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High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io
Icarus 160, 124–131 (2002) doi:10.1006/icar.2002.6955
High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io F. Marchis,1 I. de Pater,1 A. G. Davies,2 H. G. Roe,1 T. Fusco,3 D. Le Mignant,4 P. Descamps,5 B. A. Macintosh,6 and R. Prang´e7 1
Astronomy Department, University of California, Berkeley, California 94720-3411; 2 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109-8099; 3 ONERA, DOTA-E, BP 72, F-92322 Chatillon, France; 4 W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, Hawaii 96743; 5 Institut de M´ecanique C´eleste et de Calcul des Eph´emerides, UMR-CNRS 8028, Observatoire de Paris 77, avenue Denfert-Rochereau, F-75014 Paris, France; 6 Lawrence Livermore National Laboratory/IGPP, 7000 East Avenue, Livermore, California 94551; and 7 Institut d’Astrophysique Spatiale, bat. 121, Universit´e Paris Sud, 91405 Orsay Cedex, France E-mail: [email protected] Received February 2, 2002; revised June 25, 2002
Io, the innermost Galilean satellite of Jupiter, is a fascinating world. Data taken by Voyager and Galileo instruments have established that it is by far the most volcanic body in the Solar System and suggest that the nature of this volcanism could radically differ from volcanism on Earth. We report on near-IR observations taken in February 2001 from the Earth-based 10-m W. M. Keck II telescope using its adaptive optics system. After application of an appropriate deconvolution technique (MISTRAL), the resolution, ∼100 km on Io’s disk, compares well with the best Galileo/NIMS resolution for global imaging and allows us for the first time to investigate the very nature of individual eruptions. On 19 February, we detected two volcanoes, Amirani and Tvashtar, with temperatures differing from the Galileo observations. On 20 February, we noticed a slight brightening near the Surt volcano. Two days later it had turned into an extremely bright volcanic outburst. The hot spot temperatures (>1400 K) are consistent with a basaltic eruption and, being lower limits, do not exclude an ultramafic eruption. These outburst data have been fitted with a silicate-cooling model, which indicates that this is a highly vigorous eruption with a highly dynamic emplacement mechanism, akin to fire-fountaining. Its integrated thermal output was close to the total estimated output of Io, making this the largest ionian thermal outburst yet witnessed. c 2002 Elsevier Science (USA)
Key Words: Io; volcanism; infrared observations.
1. INTRODUCTION
Io’s volcanic activity, attributed to internal heating caused by tidal interactions with Jupiter and Europa (Peale et al. 1979), was first discovered from space with Voyager 1 in 1979 (Morabito et al. 1979). The most obvious manifestations of Io’s high heat flow are many volcanic hot spots and plumes. The Voyager infrared spectrometer (IRIS) scanned one-third of the surface where at least 17 hot spots were detected (McEwen et al. 1992). Typical temperatures of 300 to 650 K (Pearl and Sinton 1982) suggested that ionian volcanism may be dominated by relatively low temperature sulfur lavas (Lunine and Stevenson 1985) rather
than by silicate volcanism (at liquidus temperatures >1000 K) which is dominant on Earth. However, it was shown later that the IR spectra of several of these relatively cool hot spots could also be modeled assuming a cooling basaltic lava flow (Carr 1986, Howell 1997), and the issue remained open. Since these Voyager flybys, ground-based monitoring programs of Io’s volcanic activity were made possible by improvements in detector technology and observing techniques. One can observe Io in the infrared while in eclipse, so that it only “glows” via its hot spots (Veeder et al. 1994). Another approach is to use Io’s occultations/eclipses by Jupiter (Spencer et al. 1990) or by another satellite (Goguen et al. 1988, Descamps et al. 1992). These mutual events, which only occured every six years, were used to derive approximate positions and temperature information for some individual hot spots. Much higher temperatures (up to ∼1500 K) (Blaney et al. 1995, Spencer et al. 1997) were obtained as spatial resolution and/or sensitivity increased. More recently, adaptive optics (AO) systems offered improved spatial resolution. High-resolution images from the ADONIS AO system on the 3.6-m ESO telescope at 3.8 µm revealed the distribution and evolution of more than 20 low temperature (because of the long wavelength used) hot spots (Marchis et al. 2000, 2001). This monitoring program was carried out in conjunction with the Galileo spacecraft, which has been orbiting within the jovian system since December 1995. The groundbased program and the Galileo near-infrared mapping spectrometer (NIMS) (Lopes-Gautier et al. 1999) obtained very similar detailed maps of active volcanic centers. Temperatures derived from single-temperature blackbody fits to NIMS data are generally not useful for determining if temperatures are diagnostic of sulfur or silicate composition (Davies et al. 2000). However, other analyses revealed high-temperature components, indicative of silicate volcanism (e.g., Davies et al. 1997). This justified the fitting of sophisticated silicate lava cooling models, with liquidus temperatures in the range 1400–1500 K, to the NIMS data (Davies et al. 2000). Finally, some spectacular Galileo visible observations of Io in Jupiter’s shadow revealed 124
0019-1035/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved.
VIOLENT VOLCANIC ACTIVITY ON IO OBSERVED WITH KECK AO
the presence of very high temperature (>1700 K) hot spots (McEwen et al. 1998b), suggesting that some of the lavas are ultramafic (magnesium-rich) silicates, a type of volcanism which may be compared to the komatiite terrestrial volcanic rocks found mainly in Precambrian terrains (Williams et al. 2001). Since the early Voyager encounters our view of Io has thus changed from a body dominated by relatively cool sulfur volcanism to one where high-temperature hot spots are the norm. The presence of ultra-high temperatures (>1700 K) is particularly intriguing, with Io perhaps being a volcanic laboratory allowing us to study processes that shaped the early Earth (Matson et al. 1998). With the Galileo mission coming to an end, the future monitoring of Io’s volcanism lies in the hands of terrestrial observers. 2. OBSERVATIONS, DATA PROCESSING, AND DECONVOLUTION
We report on observations of Io in the near infrared (1–2.5 µm) using the 10-m W. M. Keck II AO system (Wizinowich et al. 2000) on 19, 20, and 22 February, 2001 UT (Table I). We used the NIRSPEC camera (McLean et al. 1998) in its imaging mode using a SCAM Rockwell HgCdTe detector sensitive in the 0.95- to 2.5-µm range. The total field of view is 4.3 arcsec with a pixel size of 16.8 milliarcsec (mas). The data were reduced in the standard way (sky subtraction, bad pixel removal, and flat-field correction), and photometric calibration was done using several photometric stars observed a few times throughout the night. The satellite magnitude (m v ∼ 5) and its angular size (∼1.2 arcsec) do not compromise the quality of the wavefront analysis. An angular resolution of ∼42 mas in H band (1.63 µm) was estimated by imaging a nearby star of a similar magnitude (obtaining its PSF, point spread function) just before and/or after the observing target. This corresponds to a spatial resolution of ∼130 km at the center of Io’s disk, roughly 2–3 times better than most of the global Galileo/NIMS observations (Dout´e et al. 2001). The angular resolution in K band (2.12 µm) is ∼50 mas. In J band (1.25 µm), resolution varies depending of the atmospheric seeing conditions (42–60 mas). Even though AO systems correct for atmospheric turbulence in real time, this correction is not perfect, and part of the energy of the PSF is lost in a halo surrounding the coherent peak. The TABLE I Information on the NISPEC/Keck AO Observations SSPa
SEP Date
Time (UT)
Filter
Long.
