Keck AO observations of Io in and out of eclipse

Icarus 169 (2004) 250–263 www.elsevier.com/locate/icarus

Keck AO observations of Io in and out of eclipse Imke de Pater,a,b,∗ Franck Marchis,a Bruce A. Macintosh,c Henry G. Roe,d David Le Mignant,e James R. Graham,a and Ashley G. Davies f a Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720, USA b Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA c Lawrence Livermore National Laboratory, Livermore, CA 94550, USA d Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA e W.M. Keck Observatory, 65-1120 Mamalahoa Hwy., Kamuela, HI 96743, USA f Jet Propulsion Laboratory, California Institute of Technology, MS 183-601, 4800 Oak Grove Drive, Pasadena, CA 91109, USA

Received 1 April 2003; revised 14 July 2003

Abstract We present adaptive optics (AO) observations of Io taken with the W.M. Keck II telescope on 18 December 2001 (UT) before the satellite went into eclipse, and while it was in Jupiter’s shadow. Making these kind of Io-in-eclipse observations, as well as the associated data reduction and analysis are challenging; hence one focus of the paper is to explain the methods and tools used for these data sets. For the sunlit images Io itself was used as the wavefront reference source, while nearby Ganymede was used as reference ‘star’ when Io was in eclipse. Observations were obtained in K -, L -, and M-bands. The sunlit images have been deconvolved using MISTRAL. The Io-in-eclipse data were deconvolved with IDAC and MISTRAL. The former gives better results, both in absolute photometry and in matching the original images. We determined the flux densities of the hot spots from the original Io-in-eclipse data with StarFinder, as well as from the deconvolved images by integrating the intensity over the relevant areas. We determined the highly anisoplanatic PSF via a FFT method from the original data, and used this in StarFinder and as a starting PSF for IDAC and MISTRAL. We derived temperatures and areal coverage of all 19 spots detected in both K - and L -band images of Io-in-eclipse. We also determined temperatures and areal coverage of the hot spots visible on the L - and M-band images of sunlit Io. Most volcanoes contain a compact hot ‘core’ (
10 km2 at 600–800 K) within a larger area at lower temperatures (e.g., ∼ 102 –104 km2 at 300–500 K). The total heat flow contributed by these active volcanoes is 0.2 W m−2 , ∼ 8% of the average global heat flow measured at 5–20 µm by Veeder et al. [J. Geophys. Res. 99 (1994) 17095].  2003 Elsevier Inc. All rights reserved. Keywords: Io; Eclipse; Adaptive optics; PSF

1. Introduction The Voyager and Galileo flybys have provided a wealth of information on Io’s volcanism through images of active plumes, lava flows, and calderas (McEwen et al., 1998, 2000; Keszthelyi et al., 2001; Turtle et al., 2001, 2004), combined with maps of the thermal emissions obtained with the near-infrared mapping spectrometer (NIMS) (Lopes-Gautier et al., 1999, 2000; Lopes et al., 2001) and the far-infrared photopolarimeter-radiometer (PPR) (Spencer et al., 2000). Despite all this information, however, we still do not understand the detailed heating and cooling processes on this * Corresponding author.

E-mail address: [email protected] (I. de Pater). 0019-1035/$ – see front matter  2003 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2003.08.025

satellite. For example, we do not know if Io’s heat flow is constant or episodic over time, nor do we know if most of the heat is dissipated in an asthenosphere just below the crust, or in Io’s deep mantle (Segatz et al., 1988; Ross et al., 1990; Lopes-Gautier et al., 1999; Veeder et al., 2004). Continued observations are needed to resolve these issues, and with the demise of the Galileo spacecraft we depend on ground-based observations for such data. Infrared data of Io have been taken since the early 1970’s, and hence provide already a database of  30 years (e.g., Hansen, 1973; Witteborn et al., 1979; Sinton et al., 1983). The early data consist of disk-integrated photometric flux measurements, where volcanic outbursts are spotted via an increase in the total infrared flux density (Veeder et al., 1994). When imaging became feasible, Io’s angular size of ∼ 1 , only slightly larger than the angular resolution ob-

AO observations of Io-in-eclipse

tained under good atmospheric seeing conditions, prevented astronomers from pinpointing hot spots on Io’s disk. Moreover, since Io’s sunlit disk is very bright at near-infrared wavelengths, the contrast between thermal hot spot emissions and reflected sunlight is generally only visible at wavelengths longwards of ∼ 3 µm. Spencer et al. (1990), therefore, imaged Io during eclipses, while the satellite was in Jupiter’s shadow, so only emission from hot spots was seen. Separation of hot spots from each other, however, was still difficult due to the relatively low spatial resolution in conventional images (0.4–0.5). Accurate information on the location and size of hot spots could be obtained by making fast photometric measurements during eclipse observations, when Io disappears or reappears from Jupiter’s shadow, or during occultations of Io by other satellites (Goguen et al., 1988; Spencer et al., 1990, 1997; Howell et al., 2001). In recent years the spatial resolution that can be achieved in ground-based observations has vastly improved via the use of speckle and adaptive optics (AO) techniques. Such high resolution observations of Io have been conducted by Macintosh et al. (2003) using speckle imaging on the 10-m Keck I telescope of Io in eclipse at 2.2 µm, and by Marchis et al. (2000, 2001) using adaptive optics techniques on sunlit Io at 3.8 µm with the ADONIS system on the 3.6-m ESO telescope. The latter program gathered data for about 4 years during the Galileo–Io encounters. Over the past 1.5 years we have obtained multi-wavelength 1–2.5 µm Keck AO observations of sunlit Io, which reveal the presence of high-temperature hot spots which would not have been visible in conventional images (Marchis et al., 2002). In this report we present adaptive optics (AO) observations of Io conducted with the near-infrared AO camera NIRC2 on the Keck II telescope. All images were taken on UT 18 December 2001, about 1 month before the failed Galileo I33 encounter, the only encounter that would have provided high resolution images of the Jupiter-facing hemisphere. We present images of this hemisphere in reflected sunlight in Kcont (2.2 µm), L - (3.8 µm), and M- (4.7 µm) bands, using Io itself as the reference source for the wavefront sensor. Our L - and M-band images show both reflected sunlight and thermal emission from volcanic hot spots. At K -band the emission is dominated by reflected sunlight, and in these particular images no hot spots can be distinguished on Io’s disk. Depending on wavelength and geocentric distance, we typically get resolutions of ∼ 120–180 km near the center of Io’s disk, better than most global Galileo/NIMS images (e.g., Douté et al., 2001). As shown by Marchis et al. (2002), at such resolutions hot spots can be isolated, the location of such spots can be precisely determined, and with observations at more than one wavelength one can constrain the hot spot temperatures. In a complementary paper, Marchis et al. (2003) present our entire series of images in Kcont, L - and M-bands taken in the period December 18– 28, 2001 (UT). A movie of these images and a preliminary report was presented by Le Mignant et al. (2002).

