0.07) (27INCAMAXTLI) at 9-10 x l0 s W/.tm-1 x l0 s W rim-1 km/NIMS pixel Culann 19øS,160øW detected not observed not observed Tc = 480 -+ 27 K P(4.7 rim): (54.0 -+ 5.0 X 108 W •m -1 I27D 0, 164øW not detected not detected detected P(4.7 rim) < 0.3 P(4.7 rim) < 0.2 P(4.7 rim) = (0.65 -+ 0.25) X 108 W •m -1 x l0 s W •m-1 x l0 s W rim-1 33,070 LOPES ET AL.: IO IN T} IE NEAR INFRARED

emissionto better than _+3ø in latitude and longitude. Maui Patera was not imaged by NIMS during the close Io fiybys. ooo The main Amirani flow appearssimilar to the Prometheus flow field at visible wavelengths,but thermal mapping from NIMS data (Plate 6) showssome significant differences. While Prometheushas a vent area that is significantlyhotter than the rest of the flow field, and consistentlylower temperatures alongthe lengthof the flow, Amirani presentsa more complex temperature distribution.Although thermal emissionis seen u along the whole north-southlength of the flow, it is concen- trated in a few discreteareas, where temperaturesreach up to IC -RKS 10 21 1000 K. This is consistentwith breakoutsof fresh lava alongthe flow. Keszthelyiet al. [this issue]reported changes in the dark- est areason the flow betweenI24 and I27, which supportthe interpretationthat the hottestareas detected by NIMS correspondto fresh materialsbreaking out of the flow. The NIMS 10 21 I thermal map is alsoconsistent with the interpretationof Kesz- thelyiet al. [this issue]that the Amirani flow field is similar to a compound flood basalt flow field on Earth, such as the Columbia River Basalts.However, the actualcomposition cannot be determinedfrom either the morphologicalanalysis or the minimumtemperatures obtained from the NIMS data. Our results, as discussedin section 4.2.1 for Prometheus, are consistentwith lavashaving eruption temperatureseither in the basalticor ultramafic range.

4.3. Hot Spots Showing Low Temperatures: Loki and Emakong -oo These two hot spotson Io differ from others describedin T detail in this paper becausetemperatures above ---400K were E not detectedby NIMS at either of thesehot spots,even at high P spatial resolution.However, in the case of Loki the NIMS E observation did not cover the whole caldera and missed the R A most active area. In the caseof Emakong the NIMS observations covered the whole caldera. U R The thermal mappingof the Loki calderawas discussedin detail by Lopes-Gautieret al. [2000a] from NIMS observations TICK . RK$ 12 duringI24 and by Spenceret al. [2000]from observationsby the PhotopolarimeterRadiometer (PPR) in I24 and I27. Both instruments showed thermal emission from the dark materials on the caldera floor but not from the light colored materials that appear to form an island (or perhapsa raft) inside the 12 caldera. A dark colored crack or valley running through the island showed the highest color temperaturesin the NIMS data (350 _+55 K). Median color temperaturesfor the dark floor material are 305 _+44 K; however,NIMS did not image the southernpart of the caldera floor, where data from PPR showedthe highestfloor temperatures(brightness tempera-

Plate 5. (opposite) Prometheus as observed by SSI and NIMS in 124 and I27. (c) Image from SSI, shownfor comparison with the NIMS thermal maps from (b) I27 and (a) I24. The NIMS maps represent color temperatures; blues and greens are lowest and yellows and pinks are highest. Both observationsshow saturation (white) near the hottest(eastern) area and the 124 map is truncatedat the left and right edges. Temperatureprofiles along the flow field are shownfor both observations.Profiles show highest temperatures at the eastern part of the flow field, near the inferredvent region.Profiles are consistentwith that expectedfor an insulatedflow field. (Note that the pixelsat the flow edge,numbered 1, showan artificial zero value which indicatesno signal is present.) The lowest temperaturethat NIMS can detect is 180 K if a pixel is filled. LOPES ET AL.: IO IN THE NEAR INFRARED 33,071