Lat.
Long.
Lat.
19 Feb. 2001 20 Feb. 2001 22 Feb. 2001
05:20–05:30 05:06–05:12 04:47–04:50
J, H, K J, H, K J, H, K +2 SW
102.8◦ 304.0◦ 348.1◦
2.8◦ 2.8◦ 2.8◦
91.6◦ 292.7◦ 336.8◦
2.8◦ 2.7◦ 2.7◦
a
SSP: sub-solar point.
125
direct effect of this imperfection is to blur the images. The amount of blurring is indicated by the Strehl ratio, which is the ratio of the maximum intensity of the recorded PSF to the maximum of the theoretical diffraction-limited PSF when both are normalized. Our PSF stars indicate a Strehl ratio of ∼20% in J band (1.26 µm) and ∼50% in K band (2.12 µm). The sharpness of our images can be further enhanced by applying an inversion process, such as a deconvolution algorithm, to the data. We used a new myopic deconvolution algorithm called MISTRAL (Myopic Iterative STep-preserving Restoration ALgorithm) and developed by ONERA (Office National d’Etudes et de Recherches Aerospatiales), especially aimed at AO observations of planetary objects (Conan et al. 1998). MISTRAL uses a stochastic approach to finding the best image reconstruction, using information about the object and the PSF. This requires several star images to get enough data about variations in the PSF. The main improvement of this technique over more classical methods is that it avoids both noise amplification and creation of sharp-edge artifacts or “ringing effects” and that it better restores the initial photometry. This algorithm has already been used on simulated and real AO observations (Conan et al. 2000, Marchis et al. 2001). 3. RESULTS
3.1. Image Analysis Our results are shown in Fig. 1. The first two rows in Fig. 1a display the 20 February JHK observations before (first row) and after (second row) deconvolution. The deconvolved images are sharper, with a resolution estimated to be 35 mas in H band (1.61 µm), i.e., equivalent to the telescope diffraction limit (105 km on Io’s surface). They display the same albedo features on the surface of the satellite as the Minnaert law reconstructed visible images based on the 10- to 23-km-resolution Galileo/SSI map (McEwen et al. 1998a). During the other two nights (Fig. 1b), several bright spots stood out with a much higher contrast. These correspond to thermal emission from active hot spots, surprisingly detected directly, against Io’s sunlit hemisphere. This never occurred at such short wavelengths during our previous four-year survey with the ADONIS AO system on the 3.6-m ESO telescope. Figure 2 displays two Keck AO images without deconvolution and a simulation of corresponding ADONIS images in the same wavelength range. The 010222A hot spot would have been detected by an AO system on a 4-m-class telescope, but none of the fainter hot spots (010219A and 010219B) on the other hemisphere can be seen on these 120-mas-resolution images. Our Keck data clearly indicate the presence of hightemperature volcanic activity whose multiwavelength detection was only possible because of the higher angular resolution provided by the 10-m Keck telescope, which was further improved by the powerful MISTRAL deconvolution method. 3.2. Hot Spot Flux Analysis Each detected hot spot has been labeled following our previous convention (Marchis et al. 2001) consisting of six numbers
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FIG. 1. (a) Jupiter-facing hemisphere observed with the Keck AO system. The basic-processed images from 20 February 2001 (first row) are displayed. The second row corresponds to the same images after applying the MISTRAL deconvolution process. Albedo features, similar to the 20-km-resolution reconstructed GALILEO/SSI image (right column) are easily detected. The last row shows the 22 February 2001 images, which are dominated by the presence of the Surt outburst. (b) Observations from 19 February 2001. Two hot spots, corresponding to Tvashtar (North) and Amirani, are clearly detected in the H and K bands. Note the bad quality of the J-band image after deconvolution, due to the poor seeing condition of this observing night.
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TABLE III Temperatures and Emission Areas from Two-Temperature Model
FIG. 2. Comparison of the 10-m Keck data with images as would have been seen with the lower resolution ADONIS AO system on the 3.6-m ESO telescope. Data in the left column correspond to real Keck AO observations obtained without deconvolution. The right column displays the corresponding images at the ESO-ADONIS resolution (0.11 in H band and 0.12 in K band). The Surt outburst would have been seen by ADONIS, but the other fainter hot spots on the opposite hemisphere cannot be detected with a 4-m-class telescope.
corresponding to the date of observation (YYMMDD), plus a letter for each source. To estimate the positions of the hot spots, the images were deprojected on a cylindrical map, using the subEarth point (SEP) from the physical ephemeris, and a Gaussian function was fitted through the emission profile. In addition to the uncertainty in the fit, the error in the derived position also includes a term introduced by the uncertainty in Io’s estimated limb for the cylindrical projection, ∼1/4 pixel. The position of each hot spot is listed in Table II and is indicated on a Galileo/Voyager visible cylindrical map (Fig. 3). These locations correspond precisely to the known active areas Amirani (010219A), Tvashtar (010219B), and Surt (010220A and 010222A). In Table II, we list the radiance of the thermal emission after subtracting the background radiation due to reflected sunlight
Hot spot name
Best T1 (K)
Best T2 (K)
A1 (km2 )
A2 (km2 )
010222A
880 ± 10
1470 ± 20
1760 ± 155
93 ± 9
and correcting for emission angle. Uncertainties come from several sources: the Poisson noise of the detector (1–5%), our estimate of the background emission (<2%), and the accuracy in the recovery of the initial photometry from the deconvolution method (<3%). The latter uncertainty has been estimated from deconvolution tests on simulated Io images (Marchis 2000). A classical deconvolution method would not recover the photometry of such bright sources as accurately, since the typical uncertainty using a Lucy–Richardson method is ∼10% for high-contrast sources (Marchis 2000). Finally, considering the uncertainties introduced by the measurement and the absolute magnitude of the photometric stars, the total observed uncertainty in the measured intensity should be ∼10% for the hot spots detected on 19 and 22 February. The flux transmitted by several broadband filters allows us to estimate the temperature and the area of the source assuming that the hot spot emission follows a classical blackbody law. The fitting process used a classical χ 2 minimization method, and the transmission profile of the filter was included in the calculation, assuming a standard transmissivity of the atmosphere (McLean et al. 1998). On 19 and 20 February, we detected the hot spots in H (1.61 µm) and K (2.12 µm) bands only. We thus fitted the spectral dependance of the emission by a single blackbody temperature for each hot spot (i.e., two parameters: temperature and area). On 22 February, the hot spot was also detected in the broadband J filter (1.26 µm) and in two narrow-band filters centered on 1.68 and 1.71 µm. A two-temperature blackbody law (i.e., four parameters: two temperatures, each with a particular areal coverage) gave a better fit than a single blackbody temperature law. Tables II and III list the temperatures and area calculated for each hot spot. 3.3. Amirani The 010219A thermal source coincides with Amirani, a wellknown hot spot discovered by Voyager (Pearl and Sinton 1982)
TABLE II Intensity, Temperature, and Emission Area of the Hot Spots Estimated from Blackbody Fits Hot spot name
Position (Long., Lat.)