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At wavelengths shortwards of ∼ 3.5 µm, hot spots are best observed when Io is in eclipse. Under such circumstances we see (practically) no reflected sunlight from the satellite, and the emission visible is essentially all thermal emission from hot spots. The Keck wavefront sensor (like most wavefront sensors) is sensitive to visible light only; and hence, while Io is in eclipse, it is generally impossible to observe the satellite with AO. On 18 December 2001 (UT) we had the relatively rare (perhaps once a year) opportunity that another Galilean satellite, Ganymede, was only 30 away from Io while the satellite was in eclipse (see Fig. 1). We

(a)

(b) Fig. 1. (a) Geometry of the jovian system during the Io eclipse (from the Planetary Ring Node: http://ringmaster.arc.nasa.gov). (b) Ganymede’s track with respect to Io on the steering mirrors.

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used Ganymede as wavefront reference star (i.e., closed the AO loop analysis on Ganymede), while obtaining science observations of Io. We obtained data in K - and L -bands which we present here together with the sunlit images.

2. Observations and data processing We present three sets of observations (Table 1), all taken on 18 December 2001 (UT) with the near-infrared AO camera NIRC2 on the Keck II telescope. The Keck AO system on the 10-m W.M. Keck II telescope is described by Wizinowich et al. (2000a, 2000b). The NIRC2 camera features a 1024 × 1024 Aladdin-3 InSb detector1 with a pixel size of 9.94 ± 0.05 marcs. Io’s diameter at the time of the observations was 1.19 . All images were processed in the conventional way: flat-fielded, sky subtracted, and bad pixels replaced by the median of surrounding pixels. Individual frames were shifted to a common center, so co-adding several frames would lead to increased signal-to-noise in the final images. We removed a low-amplitude ripple pattern, caused by readout electronics, from the L -band images of Io-in-eclipse via masking in the Fourier domain. Photometric calibration was performed on the star HD22686 (Elias et al., 1982). We converted stellar magnitudes to flux units using the spectrum of Vega (Colina et al., 1996), assumed to be a zero-magnitude star at all wavelengths, by averaging over the bandpass of our filters, and using atmospheric transmission spectra generated with the ATRAN modeling software (Lord, 1992). We derived a flux density (at 1.0 airmass) of 4.4 × 10−10 W m−2 µm−1 for Vega (0 mag star) in K -band. At L - and M-bands we obtained 5.4 × 10−11 and 2.3 × 10−11 W m−2 µm−1 , respectively. The zero-magnitude flux densities are very similar to those published by Tokunaga (2000). We observed HIP33649 and HIP30769 to determine the PSF (Point Spread Function) at the various wavelengths bands. These stars have similar brightness to Io and allow 1 The Aladdin detector was developed by Raytheon Infrared Operations (RIO), in collaboration with NOAO and USNO.

to asses the AO performance despite the fact that the reference sources (Io, Ganymede, and the star) have a different angular size. The angular resolution, or FWHM (Full Width at Half Maximum) at K - and Kcont-bands was measured to be 0.05–0.06, at L -bond it was ∼ 0.08 , and at M-band it was ∼ 0.10–0.11. The Strehl ratios (the observed PSF peak intensity divided by the maximum of the theoretical diffraction limited PSF when both are normalized) at K , L , and M were measured to be ∼ 0.15–0.20, 0.5–0.6, and ∼ 0.3, respectively. We show the basic-processed data, as well as deconvolved images in Figs. 2 and 3. The sunlit Io images were deconvolved using MISTRAL (Myopic Iterative STeppreserving Restoration ALgorithm), an algorithm developed by ONERA (Office Nationale d’Études et de Recherches Aerospatiales), especially aimed at AO observations of planetary objects (Conan et al., 2000). MISTRAL uses a stochastic approach to finding the best image reconstruction, using information about the object and the PSF. The main improvement of this technique over more classical methods is that it avoids both noise amplification and creation of sharpedges artifacts or ‘ringing effects,’ and that it better restores the initial photometry. We have successfully used this algorithm in the past for AO observations of Io obtained with the ADONIS ESO-3.6 m (Marchis et al., 2001) and Keck AO systems (Marchis et al., 2002). The images of Io-ineclipse were deconvolved both using MISTRAL and IDAC, a non-linear, iterative scheme using a conjugate gradient minimization algorithm applied to an error metric derived from the data (Jefferies and Christou, 1993). This ‘blind’ deconvolution algorithm estimates both the object (assumed not to vary with time during the observations) and PSF (varying with time) from a set of observations. The algorithm is described in more detail by Christou et al. (1998), and has been used to deconvolve ADONIS ESO 3.6-m images of Io (Marchis et al., 2000). We used both deconvolution techniques on the Io-in-eclipse data to test their similarities and differences, and to determine the best algorithm for such images. Comparison of rows 1 and 2 in Fig. 2 clearly shows the improvement in image quality/sharpness resulting from the