1 G 12 tickmarks

MAUl

Plate 6. (b) Amirani lava flow field in an imagefrom SSI taken duringthe C21 orbit. The area observedby NIMS during I27 is delineated.(c) NIMS image showinga map of color temperatures,ranging from purple (lowest)to red (hottest).No thermalemission was detected by NIMS from the flow runningwest toward Maul. Maui is the dark calderaat the southwesternedge of the SSI image (beyondthe edge of the NIMS field of view). Thermal emissionwas detectedfrom the main Amirani north-southflow, from the Amirani calderato the south of the main flow, and from an area between the caldera and the main flow that appear to be connectedby a fissuresurrounded by red deposits.Thermal emissionwas also detectedfrom a small caldera to the northwestof the main flow that appearsdark in the visibleimage. (a) A temperatureprofile along the Amirani flow (seenumbers on thermalmap for locations).Highest temperatures are -1000 K and are located near the middle of the flow, probablyrepresenting a lavabreakout. The temperatureprofile alongAmirani has larger variationsthan that along Prometheus,but both are consistentwith insulatedflows. Amirani appears to havemore breakoutsfrom insulatedflows than Prometheus.(Note that the pixelsat the flow edgeshow an artificialzero value which indicatesthat no signalis present.)

turesfrom PPR data exceeded370 K). Earlier analysisof NIMS areaswere not observedby NIMS, and PPR is not sensitiveto datafrom distantobservations [Davies et al., 2000b]has measured smallareas at high temperatures.Ground-based data [Howellet temperaturesup to -1000 K at Loki. It is possiblethat similar al., this issue;Spencer et al., 2000] indicatethat duringI24, Loki temperatureswere presentat Loki during I24, but the hottest wasin the earlystages of a typicalbrightening (major eruption). 33,072 LOPES ET AL.' IO IN THE NEAR INFRARED