Candidates
IJ
IH (GW/sr/µm)
IK
Best mono-T (K)
Area (km2 )
010219A 010219B 010220A 010222A
27 ± 3 N, 118 ± 3 W 58.5 ± 3.5 N, 124.5 ± 3 W 41 ± 3 N, 337 ± 5 W 41 ± 4.5 N, 340 ± 4.5 W
Amirani Tvashtar Surt Surt
— — — 1440 ± 20
115 ± 7 93 ± 6 12 ± 4 3470 ± 150
245 ± 11 310 ± 12 18 ± 2 5590 ± 180
990 ± 35 810 ± 15 1330 ± 270 1240 ± 20
103 ± 22 460 ± 100 10−3 (r < 50 m) 445 ± 42
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FIG. 3. Localization of the thermal sources detected on the AO images and comparison with a Galileo/Voyager map. The black square on the images indicates the position estimated with our AO observation (see Table II). We compare the position of Surt with two Voyager images and a Galileo image. Note the surface change feature in the second Voyager image. The change in appearance of this caldera may have been caused by the bright eruption seen from the ground by Sinton (1980). Its position is compatible with the bright outburst obtained from our data.
and still active during the Galileo era (Lopes-Gautier et al. 1999). The high-resolution thermal map of the volcano acquired with NIMS during the I24 and I27 flybys (October 1999 and February 2000) indicates that the active area was a 350-km-long north– south lava flow field, with a mean temperature of 420 K (Lopes et al. 2001). Our data do not display any north–south extension of the source. We estimate a higher temperature of 1000 K over an area of 100 km2 . I27 NIMS data show also that a small region in the middle of the lava flow field reached a color temperature of ∼1000 K. Since it is roughly located in one NIMS pixel (of size 10.5 km) its area should be less than or equal to 100 km2 . Our observation confirms the presence of this thermal emission, which could correspond to a lava breakout in the Amirani lava flow field. 3.4. Tvashtar The Tvashtar hot spot was first detected on 26 November 1999 from the ground (Howell et al. 2001) and by NIMS and SSI during the I25 Io flyby (McEwen et al. 2000). The eruption appeared to be a linear lava curtain. Since most of the detectors were saturated, only a lower bound of 1060 ± 60 K could be deduced from the NIMS data (Lopes et al. 2001). One year later this volcanic area was still active. A bright outburst was observed in December 2000 with the ADONIS AO system (Marchis et al. 2001), as well as with NIMS and SSI on Galileo. Two months later our data reveal the presence of a thermal source, 010219B, for which we derive a lava temperature of 810 K over an area
of 460 km2 . Assuming that the volcanic type of the Tvashtar eruption did not change over time, our estimate of 810 K may be interpreted as the average of a cooling lava flow (with a typical temperature of 400–500 K) and of an active fire fountain (with temperatures up to 1400 K). To derive better models for Tvashtar’s complex volcanic activity, one needs to analyze in detail both ground-based and Galileo NIMS/SSI data as obtained over the past two years. 3.5. Surt When we observed Io on 20 February 2001, we did not see any obvious activity on this near-Jupiter-facing hemisphere. Two days later, the images of the same hemisphere changed dramatically and revealed an extremely bright hot spot (010222A) at the position of Surt. The spectral dependance of the emission shows that this bright hot spot consisted of a 95 km2 area at an exceptionally high temperature of 1470 K and of a larger area, 1760 km2 , at 880 K. A simultaneously obtained H-band spectrum also indicates a temperature exceeding 1200 K (Fig. 4). An extrapolation of our blackbody temperature fit shows that Surt must have radiated ∼5000 GW/sr/µm at a wavelength of 5 µm, i.e., 10 times higher than the flux of the sudden short-lived eruptions normally referred to as “outbursts” and which may occur several times per year (Spencer and Schneider 1996). Previous outburst observations generally yield much lower 5-µm intensities. For example, the 1986/08/07 outburst (see Veeder et al. (1994); T = 1550 K and r = 8 km) gives a 5-µm brightness of
VIOLENT VOLCANIC ACTIVITY ON IO OBSERVED WITH KECK AO
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3.6. Characterizing the Surt Outburst
FIG. 4. Spectroscopic AO observations of the Surt outburst. The inset displays a simultaneous image of Io using the slit-viewing camera. The H-band spectra obtained were processed using standard techniques, including atmospheric correction using a standard star, followed by subtraction of the observed spectrum of sunlit Io. Thus, the presented spectrum represents only thermal emission from the hot spot. The apparent absorption features in the final spectrum are artifacts. They arise in the atmospheric correction procedure due to imperfect removal of the standard star’s spectrum. The best fit of the spectrum indicates a temperature of 1200 K for the emission area, a temperature similar to the best mono-temperature deduced from the multiwavelength images (see Table II). Ideally spectra with greater wavelength coverage (e.g., from 1 to 5 µm) should be taken to get a better estimate of the temperature and area of the eruption; however, in this case we were limited by the fast approaching occultation of Io by Jupiter.
only ∼1400 GW/sr/µm, four times fainter than the Surt outburst reported here. More recently, the 9908A outburst reached a maximum observed 4.68-µm intensity of 1800 GW/sr/µm (Howell et al. 2001). Such violent activity has been seen once before at Surt. During its first 10 orbits Galileo did not detect any activity in this area, but in June 1996 activity was reported from the ground (LopesGautier et al. 1999, Spencer et al. 1997). Even earlier, between the consecutive flybys of the Voyager spacecraft, a very bright eruption (Sinton 1980) was observed in the thermal infrared (see Fig. 3). Images of Surt taken by the two Voyager spacecraft revealed the appearance of a dark and localized caldera, the origin of which may have triggered the bright infrared eruption. The position of this caldera is similar to the position of the bright outburst observed by us. This dark caldera was not seen on the first Galileo images; it probably had been covered by plume deposits from a nearby volcano in the intervening time. Surt seems to be an “outburst-type” volcanic area, imaged for the first time. Once the images were deconvolved, we discovered very faint activity on the 20 February data at the same location (010220A) (see Fig. 1a). The detection is marginal but strongly suggestive of a precursor to the big event. It corresponds to a very small area (less than 50 m radius) at a temperature up to 1400 K, like basaltic lava flows on Earth (McEwen et al. 2000).