Table 1 Summary of observations UT time (h:min)

Filter

Wavelength (µm)

Guidestar

Sepa (arcs)

Target

CMLb (deg)

N ×tint (N × s)

07:47 07:34 07:36 08:45 08:51 08:59 09:10 09:25

Kcont L Ms Kcont L Ms K L

2.25–2.28 3.43–4.13 4.55–4.79 2.25–2.28 3.43–4.13 4.55–4.79 1.95–2.30 3.43–4.13

Io Io Io Ganymede Ganymede Ganymede Ganymede Ganymede

NA NA NA 18.2 19.9 22.7 27.3 34.2

Sunlit Io Sunlit Io Sunlit Io Sunlit Io Sunlit Io Sunlit Ioc Io in eclipse Io in eclipse

337.2 335.4 335.6 345.4 346.2 347.4 348.9 351.0

12 × 3 6 × 2.75 5 × 5.5 31 × 3 9 × 2.75 3 × 2.75 6×5 6 × 2.75

a Angular separation between Io and Ganymede (NA = not applicable). b Central meridian longitude on Io, as seen from Earth. Sub-Earth latitude is 1.83◦ . c Seeing was very poor; images were not used.

AO observations of Io-in-eclipse

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Fig. 2. Images of Io’s sunlit hemisphere at Kcont, L , and Ms bands, taken on 18 Dec. 2001 at ∼ 07:30 UT (top two rows) and near 09:00 UT (bottom row), before Io went into eclipse. The images in the top row have been processed in the usual way; the images in the second and third rows have been deconvolved using MISTRAL. The AO system was locked on Io in the top and second row; in the third row the AO system was locked on Ganymede, at a distance of ∼ 20 from Io. The images in column 4 are reconstructed from the Galileo/SSI data at a resolution of 20 km (Courtesy: P. Descamps). All images have been rotated so Io’s north pole is up.

image deconvolution process. At K -band no hot spots are seen, and variations in surface albedo dominate the brightness distribution. The similarity in albedo features between the K -band and visible light Galileo image is striking (see also Marchis et al., 2002, 2003); dark calderas clearly show up in the Keck image as well as the lighter deposits surrounding the volcanic centers. At L - and M-bands sunlight is much less intense, whereas the thermal emission from hot lava is more intense than at K -band. As a result hot spots do stand out against Io’s sunlit disk at these longer wavelengths. The deconvolved images are sharper, show many volcanic centers and, at L -band, some variations in surface albedo. Row 3 in Fig. 2 shows deconvolved images of Io’s sunlit hemisphere where Ganymede was used for wavefront sens-

ing. The quality of these images, in particular at K -band, is relatively poor compared to those in row 2, because of the anisoplanatic effect. The anisoplanatic angle, θ0 , is the angular distance for which the rms error in the corrected image is less than 1 radian. This angle is a function of the height of the turbulence layers in our atmosphere and the atmospheric seeing. At zenith this value would be close to 40 in K, assuming the coherence length in the atmosphere r0 = 20 cm at visible wavelengths. Hence θ0 , like r0 , increases as λ6/5 , where λ = wavelength; hence θ0 is larger at M-band than K band. Since the airmass A did not change significantly while Io was in eclipse, we can ignore the dependence θ ∝ A−8/5 . Hence having our reference star ∼ 20–30 away from the astronomical object lowered the Strehl ratio significantly, while enhancing the PSF halo; at K -band the Strehl ratio

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Fig. 3. Images of Io in K - (column 1) and L -bands (column 2) while the satellite was in eclipse. The top row shows the basic-processed images; row 2 shows images after deconvolution with IDAC, and row 3 shows the results after deconvolution with MISTRAL. The various hot spots are identified in the top row (see Table 2 for coordinates and common name). The IDAC and MISTRAL deconvolved images were convolved with a gaussian beam with a FWHM of 0.03 at K -, and 0.04 at L -band (see text). All images have been rotated so Io’s north pole is up. On the right side of the figure we show: a Galileo SSI reconstructed image at the time of the observations (top row), and temperature maps as derived from the K/L eclipse data (row 2), and from the L/M sunlit images (row 3). The values for the various temperatures are listed in Table 4.

was only a few percent. This results in an overall degrading of the images, as shown, even though special care was taken to provide MISTRAL with a PSF that most closely resembled the telescope response to a point source given the anisoplanatic observing conditions (see Section 3). In Fig. 3 we show Io-in-eclipse in K - and L -bands. The top row shows the basic-processed images; rows 2

and 3 show deconvolved images, as described in detail in Section 3. We indicate all volcanic hot spots on these images with letters, A–W, in order of decreasing intensity at L -band. Identification and location (ionian W. longitude and latitude) are indicated in Table 2. An X indicates that the hot spot is seen on the K -band (column 5) and L -band (column 6) images in eclipse, and in the L -band (column 7)

AO observations of Io-in-eclipse

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Table 2 Hot spot flux identification Letter A B C D E F G H I J K L M N O P Q R S T U V W