Both PPR and NIMS data from the fiybysare consistentwith , , the Loki caldera being coveredby flows of roughly uniform age, or a uniform crust on a lava lake [Spenceret al., 2000]. PPR data obtained during I27 showed changesin thermal emissionwithin the caldera floor, indicatingthat the hottest (and presumablyyounger) lavas on the southwestof the caldera in I24 had spreadto the west over most of the caldera by I27. Neither NIMS nor PPR data have been able to distinguish between two possibilitiesfor the eruptive mechanismtaking place on Loki: a partly crustedover lava lake or a calderafloor that is being flooded by lava flows. Ground-baseddata, com- binedwith Galileo data, showthat Loki undergoesintermittent brightneningsand suggesttwo possiblescenarios: a lava lake that undergoesperiodic overturningor spreadinglava flows 1 1.5 2 2.5 3 3.5 4 4.5 that are confinedby topography[Rathbun and Spencer,2000]. Wavelength (gin) The surfacethermal signaturesobserved by NIMS and PPR Figure 2. NIMS spectrum(averaged from 12 separatespec- are consistentwith large expansesof coolinglavas that can fit tra) from insidethe Emakongcaldera (from 27INREGION01 either the lava lake or the spreadingflows scenario. observation).The low-albedointerior of the Emakongcaldera While Loki hasbeen Io's mostoften observedhot spotsince is shownin the SSI image in Plate 4, as is a 4.8-/•m map of the Voyager,Emakong was not detectedas a hot spotuntil NIMS NIMS observation.The NIMS spectrumshows thermal emis- data from 125 and I27 region observationsshowed faint ther- sion,consistent with a temperatureof ---300K, within the same mal emissionfrom the dark caldera.Emakong is locatedin the pixel as the 4.1-/•m SO2 absorption.Emakong is the only re- center of the BosphorusRegio region of Io. This region has gion on Io where patchesof SO2 are seen within the same been repeatedlyobserved by NIMS prior to the flybys;how- relativelysmall areas (---25 km 2) asthermal emission. It is also ever, thermal emission from Emakong has never been the only low-albedoregion on Io that showsSO2 absorptionin the NIMS data. detected. Emakong was also observed in I24 during our 24INREGION02observation at 105km pixel-1, but no thermal emission was detected at this resolution. Unless the NIMS observationsconsistently missed periods of intenseactivity, the the initial velocityof plumesdriven by SO2. These conditions implicationof the lack of detectionat low spatialresolution is favor the coexistenceof SO2 and activevolcanism within areas that thermal emissionhas remained at a consistentlylow level. of the size of the NIMS pixelsrelative to silicatevolcanism. This could imply that activity at Emakong is different from activityat Loki or at other Io hot spots. 4.4. Hot Spots Showing High Temperatures: NIMS spectrafrom 124 and I27 show the SO2 absorption Pele and Tvashtar band at 4.1 /•m within the same pixels as thermal emission Prior to the flybys,temperatures consistent with ultramafic- (Figure 2). The Emakongcaldera is the only low-albedoloca- type volcanism(>1500 K) had only been obtainedfor Pillan tion on Io observedat high spatial resolutionthat showsthe duringa major eruptionin 1997 [McEwenet al., 1998b;Davies SO2 absorptionband. The presenceof thisband makesit hard et al., this issue].One of the major questionsabout Io's volca- to calculatea temperature,as this data point has to be ex- nismis whether high-temperaturelavas are presentat manyor cluded. Using only three bands, data from both I25 and I27 only a few of the hot spots.Since lava cools quickly upon observationsare consistentwith T c --- 300 K. When data from eruption, it is possiblethat eruptiontemperatures at any given the two observationsare combinedand averaged(to improve hot spotare muchhigher than that measuredat a point in time signal/noise),we obtain T c = 344 _+ 60 K. These results from orbit. In addition, observationshaving spatial resolution indicate that the Emakong caldera contains,within the same muchlarger than the sizesof the hot spotsmake it difficultto pixelareas (---25 km2), cold patches where SO2 has condensed, detect high temperaturesbecause high temperaturesare usu- as well as volcanicallyactive areas at a minimum temperature ally associatedwith very small areas. Hence, as the pixel size of 300 K. increases,the signal from the hottest componentsbecomes It is unlikelythat