We have fitted our Surt data with a silicate lava cooling model (Davies 1996), originally developed to interpret ground-based ionian observations and subsequently used to interpret Galileo/ NIMS spectra (Davies et al. 2000). The model calculates the distribution of temperatures and areas seen during an eruption as a function of time. Fitting observational data with the model output (an integrated thermal spectrum) thus produces a range of emitting surface ages. Assuming a basaltic composition for the erupting magma, the model fit to the Surt data produces a range of areas at temperatures continuously ranging from a fraction of a square kilometer at 1475 ± 0.5 K down to 8.9 km2 at 1030 ± 0.5 K. The total area of the entire thermal anomaly is approximately 800 km2 . The 1030-K area, the oldest area contributing to the observed thermal output, is less than 100s old. This suggests that the eruption style is extremely dynamic, indicative of a highly vigorous style of lava emplacement, such as large fire fountains. Such activity was seen at Tvashtar by Galileo SSI in 1999 (McEwen et al. 2000). More surprisingly, recent data collected during the Galileo/SSI I31 flyby (6 August, 2001) do not indicate any obvious changes in surface reflectivity in the Surt area, including no new low-albedo area (A. McEwen, private communication) as would be expected after an eruption. 4. CONCLUSION
The detection of these ionian eruptions at near-infrared wavelengths (<2.5 µm) is surprising and implies that we observed highly energetic hot spots which were never seen in our previous four-year survey using the 3.6-m telescope AO system. Comparing the thermal signature of the Surt eruption with others seen by Galileo NIMS (Davies 2001), one can see that the thermal output [(7.8 ± 0.6) × 1013 W] of this outburst is considerably larger than any other eruption seen by the Galileo spacecraft (see Fig. 5). This is in fact the most energetic outburst ever observed on Io. It is the largest in the range of outbursts detected from the ground in the thermal and far infrared (3–20 µm) after the Voyager encounters. Note that the majority of those outbursts were observed only by photometric measurements, which do not provide a very precise location of the emission area. Therefore the nature of the activity which corresponds to such outbursts has remained uncertain. The Surt outburst with an output of (7.8 ± 0.6) × 1013 W nearly matches the total thermal output from Io integrated over all wavelengths (1014 W, assuming an average global heat flow of 2.5 W · m−2 ; Veeder et al. 1994). The other eruptions, with a total output of 10.1 × 1012 W for Tvashtar and 5.0 × 1012 W for Amirani, are more energetic than the Pillan eruption observed during the C9 flyby (3.6 × 1012 W), the brightest eruption seen by Galileo. Adding the three eruption intensities, we derive a mean total output for the February 2001 period of 2.2 W · m−2 . These three extremely bright and short-lived hot spots can account for the estimated thermal heat flow of 2.5 W · m−2
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FIG. 5. Best-fit spectra to the observation. Crosses correspond to the observed intensity in broadband filters (see Table II). Squares indicate the narrow-band intensities of Surt centered at 1.68 and 1.71 µm. The blue and purple lines are the best mono-temperature blackbody fits for Amirani and Tvashtar. The bold red line is the model spectrum of Surt derived from the silicate-cooling flow model. The classical two-temperature model is overlaid with a red solid line with its two components dashed and dotted. For comparison, we display (in green) spectra of various active volcanoes as deduced from the Galileo/NIMS observations. The outburst detected by the AO observations clearly corresponds to a much stronger eruption. The total output of each eruption is indicated (in GW).
(Veeder et al. 1994) without even considering the output of the many cooler sources not detectable at short wavelengths. High angular resolution provided by adaptive optics systems on 8- to 10-m-class telescopes is a promising tool for monitoring the volcanic activity of Io from the ground with a spatial resolution better than the global Galileo/NIMS observations. With the Galileo mission coming to an end in November 2002, future monitoring of Io’s volcanism lies in the hands of terrestrial observers. Further instrumental improvements, such as a wider wavelength range (1–5 µm) and the ability to make spectroscopic AO observations, will allow us to better constrain the nature, temperature, and evolution of ionian eruptions. ACKNOWLEDGMENTS Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to R. Howell and J. Spencer for reviewing this article. This work has been supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement AST-9876783. Ashley Davies is supported by a NASA Planetary Geology and Geophysics grant 344-30-23-09. Part of this work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48
and the France–Berkeley Fund. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations of these ionian volcanoes would have been possible.