Hot spot Loki Dazhbog Svarog Surt Janus Ulgen Masubi Sengen Uta Fuchi

Euboea Mihr Nusku

Kanehekili

W. longitude φ (deg) 311 ± 2 313 ± 3 291 ± 4 339 ± 2 39 ± 2 284 ± 4 59 ± 5 311 ± 2 302 ± 2 23 ± 2 328 ± 2 326 ± 1 305 ± 2 1±1 357 ± 1 304 ± 2 6±4 353 ± 3 10 ± 1 37 ± 2 14 ± 2 51 ± 4 41 ± 3

Latitude θ (deg)

K-band, eclipse UT 09:10

L-band, eclipse UT 09:25

L-band, sunlit UT 07:34

M-band, sunlit UT 07:36

X X X X X X X X X X X X X X X X X

X X Xa X X X X X Xa X X Xa X X X X X X X X X X X

X X

X X

X X X

X X X

X X X X

X

9±1 49 ± 2 48 ± 1 41 ± 1 −8 ± 1 −42 ± 1 −48 ± 1 −33 ± 1 −44 ± 1 −39 ± 1 25 ± 1 34 ± 1 −61 ± 2 9±1 −49 ± 1 −18 ± 1 −68 ± 2 −7 ± 1 −15 ± 1 −20 ± 1 31 ± 1 34 ± 1 46 ± 1

X X

Xb X Xb Xb

X X X Xb X Xb Xb X X Xb

An X in column 5–8 indicates that the hot spot is seen on this image. Approximate West longitude and latitude, as derived from the Io-in-eclipse data, are recorded for each volcano. Io rotates ∼ 17◦ in 2 h, so that Janus is on the East (left) limb at 07:30 UT, and Daedalus is visible at W. longitude = 274◦ , latitude = 20◦ . a Only seen after deconvolution with IDAC. b Only visible by analogy with Io-in-eclipse maps.

and M-band (column 8) images taken two hours earlier in sunlight. Before deconvolution, we see 16 sources in the Ioin-eclipse images in both K - and L -bands; 3 sources are seen in K and not in L , while 4 other sources are only seen in L and not in K . Sources seen in L and not in K are likely low-temperature sources, whereas sources seen in K and not in L result from a poorer angular resolution in L . These last sources are seen in L -band after deconvolution with IDAC (Section 3), bringing the total number of sources seen at both K - and L -bands up to 19.

3. Hot spot intensities The images in Figs. 2 and 3 provide information on the number, location, and temperature of hot spots. A qualitative comparison of the images at the different wavelengths provides information on the relative temperatures and areal coverage of the volcanoes: for example, Dazhbog relative to Surt is much brighter in L - then in K -band (Fig. 3), suggesting that Dazhbog at this time had a much lower temperature than Surt. In order to determine precise temperatures, we need to extract the flux density of each spot at the different wavelengths. For the sunlit L - and M-band images from ∼ 7:30 UT we determined the flux density from the Mistral deconvolved images simply by integrating the total flux density over the source region and subtracting the background.

The flux densities thus derived are accurate to ∼ 5–10%; including the photometric accuracy, we assign an error of 10–15% to all derived intensities. All hot spot intensities are corrected for foreshortening effects: assuming Io’s disk to be smooth and perfectly spherical, spots near the limb will be foreshortened by cos(θ − θE ) cos(CML − φ). Table 2 provides the longitude φ and latitude θ for each volcano, while the central meridian longitude, CML, is listed in Table 1. The sub-Earth latitude, θE = 1.83◦ . Topography on scales of a few 100 m, however, may play a major role in determining the effective area visible to us (Marchis et al., 2001). The individual hot spot intensities at L - and M-bands are listed in Table 3, columns 9 and 10, respectively. When Io is seen in eclipse, practically no sunlight is reflected off the disk, and we can assume that all emission is thermal radiation from hot spots and/or lava flows. Io’s disk-averaged surface temperature from solar insolation alone is ∼ 130 K (Nash et al., 1986). Integrated over Io’s disk as projected onto the sky, this surface temperature contributes a total flux density of 3.7 × 10−12 GW sr−1 µm−1 at 2.2 µm, 4.8 × 10−4 GW sr−1 µm−1 at 3.8 µm, and 3.2 × 10−2 GW sr−1 µm−1 at 4.7 µm. This is negligible compared to individual hot spot contributions. Hence the observed brightness distribution is simply the convolution of the PSF with the hot spots, which are in all likelihood unresolved in our images. Thus it would be straightforward to determine the hot spot flux densities if the PSF is known. Unfortu-

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Table 3 Hot spot flux densities Hot spot

Volcano

Flux K eclipse StarFinder

Flux L eclipse StarFinder

Flux K eclipse IDAC

Flux L eclipse IDAC

Flux K eclipse MISTRAL

Flux L eclipse MISTRAL

Flux L sunlit MISTRAL

Flux M sunlit MISTRAL

A B C D E F G H I J K L M N O P Q U V

Loki Dazhbog Svarog Surt Janus Ulgen Masubi Sengen

1.28 0.34

19.53 10.40

20.01 9.00

14.8 12.4

45.1 37.1

2.60 2.68 5.09 4.28 1.42

2.5 2.3 3.5

4.7 2.3 7.6

1.1

1.5

0.14 0.28

1.38 0.84

1.20 1.00 0.60 0.857 0.18 0.11 0.09 0.28

2.80 3.23 3.85 5.01

Uta Fuchi

1.4 1.13

3.1 2.18

Euboea Mihr Nusku

0.53 0.11 0.22 0.22 0.18

1.57 0.53 0.71 0.63 0.92

16.60 9.71 0.43 2.60 2.80 4.09 4.28 1.19 0.65 1.44 1.10 0.59 1.26 0.38 0.87 1.30 0.33 0.10 0.04