SO2patches would have survivedif activity increasinglydominated by that from the cooler components. at Emakonghad beenat highlevels recently, with lavasat high One of the prime objectivesof the flyby observationswas to temperaturescovering large areas of the caldera. A possible attempt detectionof high temperaturesat different hot spots. scenariofor the eruption style at Emakongis a low discharge The nighttime observationswere particularlysuited for mea- rate, patchy eruption of silicatesin the caldera, with lavas surementsof the highesttemperatures since two-component spreadingslowly and cooling once they arrive at the surface fits can be used in the absenceof reflected sunlight.Single- and coveringrelatively small areas. Another scenariois that temperature determinationsfor daytime observations(ob- Emakong is the site of sulfur volcanism.Williams et al. [this tainedfrom the longerwavelengths in the NIMS range)tend to issue]proposed that the bright flows surroundingEmakong be dominated by the lower temperaturesthat cover larger may be sulfur flows. If sulfur volcanismis taking place inside areaswithin the pixels. the caldera,the temperaturesof the active sulfur flowswould 4.4.1. Pele. NIMS obtained a nighttime observationof remain below ---400 K, consistent with what NIMS has ob- Pele in I27. A brightnesstemperature map (Plate 7) at 1 /•m served. The relatively low temperature of sulfur volcanism showedthat very high temperaturesare present in a broad comparedto silicateswould allow the surfaceto approachthe band acrossthe southernhalf of the caldera,particularly in the SO2 solidificationtemperature more quicklyand would lower southeastcorner. In the cornerwhere the highesttemperatures LOPES ET AL.: IO IN THE NEAR INFRARED 33,073 are suspected,all wavelengthslonger than 1/•m are saturated, the highesttemperature within the pixel. The NIMS-derived so precisedetermination of the highesttemperatures cannot temperature is consistent with the range reported from be made. We also excludedpixels where the signalat 1/•m is ground-basedobservations, which imply that lavasmay be ul- too low to be significantlyabove the noiselevel. The remaining tramafic in composition. pixelsfall into a regionadjacent to the saturatedarea referred Data returned from 127 show that activity in the region to above.These pixelsare shownin Plate 8. The spectrafrom observedhad fallen to a lower level by February 2000. NIMS thesepixels were used to determine the highesttemperatures pixelsfrom the same and adjacentlocations as in 125 do not that NIMS could measure from this observation. showsaturation and yield one-componenttemperature fits be- We usedthe Planckfunction to obtain two-temperaturefits tween 500 and 600 K. to these individual spectra.This was done because(1) the spectracould be better fitted by two temperaturesrather than a singletemperature and (2) previouswork usingdistant ob- 5. Implications for Io's Volcanism servationsof Pele [Davieset al., this issue]indicates that the The Pillan eruptionof 1997gave the first indicationthat very spectra are better fitted by two components. The two- high temperatures,consistent with ultramaficvolcanism, are componentfits were performed using presenton Io [McEwenet al., 1998b].One questionthat arose from the Pillan eruptionwas whether this volcanowas unique flB(,k, T•) + f2B(,k, T2): I(,k), (4) in terms of showingtemperatures consistent with ultramafic where X is wavelength,f• andf2 are the fractionalareas of the lavasor whetherthese lavas are commonon Io and simplynot two components,and T• and T 2 are their temperatures,B(X, normally observed. Temperatures derived from remote- T) is Planckfunction and I(X) is the observedintensity. The fit sensingobservations have to be consideredminimum temper- was performedusing the method of simulatedannealing [Me- aturesas magma cools quickly once it reachesthe surface[e.g., tropoliset al., 1953;Press et al., 1986]. Flynn and Mouginis-Mark,1992]. Pillan's 1997 eruption was Figure 3 showsspectra where NIMS is movingslowly over a unusualbecause it had a sufficientlyhigh masseruption rate to smallregion of very high temperatures.The two-componentfit cover relatively large areas on the surface with high- gavereasonable determination of temperaturesfor 15 separate temperaturelavas, which could be detectedfrom distant ob- spectrawhich are all