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VIOLENT VOLCANIC ACTIVITY ON IO OBSERVED WITH KECK AO Davies, A. G., R. Lopes-Gautier, W. D. Smythe, and R. W. Carlson 2000. Silicate cooling model fits to Galileo NIMS data of volcanism on Io. Icarus 148, 211– 225. Descamps, P., J. E. Arlot, W. Thuillot, F. Colas, D. T. Vu, P. Bouchet, and O. Hainaut 1992. Observations of the volcanoes of Io, Loki and Pele, made in 1991 at the ESO during an occultation by Europa. Icarus 100, 235– 244. Dout´e, S., B. Schmitt, R. Lopes-Gautier, R. Carlson, L. Soderblom, J. Shirley, and The Galileo NIMS Team 2001. Mapping SO2 frost on Io by the modeling of NIMS hyperspectral images. Icarus 149, 107–132. Goguen, J. D., and 10 colleagues 1988. Io hot spots—Infrared photometry of satellite occultations. Icarus 76, 465–484. Howell, R., J. Spencer, J. Goguen, F. Marchis, R. Prang´e, T. Fusco, D. Blaney, G. Veeder, J. Rathbun, G. Orton, A. Grocholski, and J. Stansberry 2001. Ground-based observations of volcanism on io in 1999. J. Geophys. Res. 106, 33,129–33,139. Howell, R. R. 1997. Thermal Emission from Lava Flows on Io. Icarus 127, 394–407. Lopes, R. M. C., L. W. Kamp, S. Dout´e, W. D. Smythe, R. W. Carlson, A. S. McEwen, P. E. Geissler, S. W. Kieffer, F. E. Leader, A. G. Davies, E. Barbinis, R. Mehlman, M. Segura, J. Shirley, and L. A. Soderblom 2001. Io in the near infrared: Near-Infrared Mapping Spectrometer (NIMS) results from the Galileo flybys in 1999 and 2000. J. Geophys. Res. 106, 33053–33078. Lopes-Gautier, R., A. S. McEwen, W. B. Smythe, P. E. Geissler, L. Kamp, A. G. Davies, J. R. Spencer, L. Keszthelyi, R. Carlson, F. E. Leader, R. Mehlman, L. Soderblom, and The Galileo NIMS & SSI Teams 1999. Active volcanism on Io: Global distribution and variations in activity. Icarus 140, 243–264. Lunine, J. I., and D. J. Stevenson 1985. Physics and chemistry of sulfur lakes on Io. Icarus 64, 345–367. Marchis, F. 2000. Imagerie a` haute r´esolution angulaire des objets du syst`eme solaire en Optique Adaptative. Application l’´etude du volcanisme de Io. Ph.D. thesis, Universit´e Toulouse III. Marchis, F., R. Prang´e, and J. Christou 2000. Adaptive optics mapping of Io’s volcanism in the thermal IR (3.8 µm). Icarus 148, 384–396. Marchis, F., R. Prang´e, and T. Fusco 2001. A survey of Io’s vocanism by adaptive optics observations in the 3.8-µm thermal band (1996–1999). J. Geophys. Res. 106, 33141–33160. Matson, D. L., D. L. Blaney, T. V. Johnson, G. J. Veeder, and A. G. Davies 1998. Io and the early Earth. In Lunar and Planetary Institute Conference, Vol. 29, pp. 1650–1651. Lunar Planet. Inst., Houston. McEwen, A. S., N. R. Isbell, and J. C. Pearl 1992. Io thermophysics: New models with Voyager I thermal IR Spectra. In Lunar and Planetary Institute Conference Abstracts, Vol. 23, pp. 881–882. McEwen, A. S., L. Keszthelyi, P. Geissler, D. P. Simonelli, M. H. Carr, T. V. Johnson, K. P. Klaasen, H. H. Breneman, T. J. Jones, J. M. Kaufman, K. P.
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High-Resolution Keck Adaptive Optics Imaging of Violent Volcanic Activity on Io F. Marchis,1 I. de Pater,1 A. G. Davies,2 H. G. Roe,1 T. Fusco,3 D. Le Mignant,4 P. Descamps,5 B. A. Macintosh,6 and R. Prang´e7 1
Astronomy Department, University of California, Berkeley, California 94720-3411; 2 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109-8099; 3 ONERA, DOTA-E, BP 72, F-92322 Chatillon, France; 4 W. M. Keck Observatory, 65-1120 Mamalahoa Highway, Kamuela, Hawaii 96743; 5 Institut de M´ecanique C´eleste et de Calcul des Eph´emerides, UMR-CNRS 8028, Observatoire de Paris 77, avenue Denfert-Rochereau, F-75014 Paris, France; 6 Lawrence Livermore National Laboratory/IGPP, 7000 East Avenue, Livermore, California 94551; and 7 Institut d’Astrophysique Spatiale, bat. 121, Universit´e Paris Sud, 91405 Orsay Cedex, France E-mail: [email protected] Received February 2, 2002; revised June 25, 2002
Io, the innermost Galilean satellite of Jupiter, is a fascinating world. Data taken by Voyager and Galileo instruments have established that it is by far the most volcanic body in the Solar System and suggest that the nature of this volcanism could radically differ from volcanism on Earth. We report on near-IR observations taken in February 2001 from the Earth-based 10-m W. M. Keck II telescope using its adaptive optics system. After application of an appropriate deconvolution technique (MISTRAL), the resolution, ∼100 km on Io’s disk, compares well with the best Galileo/NIMS resolution for global imaging and allows us for the first time to investigate the very nature of individual eruptions. On 19 February, we detected two volcanoes, Amirani and Tvashtar, with temperatures differing from the Galileo observations. On 20 February, we noticed a slight brightening near the Surt volcano. Two days later it had turned into an extremely bright volcanic outburst. The hot spot temperatures (>1400 K) are consistent with a basaltic eruption and, being lower limits, do not exclude an ultramafic eruption. These outburst data have been fitted with a silicate-cooling model, which indicates that this is a highly vigorous eruption with a highly dynamic emplacement mechanism, akin to fire-fountaining. Its integrated thermal output was close to the total estimated output of Io, making this the largest ionian thermal outburst yet witnessed. c 2002 Elsevier Science (USA)
Key Words: Io; volcanism; infrared observations.
1. INTRODUCTION
Io’s volcanic activity, attributed to internal heating caused by tidal interactions with Jupiter and Europa (Peale et al. 1979), was first discovered from space with Voyager 1 in 1979 (Morabito et al. 1979). The most obvious manifestations of Io’s high heat flow are many volcanic hot spots and plumes. The Voyager infrared spectrometer (IRIS) scanned one-third of the surface where at least 17 hot spots were detected (McEwen et al. 1992). Typical temperatures of 300 to 650 K (Pearl and Sinton 1982) suggested that ionian volcanism may be dominated by relatively low temperature sulfur lavas (Lunine and Stevenson 1985) rather
than by silicate volcanism (at liquidus temperatures >1000 K) which is dominant on Earth. However, it was shown later that the IR spectra of several of these relatively cool hot spots could also be modeled assuming a cooling basaltic lava flow (Carr 1986, Howell 1997), and the issue remained open. Since these Voyager flybys, ground-based monitoring programs of Io’s volcanic activity were made possible by improvements in detector technology and observing techniques. One can observe Io in the infrared while in eclipse, so that it only “glows” via its hot spots (Veeder et al. 1994). Another approach is to use Io’s occultations/eclipses by Jupiter (Spencer et al. 1990) or by another satellite (Goguen et al. 1988, Descamps et al. 1992). These mutual events, which only occured every six years, were used to derive approximate positions and temperature information for some individual hot spots. Much higher temperatures (up to ∼1500 K) (Blaney et al. 1995, Spencer et al. 1997) were obtained as spatial resolution and/or sensitivity increased. More recently, adaptive optics (AO) systems offered improved spatial resolution. High-resolution images from the ADONIS AO system on the 3.6-m ESO telescope at 3.8 µm revealed the distribution and evolution of more than 20 low temperature (because of the long wavelength used) hot spots (Marchis et al. 2000, 2001). This monitoring program was carried out in conjunction with the Galileo spacecraft, which has been orbiting within the jovian system since December 1995. The groundbased program and the Galileo near-infrared mapping spectrometer (NIMS) (Lopes-Gautier et al. 1999) obtained very similar detailed maps of active volcanic centers. Temperatures derived from single-temperature blackbody fits to NIMS data are generally not useful for determining if temperatures are diagnostic of sulfur or silicate composition (Davies et al. 2000). However, other analyses revealed high-temperature components, indicative of silicate volcanism (e.g., Davies et al. 1997). This justified the fitting of sophisticated silicate lava cooling models, with liquidus temperatures in the range 1400–1500 K, to the NIMS data (Davies et al. 2000). Finally, some spectacular Galileo visible observations of Io in Jupiter’s shadow revealed 124
0019-1035/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved.