1.33 0.48

1.14 1.09 1.08 0.91 0.28

1.22 0.32 0.14 1.08 0.85 0.65 0.77 0.26 0.26 0.09 0.27 0.06 0.14 0.01 0.18 0.16 0.16 0.04 0.01

0.62 0.04 0.25 0.28 0.18 0.05 0.02

0.87 0.29 0.83

4.95 4.66

66.2 31.7

4.96

30.0

4.86

35.7

Totala Observeda

Hot spots

1.48 0.27

Flux densities are recorded in GW sr−1 µm−1 , corrected for viewing angle. Typical errors are 10–15%. a Flux densities listed in last two rows have not been corrected for viewing aspect.

nately, because the reference star (Ganymede) was ∼ 30 away from Io, and moving with respect to Io, the anisoplanatic effect is large and varied over time, so that observations of stars do not resemble the PSF. We note, however, that although the anisoplanatic effect is large and varying with time, the PSF is pretty constant over Io’s ∼ 1 disk at each instant. 3.1. StarFinder We used the routine StarFinder, developed for stellar field analysis in IDL (Diolaiti et al., 2000), to identify hot spots and determine the relative flux densities of the volcanic areas. This procedure works well for crowded star fields (i.e., volcanic hot spots in our case) if the PSF known. Unfortunately, as mentioned above, the PSF during observations where Ganymede was used for wavefront sensing is not known. In principle one can use StarFinder to extract the PSF from the observations, if indeed the field contains a large number of stars. This procedure did not work, perhaps because we detected only three bright volcanoes and many weaker ones. We use an alternative method, which has been developed by Graham (2002, private communication). We have constructed a model for the PSF which we constrain from the data themselves. The approach is based on noting that a star field can be represented by a sum of randomly located, appropriately normalized delta functions. Although the volcanoes on Io are confined to the satellite’s disk, they appear pretty random when considering the 256 × 256 (2.56 ) sub-

image. In addition, they are almost certainly unresolved, and hence Io-in-eclipse resembles a star field. The observed image can be thought of as a random number of delta functions convolved with the PSF. In the Fourier domain this convolution can be written as the product of the Fourier transform of the PSF, i.e., the optical transfer function (OTF), and a field, exp[iφ(k)], where the phase, φ(k), is a gaussian random variable. Since the expectation value, is exp[iφ(k)] = exp[−(1/2)φ(k)2 ] for a large enough ensemble of stars, the Fourier transform of our image tends to the OTF times this constant. We assume that the OTF can be approximated by a phenomenological model for the PSF arising from the adaptive optics system (the modulation transfer function, MTF): a seeing halo, a near-diffraction limited core and a white noise floor. This assumes azimuthal symmetry, a limitation which will be addressed later. The amplitude versus spatial frequency of the power spectrum of the image is shown in Fig. 4 (dots), with superposed the best-fit model (lines) MTF: the broad seeing halo at spatial frequencies 0 < k/pixel < 0.05, the neardiffraction limited core at k < 0.2, and white noise background level at all frequencies. The sum of all components is indicated by the solid line. This MTF function has been Fourier transformed back to the image plane to yield a PSF that most closely resembles that of the data. As mentioned earlier, we assumed azimuthal symmetry in deriving this PSF. It is clear, however, from the Io images that the core of the PSF is markedly asymmetric, very similar to an image of a star where the star itself is used for wavefront sensing. A star image, however, lacks the pronounced broad halo,

AO observations of Io-in-eclipse

257

(a) (a)

(b)

(b) Fig. 4. The modulation transfer function (MTF) at K -band (a) and L -band (b), with superposed fits for the seeing halo, the near-diffraction limited core, and a white noise floor. The scale along the Y -axis depends on exp[iφ(k)] (see text), but can be considered arbitrary for our purpose.

which is introduced by the anisoplanatism. We therefore constructed our PSF by using the star PSF as the core of our PSF, with the halo as constructed from the MTF. The anisoplanatism further tends to cause the halo to be elongated in the direction of the guidestar. We clearly see this effect in the K -band, but not at L -band. Assuming that the convolution of our point-source volcanoes with the PSF equals our observations, we obtained a best fit for the ratio b/a = 0.65 (b is the short halo axis, a the long axis), and the halo being slightly depressed (by 0.85) compared to the originally MTF derived halo. The resulting PSFs at K - and L -bands are shown in Fig. 5. We used StarFinder to extract the flux density of each volcano at each wavelength, with the PSF as derived above. We detected about 20 hot spots at each wavelength. The total flux density in the hot spots as determined with StarFinder is listed in the second-to-last row in Table 3, columns 3

Fig. 5. The PSF as derived from the data at K -band (a) and L -band (b) (see text for details). The image is 2.56 arcs across.

and 4 (4.95 and 66.2 GW sr−1 µm−1 ), respectively. For comparison, the last row lists the total observed flux densities, as determined by integrating over the emission area on the original basic-processed images (Fig. 3, top row; 4.66 and 31.7 GW sr−1 µm−1 , respectively). Because of the large discrepancy between observed and extracted values, we scaled all hot spot intensities to force the total flux density to be the same as the observed value. After this scaling (i.e., division by a factor of ∼ 2 at L -band), and after correction for foreshortening effects, we listed the intensities of the 14 hot spots detected at both K - and L -wavelengths in Table 3, columns 3 and 4, respectively. We judge the reliability of the derived flux densities in two ways: (1) As mentioned above, we compare the total flux density from all hot spots retrieved via StarFinder with the total flux density that was observed. The total flux density of all hot spots detected with StarFinder in K -band appears to be very similar to the total observed flux density. As mentioned above, the StarFinder flux density at L -band was about two times larger than the observed value.