overlapping.Three examplesof fits to servations. It was fortuitous that Galileo was able to observe individualspectra are shownin Figure 3. Although there are the eruptionwhile the masseruption rate was high. Activity at only six points in each spectrum(because of saturationof Pillan hasbeen observedas early as Galileo'sorbit G2 [Lopes- middlewavelengths) the consistencyof the temperaturesand Gautieret al., 1999],but temperaturesindicating clearly ultra- of the overall shapesof the spectraincrease confidence in the mafic compositionscould not be measuredfor orbits other results.We derived a median temperaturefor the hot compo- than C9 (June 1997) [Davieset al., this issue]. nent of T• = 1760 ___210 K. This rangeimplies that the Pele McEwen et al. [1998b] first proposedthe Pillan lavasto be caldera has small areaswhere the lavas are at temperatures ultramaficon the basisof temperaturedeterminations of up to consistentwith ultramafic compositions.This temperature 1825 K. Terrestrialbasalts typically erupt at 1300 K to 1450 K range for Pele overlapswith the range of 1440 +_ 150 K pro- [Morse,1980], while terrestrial ultramaficlavas (komatiites) posedby Zolotovand Fegley[2000] on the basisof sulfur and were erupted between 1700 K and 1900 K [Hess, 1989; oxygenfugacities in the Pele plume from HST data obtainedin Herzberg,1992]. The NIMS 127 data detected high temper- 1996. atures at Pele that are consistent with ultramafic volcanism. 4.4.2. Tvashtar. This chain of calderaswas observedby Although superheatedbasalts are also a possibility,we agree NIMS and SSI in 125 and 127.Fortuitously, during orbit 125,Io with the discussionpresented by McEwen et al. [1998b] that was observedfrom the ground almostsimultaneously with the ultramafic compositionis a more feasible explanationfor Io Galileo observations[Howell et al., thisissue; D. S. Acton et al., lava temperatures on Io. unpublishedmanuscript, 2000]. Both ground-basedand Gali- Pele and Pillan may not be unique. It is feasiblethat such leo observationsrevealed a region of very high temperatures high temperaturesare common,but as yet undetected,on Io. that causedpixels in SSI observationsto bleed [McEwenet al., Higher-resolutionobservations by NIMS have increasedde- 2000] and pixels in the NIMS observationsto saturate.The tectionof smallareas at high temperatures,though none of the ground-basedand SSI observationswere interpretedas images temperaturedeterminations reported here (exceptfor Pele) of a fire fountain erupting hot lavas [McEwen et al., 2000]. demand ultramafic compositions.However, the bulk of the Temperaturesestimated from the ground-basedobservations flybydata consistedof daytimeobservations. Nighttime obser- rangedfrom 1300 to 1900 K (D. S. Acton et al., unpublished vations, such as that of Pele in 127 by NIMS, are the most manuscript,2000). The observationby NIMS taken in 125 suitablefor determinationof high temperatures,as signal in (25INGIANTS01) coversthe easternpart of the activecaldera, the shortwavelengths in the NIMS range containsonly thermal and the pixelsat mostwavelengths are saturated.The highest emissionand no reflectedsunlight. Most of the hot spotsob- temperaturederived from a single-componentfit (usinga non- served during the Galileo flybys were in sunlight. In these saturatedpixel) yieldedT c = 1060 +_ 60 K (area of 0.003 cases,the estimatedtemperatures (up to ---1000K) were ob- km2).This estimate is a lowerlimit on the eruptiontempera- tained only usingthe longerwavelengths in the NIMS range, ture for two reasons.First, it was derivedfrom a pixel that is which are dominated by emissionfrom regionsat lower tem- mostlikely cooler than the hottestpixel in the observation(the peratureswhich are typically orders of magnitude larger in pixel over the hottestarea observedby SSI showedsaturation area than the highesttemperature regions.Therefore temper- at most wavelengthsin the NIMS observation).Second, the atures determined from daytime NIMS observationsprovide observationwas taken in daytime,and the single-componentfit only a minimum magmatemperature estimate, and our results to the longer wavelengthswill not be representativeof small do not rule out ultramafic compositionsfor Prometheus, areas having high temperaturesand thus will underestimate Amirani, Tvashtar,and other hot spots. 33,074 LOPES ET AL.: IO IN THE NEAR INFRARED