VIOLENT VOLCANIC ACTIVITY ON IO OBSERVED WITH KECK AO
the presence of very high temperature (>1700 K) hot spots (McEwen et al. 1998b), suggesting that some of the lavas are ultramafic (magnesium-rich) silicates, a type of volcanism which may be compared to the komatiite terrestrial volcanic rocks found mainly in Precambrian terrains (Williams et al. 2001). Since the early Voyager encounters our view of Io has thus changed from a body dominated by relatively cool sulfur volcanism to one where high-temperature hot spots are the norm. The presence of ultra-high temperatures (>1700 K) is particularly intriguing, with Io perhaps being a volcanic laboratory allowing us to study processes that shaped the early Earth (Matson et al. 1998). With the Galileo mission coming to an end, the future monitoring of Io’s volcanism lies in the hands of terrestrial observers. 2. OBSERVATIONS, DATA PROCESSING, AND DECONVOLUTION
We report on observations of Io in the near infrared (1–2.5 µm) using the 10-m W. M. Keck II AO system (Wizinowich et al. 2000) on 19, 20, and 22 February, 2001 UT (Table I). We used the NIRSPEC camera (McLean et al. 1998) in its imaging mode using a SCAM Rockwell HgCdTe detector sensitive in the 0.95- to 2.5-µm range. The total field of view is 4.3 arcsec with a pixel size of 16.8 milliarcsec (mas). The data were reduced in the standard way (sky subtraction, bad pixel removal, and flat-field correction), and photometric calibration was done using several photometric stars observed a few times throughout the night. The satellite magnitude (m v ∼ 5) and its angular size (∼1.2 arcsec) do not compromise the quality of the wavefront analysis. An angular resolution of ∼42 mas in H band (1.63 µm) was estimated by imaging a nearby star of a similar magnitude (obtaining its PSF, point spread function) just before and/or after the observing target. This corresponds to a spatial resolution of ∼130 km at the center of Io’s disk, roughly 2–3 times better than most of the global Galileo/NIMS observations (Dout´e et al. 2001). The angular resolution in K band (2.12 µm) is ∼50 mas. In J band (1.25 µm), resolution varies depending of the atmospheric seeing conditions (42–60 mas). Even though AO systems correct for atmospheric turbulence in real time, this correction is not perfect, and part of the energy of the PSF is lost in a halo surrounding the coherent peak. The TABLE I Information on the NISPEC/Keck AO Observations SSPa
SEP Date
Time (UT)
Filter
Long.
Lat.
Long.
Lat.
19 Feb. 2001 20 Feb. 2001 22 Feb. 2001
05:20–05:30 05:06–05:12 04:47–04:50
J, H, K J, H, K J, H, K +2 SW
102.8◦ 304.0◦ 348.1◦
2.8◦ 2.8◦ 2.8◦
91.6◦ 292.7◦ 336.8◦
2.8◦ 2.7◦ 2.7◦
a
SSP: sub-solar point.
125
direct effect of this imperfection is to blur the images. The amount of blurring is indicated by the Strehl ratio, which is the ratio of the maximum intensity of the recorded PSF to the maximum of the theoretical diffraction-limited PSF when both are normalized. Our PSF stars indicate a Strehl ratio of ∼20% in J band (1.26 µm) and ∼50% in K band (2.12 µm). The sharpness of our images can be further enhanced by applying an inversion process, such as a deconvolution algorithm, to the data. We used a new myopic deconvolution algorithm called MISTRAL (Myopic Iterative STep-preserving Restoration ALgorithm) and developed by ONERA (Office National d’Etudes et de Recherches Aerospatiales), especially aimed at AO observations of planetary objects (Conan et al. 1998). MISTRAL uses a stochastic approach to finding the best image reconstruction, using information about the object and the PSF. This requires several star images to get enough data about variations in the PSF. The main improvement of this technique over more classical methods is that it avoids both noise amplification and creation of sharp-edge artifacts or “ringing effects” and that it better restores the initial photometry. This algorithm has already been used on simulated and real AO observations (Conan et al. 2000, Marchis et al. 2001). 3. RESULTS
3.1. Image Analysis Our results are shown in Fig. 1. The first two rows in Fig. 1a display the 20 February JHK observations before (first row) and after (second row) deconvolution. The deconvolved images are sharper, with a resolution estimated to be 35 mas in H band (1.61 µm), i.e., equivalent to the telescope diffraction limit (105 km on Io’s surface). They display the same albedo features on the surface of the satellite as the Minnaert law reconstructed visible images based on the 10- to 23-km-resolution Galileo/SSI map (McEwen et al. 1998a). During the other two nights (Fig. 1b), several bright spots stood out with a much higher contrast. These correspond to thermal emission from active hot spots, surprisingly detected directly, against Io’s sunlit hemisphere. This never occurred at such short wavelengths during our previous four-year survey with the ADONIS AO system on the 3.6-m ESO telescope. Figure 2 displays two Keck AO images without deconvolution and a simulation of corresponding ADONIS images in the same wavelength range. The 010222A hot spot would have been detected by an AO system on a 4-m-class telescope, but none of the fainter hot spots (010219A and 010219B) on the other hemisphere can be seen on these 120-mas-resolution images. Our Keck data clearly indicate the presence of hightemperature volcanic activity whose multiwavelength detection was only possible because of the higher angular resolution provided by the 10-m Keck telescope, which was further improved by the powerful MISTRAL deconvolution method. 3.2. Hot Spot Flux Analysis Each detected hot spot has been labeled following our previous convention (Marchis et al. 2001) consisting of six numbers
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FIG. 1. (a) Jupiter-facing hemisphere observed with the Keck AO system. The basic-processed images from 20 February 2001 (first row) are displayed. The second row corresponds to the same images after applying the MISTRAL deconvolution process. Albedo features, similar to the 20-km-resolution reconstructed GALILEO/SSI image (right column) are easily detected. The last row shows the 22 February 2001 images, which are dominated by the presence of the Surt outburst. (b) Observations from 19 February 2001. Two hot spots, corresponding to Tvashtar (North) and Amirani, are clearly detected in the H and K bands. Note the bad quality of the J-band image after deconvolution, due to the poor seeing condition of this observing night.