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Fig. 6. Difference maps of Io-in-eclipse data. The top row shows the difference between the original observed image (top row in Fig. 3) and an image constructed from the point sources extracted by StarFinder, convolved with the PSF (those displayed in Fig. 5). Row 2 shows the difference of the original images with the deconvolved IDAC image, convolved with the PSF as determined by IDAC. Row 3 shows the difference image between the original images and the deconvolved MISTRAL images, convolved with the MISTRAL determined PSF. The minima and maxima, compared to the peak intensities in the original images, are indicated for each figure. All images have been rotated so Io’s north pole is up.

AO observations of Io-in-eclipse

(2) We inspect the difference between the original map and the ‘model’ map, where the ‘model’ map consists of the retrieved StarFinder intensities convolved with the PSF (Fig. 5). In Fig. 6a we show the original Io map (from Fig. 3, top row) minus this ‘model’ StarFinder map. These difference maps were not scaled or normalized to the observed flux density. Peak values in K vary from −14% to +17% of the maximum value in the observed K map. At L -band the numbers range from −15% to +5% of the observed maximum at L -band. 3.2. Deconvolution methods An alternative technique to determine photometric flux densities from images is through deconvolution of the data, such as we have done for the sunlit Io images. Below we describe our application and results of using IDAC and MISTRAL on the Io-in-eclipse data. As mentioned above, on these images we primarily see thermal emission from hot spots on the satellite, which are likely point sources with respect to the angular resolution of the images. Hence we expect our deconvolved image to consist of a number of point sources, which when convolved with the PSF should equal the images as observed, i.e., those displayed in the top row of Fig. 3. IDAC deconvolves a series of images, and determines the object (assumed not to vary over time) and a series of PSFs (one per image in the original series of images) that best matches the data. The results of the algorithm depend on the initial ‘guess’ of the PSF, provided by the user (usually an average star image), and a ‘cutoff’ or maximum in the spatial frequencies, k/pixel, of the deconvolved image. Usually this number is chosen to be approximately equal to the FWHM of the telescope, or k/pixel ≈ 0.2. However, we know that Io-in-eclipse images consist basically of a number of point sources, and we found that it is best to choose k/pixel ≈ 1, i.e., allow the algorithm to ‘overresolve’ the image. It is further essential to provide a PSF with an extended halo, one which closely matches the ‘observed’ PSF, including the anisoplanatic effect. We used the PSF as determined in Section 3.1 (Fig. 5). Our final IDAC deconvolved images are shown in row 2 of Fig. 3. Since the algorithm deconvolved the images down to a series of delta functions at an angular scale much smaller than the FWHM of the telescope, we convolved the IDAC images with the center of a gaussian beam2 with a FWHM of 0.03 at K -band and 0.04 at L band, i.e., a factor of 2 smaller than the ‘nominal’ resolution of the telescope. Most hot spots have been recovered with IDAC, and several spots which went unnoticed in the original L -band image were recovered after deconvolution; the

2 To avoid gaussian wings from individual sources to interfere with nearby sources, we convolved the images with only the center of a gaussian beam, i.e., where the intensity of the beam > 10% of the peak value.

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intensities3 of the 19 spots seen at both K - and L -bands are listed in Table 3, columns 5 and 6, respectively. As in the StarFinder intensities, these individual hot spot intensities were first normalized to the observed value, and then corrected for foreshortening effects. Note that the total flux density recovered by IDAC was very similar to the observed value (they differ by ∼ 6%). A difference map (original from Fig. 3, top row, minus IDAC map convolved with the PSF) is shown in Fig. 6b. The results obtained via MISTRAL are also best when providing MISTRAL with a PSF that closely resembles the observed anisoplanatic PSF. The resulting images, after convolution with the core of a gaussian beam with a FWHM of 0.03 at K - and 0.04 at L -band, are shown in row 3 of Fig. 3. Difference images (no scaling) are displayed in row 3 of Fig. 6. The total flux recovered in the MISTRAL images exceeds the observed values by 4% at K - and 12% at L -band. Difference maps, however, show quite large residual values. Note that, as for StarFinder and IDAC, the individual hot spot intensities listed in Table 3 were first normalized to the total observed flux, and then corrected for foreshortening effects.

4. Discussion Table 3 lists the flux densities of all sources detected using the techniques described above. We only list sources that were detected at both bands. The values derived according to the three techniques are very similar. However, based upon a comparison of the Io-in-eclipse residual maps and of the difference between the total observed Io-in-eclipse flux with that retrieved using the three different methods, we trust the IDAC derived values best. Moreover, with IDAC we were able to retrieve almost all sources. In the following we will use these IDAC-derived numbers for the Io-in-eclipse data, and the MISTRAL numbers for the sunlit images (both are printed bold-faced in Table 3). The sunlit L values agree quite well with the Io-ineclipse L intensities, despite the fact that the viewing geometry is slightly different (note that the intensities were corrected for foreshortening effects). The largest discrepancy is seen for Dazhbog; this difference is likely caused by Dazhbog’s close proximity to Io’s limb, where errors in pinpointing its location, and hence errors in the derived foreshortening effect, are large. The observed ratio in the K/L and L/M flux density give the color temperature, which yields a fairly accurate measurement of the physical temperature if the emitting lava is uniform in temperature. The areal coverage determines the 3 We used several different procedures to determine the intensities of the individual hot spots on the Io-in-eclipse images. Best results were obtained by using either TVSTAT in AIPS, a software package developed and distributed by the National Radio Astronomy Observatory (NRAO), or by integrating over the source region within IDL.