::.

ß ...

... LOPES ET AL.: IO IN THE NEAR INFRARED 33,075 33,076 LOPES ET AL.: IO IN THE NEAR INFRARED

T1 - 1587K A1 -0.000052 km 2, T2- 543K, A2 -0.021 km2

i .... i .... i , . ' '' [ i , , ,

•0o • 0

T1 - 1 63 0.034 km2

1 2 3 4 5

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T1 - 2154K A1 - 0.000006 km 2, T2 - 907K, A2 - 0.003 km2

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Figure 3. Typicalexamples of two componentfits to NIMS spectraof Pele.Two-component fits were done to 15 NIMS spectrafrom the sameregion which all overlappedin areal coverage.The middlewavelengths were saturatedin all the spectra.The medianhot componentderived from the 15 spectrayielded T• = 1760 _+ 210 K. LOPES ET AL.: IO IN THE NEAR INFRARED 33,077

Our results rule out low-temperaturevolcanism (such as References sulfur) for mostof the observedhot spotsbut generallyrepre- Carlson,R. W., P. R. Weissman,W. D. Smythe,J. C. Mahoney, and sent minimum temperature measurementsbecause of effects the NIMS Science and Engineering Teams, Near-Infrared Spec- of coolingrates and spatialresolution. We set no upper limits trometer Experimenton Galileo, SpaceSci. Rev., 60, 457-502, 1992. on temperatureof smallexposed areas within the observedhot Carlson, R. W., et al., The distribution of sulfur dioxide and other infrared absorberson the surfaceof Io in 1997, Geophys.Res. Lett., spotsnor temperatureswithin crustedlava lakes or tube-fed 24, 2479-2482, 1997. flows,where mostof the exposedsurface can consistof cooling Carlson,R. W., et al., Radiolyticinfluence on the surfacecomposition lavas that are several hundred degreescooler than eruption of the Galilean satellites,paper presentedat Jupiter:Planet, Satel- temperatures. Moreover, if the eruption has temporarily lites, and MagnetosphereConference, Lab. for Atmos. and Space Phys.,Univ. of Colo., Boulder, Colo., June 25-30, 2001. halted during the time of the observation,high temperatures Carr, M. H., A. S. McEwen, K. A. Howard, F. C. Chuang,P. Thomas, would not be present and only temperaturesconsistent with P. Schuster, J. Oberst, G. Neukum, G. Schubert, and the Galileo thoseof coolinglavas would be detected.It is thereforenot yet ImagingTeam, Mountainsand calderason Io: Possibleimplications possibleto infer differencesin magmatemperature or compo- for lithosphericstructure and magmageneration, Icarus, 135, 146- 165, 1998. sitionfor varioushot spotsfrom the availabletemperature data Crisp, J., and S. Baloga, A model for lava flows with two thermal that we havefor Io, thoughsuch inferences may be valid on the components,J. Geophys.Res., 95, 1255-1270, 1990. basis of morphologicaldata or unusual characteristics.One Davies,A. G., A. S. McEwen, R. Lopes-Gautier,L. Keszthelyi,R. W. Carlson, and W. D. Smythe,Temperature and area constraintsof example is the interior of the Emakong caldera, where low the South Volund volcano on Io from the NIMS and SSI instru- thermal emissioncombined with the presence of SO2 in a ments during the Galileo G1 orbit, Geophys.Res. Lett., 24, 2447- volcanicallyactive region supportthe conclusionsof Williams 2450, 1997. et al. [this issue]that Emakong and its surroundingsmay be a Davies, A. G., R. Lopes-Gautier,W. D. Smythe,and R. W. Carlson, site of sulfurvolcanism. So far, Emakong is unique amongIo's Silicatecooling model fits to Galileo NIMS data of volcanismon Io, Icarus, 148, 211-225, 2000a. hot spotsas the only locationwhere NIMS has detectedSO2 Davies,A. G., L. P. Keszthelyi,R. Lopes-Gautier,W. D. Smythe,L. andthermal emission within the samepixel (-20-25 km2).It Kamp, R. W. Carlson,and the Galileo NIMS and SSI Teams,Erup- is unclear whether Emakong is a unique hot spot in this re- tion evolutionof major volcanoeson Io: Galileo takes a closelook, spect.NIMS hasonly observed-5% of Io's surfaceat resolu- Lunar Planet. Sci. Conf. [CD-ROM], XXXI, Abstract 1754, 2000b. Davies, A. G., L. P. Keszthelyi,D. A. Williams, C. B. Phillips,A. S. tions better than -25 km/NIMS pixel, and it is possiblethat McEwen, R. Lopes-Gautier, W. D. Smythe, L. W. Kamp, L. A. thermal emissionat levelssimilar to that at Emakonghave not Soderblom,and R. W. Carlson,Thermal signature,eruption style, been so far detected. and eruptionevolution at Pele andPillan on Io, J. Geophys.Res., this issue. The three flybysin 1999and 2000 coveredonly a verylimited Doutd, S., R. Lopes-Gautier,W. D. Smythe,L. W. Kamp, R. 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