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TABLE III Temperatures and Emission Areas from Two-Temperature Model
FIG. 2. Comparison of the 10-m Keck data with images as would have been seen with the lower resolution ADONIS AO system on the 3.6-m ESO telescope. Data in the left column correspond to real Keck AO observations obtained without deconvolution. The right column displays the corresponding images at the ESO-ADONIS resolution (0.11 in H band and 0.12 in K band). The Surt outburst would have been seen by ADONIS, but the other fainter hot spots on the opposite hemisphere cannot be detected with a 4-m-class telescope.
corresponding to the date of observation (YYMMDD), plus a letter for each source. To estimate the positions of the hot spots, the images were deprojected on a cylindrical map, using the subEarth point (SEP) from the physical ephemeris, and a Gaussian function was fitted through the emission profile. In addition to the uncertainty in the fit, the error in the derived position also includes a term introduced by the uncertainty in Io’s estimated limb for the cylindrical projection, ∼1/4 pixel. The position of each hot spot is listed in Table II and is indicated on a Galileo/Voyager visible cylindrical map (Fig. 3). These locations correspond precisely to the known active areas Amirani (010219A), Tvashtar (010219B), and Surt (010220A and 010222A). In Table II, we list the radiance of the thermal emission after subtracting the background radiation due to reflected sunlight
Hot spot name
Best T1 (K)
Best T2 (K)
A1 (km2 )
A2 (km2 )
010222A
880 ± 10
1470 ± 20
1760 ± 155
93 ± 9
and correcting for emission angle. Uncertainties come from several sources: the Poisson noise of the detector (1–5%), our estimate of the background emission (<2%), and the accuracy in the recovery of the initial photometry from the deconvolution method (<3%). The latter uncertainty has been estimated from deconvolution tests on simulated Io images (Marchis 2000). A classical deconvolution method would not recover the photometry of such bright sources as accurately, since the typical uncertainty using a Lucy–Richardson method is ∼10% for high-contrast sources (Marchis 2000). Finally, considering the uncertainties introduced by the measurement and the absolute magnitude of the photometric stars, the total observed uncertainty in the measured intensity should be ∼10% for the hot spots detected on 19 and 22 February. The flux transmitted by several broadband filters allows us to estimate the temperature and the area of the source assuming that the hot spot emission follows a classical blackbody law. The fitting process used a classical χ 2 minimization method, and the transmission profile of the filter was included in the calculation, assuming a standard transmissivity of the atmosphere (McLean et al. 1998). On 19 and 20 February, we detected the hot spots in H (1.61 µm) and K (2.12 µm) bands only. We thus fitted the spectral dependance of the emission by a single blackbody temperature for each hot spot (i.e., two parameters: temperature and area). On 22 February, the hot spot was also detected in the broadband J filter (1.26 µm) and in two narrow-band filters centered on 1.68 and 1.71 µm. A two-temperature blackbody law (i.e., four parameters: two temperatures, each with a particular areal coverage) gave a better fit than a single blackbody temperature law. Tables II and III list the temperatures and area calculated for each hot spot. 3.3. Amirani The 010219A thermal source coincides with Amirani, a wellknown hot spot discovered by Voyager (Pearl and Sinton 1982)
TABLE II Intensity, Temperature, and Emission Area of the Hot Spots Estimated from Blackbody Fits Hot spot name
Position (Long., Lat.)
Candidates
IJ
IH (GW/sr/µm)
IK
Best mono-T (K)
Area (km2 )
010219A 010219B 010220A 010222A
27 ± 3 N, 118 ± 3 W 58.5 ± 3.5 N, 124.5 ± 3 W 41 ± 3 N, 337 ± 5 W 41 ± 4.5 N, 340 ± 4.5 W
Amirani Tvashtar Surt Surt
— — — 1440 ± 20
115 ± 7 93 ± 6 12 ± 4 3470 ± 150
245 ± 11 310 ± 12 18 ± 2 5590 ± 180
990 ± 35 810 ± 15 1330 ± 270 1240 ± 20
103 ± 22 460 ± 100 10−3 (r < 50 m) 445 ± 42
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FIG. 3. Localization of the thermal sources detected on the AO images and comparison with a Galileo/Voyager map. The black square on the images indicates the position estimated with our AO observation (see Table II). We compare the position of Surt with two Voyager images and a Galileo image. Note the surface change feature in the second Voyager image. The change in appearance of this caldera may have been caused by the bright eruption seen from the ground by Sinton (1980). Its position is compatible with the bright outburst obtained from our data.
and still active during the Galileo era (Lopes-Gautier et al. 1999). The high-resolution thermal map of the volcano acquired with NIMS during the I24 and I27 flybys (October 1999 and February 2000) indicates that the active area was a 350-km-long north– south lava flow field, with a mean temperature of 420 K (Lopes et al. 2001). Our data do not display any north–south extension of the source. We estimate a higher temperature of 1000 K over an area of 100 km2 . I27 NIMS data show also that a small region in the middle of the lava flow field reached a color temperature of ∼1000 K. Since it is roughly located in one NIMS pixel (of size 10.5 km) its area should be less than or equal to 100 km2 . Our observation confirms the presence of this thermal emission, which could correspond to a lava breakout in the Amirani lava flow field. 3.4. Tvashtar The Tvashtar hot spot was first detected on 26 November 1999 from the ground (Howell et al. 2001) and by NIMS and SSI during the I25 Io flyby (McEwen et al. 2000). The eruption appeared to be a linear lava curtain. Since most of the detectors were saturated, only a lower bound of 1060 ± 60 K could be deduced from the NIMS data (Lopes et al. 2001). One year later this volcanic area was still active. A bright outburst was observed in December 2000 with the ADONIS AO system (Marchis et al. 2001), as well as with NIMS and SSI on Galileo. Two months later our data reveal the presence of a thermal source, 010219B, for which we derive a lava temperature of 810 K over an area
of 460 km2 . Assuming that the volcanic type of the Tvashtar eruption did not change over time, our estimate of 810 K may be interpreted as the average of a cooling lava flow (with a typical temperature of 400–500 K) and of an active fire fountain (with temperatures up to 1400 K). To derive better models for Tvashtar’s complex volcanic activity, one needs to analyze in detail both ground-based and Galileo NIMS/SSI data as obtained over the past two years. 3.5. Surt When we observed Io on 20 February 2001, we did not see any obvious activity on this near-Jupiter-facing hemisphere. Two days later, the images of the same hemisphere changed dramatically and revealed an extremely bright hot spot (010222A) at the position of Surt. The spectral dependance of the emission shows that this bright hot spot consisted of a 95 km2 area at an exceptionally high temperature of 1470 K and of a larger area, 1760 km2 , at 880 K. A simultaneously obtained H-band spectrum also indicates a temperature exceeding 1200 K (Fig. 4). An extrapolation of our blackbody temperature fit shows that Surt must have radiated ∼5000 GW/sr/µm at a wavelength of 5 µm, i.e., 10 times higher than the flux of the sudden short-lived eruptions normally referred to as “outbursts” and which may occur several times per year (Spencer and Schneider 1996). Previous outburst observations generally yield much lower 5-µm intensities. For example, the 1986/08/07 outburst (see Veeder et al. (1994); T = 1550 K and r = 8 km) gives a 5-µm brightness of
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3.6. Characterizing the Surt Outburst
FIG. 4. Spectroscopic AO observations of the Surt outburst. The inset displays a simultaneous image of Io using the slit-viewing camera. The H-band spectra obtained were processed using standard techniques, including atmospheric correction using a standard star, followed by subtraction of the observed spectrum of sunlit Io. Thus, the presented spectrum represents only thermal emission from the hot spot. The apparent absorption features in the final spectrum are artifacts. They arise in the atmospheric correction procedure due to imperfect removal of the standard star’s spectrum. The best fit of the spectrum indicates a temperature of 1200 K for the emission area, a temperature similar to the best mono-temperature deduced from the multiwavelength images (see Table II). Ideally spectra with greater wavelength coverage (e.g., from 1 to 5 µm) should be taken to get a better estimate of the temperature and area of the eruption; however, in this case we were limited by the fast approaching occultation of Io by Jupiter.