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absolute value of the flux densities. The values (temperature in K, radius in km, assumed to be a circular source) in columns 3 and 4 of Table 4 were derived from the K/L ratios of the Io-in-eclipse data, and in columns 5 and 6 from the L/M ratios of sunlit Io. Typical photometric erros are 10– 15%. The error-bars in Table 4 show how the temperatures change if the K intensity is raised (lowered) by 10% and the L intensity simultaneously lowered (raised) by 10%. We similarly derived the uncertainties listed for the L/M images. We further calculated the intensity of each hot spot expected at M-band, given the temperature and areal coverage as derived from the K/L ratios. For all hot spots (except Janus) the observed M-band intensity is a factor of 2–5 higher than expected based upon the K/L values, indicative of a broad and continuous range of temperatures on actual lava flows, i.e., low-temperature flows that are only seen at L/M bands, in addition to higher-temperature lava. Overall, as one might expect, the high temperature hot spots are usually confined to small areas, in contrast to the large low-temperature volcanic regions. We should note here that even higher temperature vents, if confined to sub-km areas, would escape our detection. We used the temperatures in Table 4 to calculate the total power output from the Jupiter-facing hemisphere. The temperatures/areas as derived from the K/L data result in a total volcanic heat flow of ∼ 720 GW sr−1 , and the temperatures/areas derived from the L/M data result in ∼ 3430 GW sr−1 . The total heat flow due to portions of these active volcanoes is thus about 0.2 W m−2 , or ∼ 8% of the average global heat flow of 2.5 W m−2 as derived from 5– 20 µm data by Veeder et al. (1994). Our K/L and L/M

band values suggest that the high temperature volcanoes, detectable at the short wavelengths, contribute ∼ 20% of the 0.2 W m−2 energy detected by us, i.e., less than 2% of the average global heat flow. Moreover, we note that even at these short wavelengths, the total energy is dominated by the low-temperature sources Loki, Dazhbog and Mihr, simply because they cover much larger volcanic areas. The blue-green-red images in Fig. 3 show a map of the temperatures derived for all hot spots. We simply convolved the temperatures in Table 4 with a gaussian beam (center of Gaussians only—see footnote 1) of 0.03 . Note that these maps merely depict the temperature. Loki, the brightest volcano on the observed Io images appears dim on the temperature maps, indicative of an extensive cool surface. Relative to L - and M-bands, there is little thermal emission at short wavelengths, indicating that the total area at high temperatures is relatively much smaller than the large cool area. This is consistent with previous observations of Loki obtained by Galileo NIMS (Davies et al., 1999), and is an indication of a relatively quiescent eruption style, without large fire-fountaining or turbulent flows (Davies, 2001). Galileo imaged the eruption of Dazhbog in August 2001. This eruption appears to have emplaced flows over a wide area that have subsequently cooled, and it is these relatively cool surfaces that dominate the thermal output from Dahzbog. One of the hottest volcanoes detected by us is Surt, a relatively compact region the core of which is at ∼ 780 K. This volcano erupted in 2001, between February 20 and 22 (UT), and was the largest volcanic outburst ever recorded on Io (Marchis et al., 2002). As in Marchis et al. (2002), we obtained model fits to the spectrum of Surt of a cooling basaltic

Table 4 Hot spot temperatures Hot spot

Volcano

Temp. K/La (K)

Radiusb K/L (km)

A B C D E F G H I J K L M N O P Q U V

Loki Dazhbog Svarog Surt Janus Ulgen Masubi Sengen

532 ± 20 461 ± 14 760 ± 40 781 ± 45 713 ± 35 626 ± 26 622 ± 26 662 ± 30 780 ± 42 518 ± 20 682 ± 30 574 ± 22 584 ± 24 435 ± 14 655 ± 30 594 ± 25 808 ± 45 777 ± 42 724 ± 40

6.5 ∓ 3.5 8.7 ∓ 4.7 0.23 ∓ 0.18 0.8 ∓ 0.5 1.1 ∓ 0.6 1.8 ∓ 1.0 2.0 ∓ 1.1 0.9 ∓ 0.5 0.4 ∓ 0.2 2.1 ∓ 1.2 0.8 ∓ 0.4 0.9 ∓ 0.5 1.3 ∓ 0.7 2.2 ∓ 1.2 0.8 ∓ 0.4 4.9 ∓ 1.5 0.25 ∓ 0.15 0.16 ∓ 0.08 0.14 ∓ 0.08

Uta Fuchi

Euboea Mihr Nusku

Temp. L/Ma (K)

Radiusb L/M (km)

339 ± 30 342 ± 30

48 ∓ 18 42 ∓ 15

438 ± 44 697 ± 100 403 ± 40

5.6 ∓ 2.0 1.1 ∓ 0.4 9.6 ∓ 3.4

541 ± 70

1.6 ∓ 0.6

380 ± 40 432 ± 45

8.2 ∓ 3 4.0 ∓ 1.4

a Hot spot temperatures and areas were derived using the bold-faced numbers in Table 3. b Radius of the hot spot is the radius of a circle with the same surface area as required to match the data. The data have been corrected for viewing angle.

The error-bars reflect the change in temperature and radius when the volcanic fluxes are simultaneously increased at the shorter, and decreased at the longer wavelength by 10%, or vice versa.