only ∼1400 GW/sr/µm, four times fainter than the Surt outburst reported here. More recently, the 9908A outburst reached a maximum observed 4.68-µm intensity of 1800 GW/sr/µm (Howell et al. 2001). Such violent activity has been seen once before at Surt. During its first 10 orbits Galileo did not detect any activity in this area, but in June 1996 activity was reported from the ground (LopesGautier et al. 1999, Spencer et al. 1997). Even earlier, between the consecutive flybys of the Voyager spacecraft, a very bright eruption (Sinton 1980) was observed in the thermal infrared (see Fig. 3). Images of Surt taken by the two Voyager spacecraft revealed the appearance of a dark and localized caldera, the origin of which may have triggered the bright infrared eruption. The position of this caldera is similar to the position of the bright outburst observed by us. This dark caldera was not seen on the first Galileo images; it probably had been covered by plume deposits from a nearby volcano in the intervening time. Surt seems to be an “outburst-type” volcanic area, imaged for the first time. Once the images were deconvolved, we discovered very faint activity on the 20 February data at the same location (010220A) (see Fig. 1a). The detection is marginal but strongly suggestive of a precursor to the big event. It corresponds to a very small area (less than 50 m radius) at a temperature up to 1400 K, like basaltic lava flows on Earth (McEwen et al. 2000).
We have fitted our Surt data with a silicate lava cooling model (Davies 1996), originally developed to interpret ground-based ionian observations and subsequently used to interpret Galileo/ NIMS spectra (Davies et al. 2000). The model calculates the distribution of temperatures and areas seen during an eruption as a function of time. Fitting observational data with the model output (an integrated thermal spectrum) thus produces a range of emitting surface ages. Assuming a basaltic composition for the erupting magma, the model fit to the Surt data produces a range of areas at temperatures continuously ranging from a fraction of a square kilometer at 1475 ± 0.5 K down to 8.9 km2 at 1030 ± 0.5 K. The total area of the entire thermal anomaly is approximately 800 km2 . The 1030-K area, the oldest area contributing to the observed thermal output, is less than 100s old. This suggests that the eruption style is extremely dynamic, indicative of a highly vigorous style of lava emplacement, such as large fire fountains. Such activity was seen at Tvashtar by Galileo SSI in 1999 (McEwen et al. 2000). More surprisingly, recent data collected during the Galileo/SSI I31 flyby (6 August, 2001) do not indicate any obvious changes in surface reflectivity in the Surt area, including no new low-albedo area (A. McEwen, private communication) as would be expected after an eruption. 4. CONCLUSION
The detection of these ionian eruptions at near-infrared wavelengths (<2.5 µm) is surprising and implies that we observed highly energetic hot spots which were never seen in our previous four-year survey using the 3.6-m telescope AO system. Comparing the thermal signature of the Surt eruption with others seen by Galileo NIMS (Davies 2001), one can see that the thermal output [(7.8 ± 0.6) × 1013 W] of this outburst is considerably larger than any other eruption seen by the Galileo spacecraft (see Fig. 5). This is in fact the most energetic outburst ever observed on Io. It is the largest in the range of outbursts detected from the ground in the thermal and far infrared (3–20 µm) after the Voyager encounters. Note that the majority of those outbursts were observed only by photometric measurements, which do not provide a very precise location of the emission area. Therefore the nature of the activity which corresponds to such outbursts has remained uncertain. The Surt outburst with an output of (7.8 ± 0.6) × 1013 W nearly matches the total thermal output from Io integrated over all wavelengths (1014 W, assuming an average global heat flow of 2.5 W · m−2 ; Veeder et al. 1994). The other eruptions, with a total output of 10.1 × 1012 W for Tvashtar and 5.0 × 1012 W for Amirani, are more energetic than the Pillan eruption observed during the C9 flyby (3.6 × 1012 W), the brightest eruption seen by Galileo. Adding the three eruption intensities, we derive a mean total output for the February 2001 period of 2.2 W · m−2 . These three extremely bright and short-lived hot spots can account for the estimated thermal heat flow of 2.5 W · m−2
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FIG. 5. Best-fit spectra to the observation. Crosses correspond to the observed intensity in broadband filters (see Table II). Squares indicate the narrow-band intensities of Surt centered at 1.68 and 1.71 µm. The blue and purple lines are the best mono-temperature blackbody fits for Amirani and Tvashtar. The bold red line is the model spectrum of Surt derived from the silicate-cooling flow model. The classical two-temperature model is overlaid with a red solid line with its two components dashed and dotted. For comparison, we display (in green) spectra of various active volcanoes as deduced from the Galileo/NIMS observations. The outburst detected by the AO observations clearly corresponds to a much stronger eruption. The total output of each eruption is indicated (in GW).
(Veeder et al. 1994) without even considering the output of the many cooler sources not detectable at short wavelengths. High angular resolution provided by adaptive optics systems on 8- to 10-m-class telescopes is a promising tool for monitoring the volcanic activity of Io from the ground with a spatial resolution better than the global Galileo/NIMS observations. With the Galileo mission coming to an end in November 2002, future monitoring of Io’s volcanism lies in the hands of terrestrial observers. Further instrumental improvements, such as a wider wavelength range (1–5 µm) and the ability to make spectroscopic AO observations, will allow us to better constrain the nature, temperature, and evolution of ionian eruptions. ACKNOWLEDGMENTS Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to R. Howell and J. Spencer for reviewing this article. This work has been supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement AST-9876783. Ashley Davies is supported by a NASA Planetary Geology and Geophysics grant 344-30-23-09. Part of this work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48
and the France–Berkeley Fund. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations of these ionian volcanoes would have been possible.
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