AO observations of Io-in-eclipse

lava flow using the Davies (1996) model. The results are shown in Fig. 7. Each curve shows the range of areas and temperatures produced by the best model fit. The thermal spectrum of the volcano is the sum of all of the areas, each at a different temperature from the eruption temperature of 1475 K, the liquidus temperature for basalt. A bin size of 1 K is used. The distributions and ages of the surfaces reveal much about the style of volcanism taking place. Curve A is the fit to the February 2001 Surt outburst (Marchis et al., 2002). The total area of this anomaly is over 800 km2 , and the ages of the surface are very young, indicative of a lava fountain episode. This volcano is apparently still active in December 2001, as evidenced from the high temperature observed. Model fits to the December 2001 epoch are shown by curve B. Compared to the initial eruption, the modeling suggests that Surt has cooled considerably, covering an area of 195 km2 and a much broader temperature range, down to ∼ 300 K, indicative of flows with a surface age of ∼ 1 month. Janus is particularly intriguing as the derived areas and temperatures are remarkably similar from both K/L and L/M ratios, suggesting that the entire hot spot may be visible at these wavelengths. This implies a small uniformly hot surface and a lack of a broader cooler region at Janus. On Fig. 7, curve C, we show a model fit of a cooling lava flow to the data. These calculations suggest a small, vigorous eruption covering only 4 km2, but all of the surface is at high temperatures. The eruption is not as vigorous as the Surt outburst from February. There are a number of possible explanations for Janus’ thermal spectrum. The eruption may have been caught at a very early stage, and the emplaced lavas are very young (Fig. 7), and therefore still hot. The erupted material may have a high-enough gas content to cause fragmentation

Fig. 7. Fits of Davies’ (1996) cooling lava flow model to the multi-wavelength AO data at Surt, Janus, and Loki. Each curve shows the range of areas and temperatures produced by the best model fit. The thermal spectrum of the volcano is the sum of all of the areas, each at a different temperature from the eruption temperature of 1475 K. Curve A is the fit to the February 2001 Surt outburst (for comparison; from Marchis et al., 2002). Curve B is our fit to the Surt data presented in this paper. Curve C is a fit to our Janus data, and curve D to Loki.

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of the magma column into fire fountains, such that the resulting thermal emission is dominated by high-temperature (> 1000 K) areas. Such eruptions have been seen, e.g., at Tvashtar (Keszthelyi et al., 2001; Wilson and Head, 2001), Pillan (Davies et al., 2001), perhaps at Loki (Davies, 1996), and Surt (Marchis et al., 2002). Alternatively, Janus might be the site of a lava lake or lava pool where the surface crust is being constantly disrupted, perhaps by gases exsolving from the lava. This process has been used to explain the long-lived eruption at Pele and the preponderance of thermal emission towards short infrared wavelengths (Davies et al., 2001): cool crust is rapidly recycled into the lava lake and thermal emission is dominated by lava at high temperatures, exposed at the plume eruption location and where the crust is being disrupted and destroyed. Based upon a combination of Galileo and Cassini data, Radebaugh et al. (2004) suggest that Pele also exhibits lava erupting in high fountains within rapidly overturning lava lake. For comparison, we also show a (preliminary) model fit to our observations of Loki (curve D), a relatively quiescent lava lake at the time of our observations (see Rathbun et al., 2002, for a long-term time evolution of Loki’s volcanic activity). The total emitting area is 1930 km2 , and total energy output is 2280 GW sr−1 . As mentioned earlier, Loki’s emission dominates the total emission received by us. The model calculations suggest that we see temperatures down to 300 K, at an age of up to ∼ 1 month.

5. Conclusions We presented results of adaptive optics observations of Io obtained with the W.M. Keck II telescope on UT 18 December 2001, before the satellite went into eclipse and while it was in eclipse. We deconvolved the sunlit images using MISTRAL, which allowed us to obtain reliable flux density estimates at L - and M-bands. Observations of Io-in-eclipse are challenging, both in acquisition (we used Ganymede for wavefront sensing) and analysis. Ganymede and Io move with respect to each other, which limits the integration time to a few seconds at most. Anisoplanatic effects become important, in particular at the shortest wavelengths. This makes extraction of trustworthy flux densities quite challenging because the PSF is essentially unknown. We solved this problem by deriving the PSF from the data themselves via a Fourier transform algorithm. We applied the deconvolution techniques IDAC and MISTRAL to the Io-in-eclipse data. Although both algorithms reveal many compact sources, the IDAC algorithm recovers more sources and is photometrically more accurate. It appeared essential for both IDAC and MISTRAL to use the anisoplanatic PSF, in particular because of its large halo which both deconvolution algorithms usually tend to suppress. The Io-in-eclipse images reveal numerous faint hot spots that have too low a contrast to be visible on the sunlit im-

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ages; in fact, none of the 19 hot spots detected in K are visible on the sunlit K-band images. We determined the temperature and areal coverage of each hot spot both from the Io-in-eclipse and sunlit-Io images. Using the K/L and L/M intensities enabled us to characterize many of the hot spots. The K/L band ratios suggest that most volcanoes contain a compact hot component, 600–800 K over a
10 km2 area. The L/M band ratios show that there is also a lower temperature component (∼ 300–500 K) which covers a much larger area, ∼ 102 –∼ 104 km2 . This component dominates the total energy output of the volcanoes by a factor of ∼ 5 : 1, while the total heat flow from these active volcanoes is ∼ 8% of the average global heat flow as derived by Veeder et al. (1994). We discussed the volcanoes Loki, Surt, Dazhbog and Janus in detail. Surt, which had a violent eruption in February 2001 (Marchis et al., 2002), is still active. In December 2001, it is characterized by a very small eruption area, and large (up to ∼ 200 km2) cooler (down to 300 K) lava flows. Janus on the other hand appears to be a small (4 km2 ) vigorous eruption site, while model fits to our Loki data agree with Loki being a relatively quiescent large (1930 km2 ) lava lake, that dominates the infrared emission from the Jupiterfacing hemisphere (2280 GW sr−1 ). With the demise of the Galileo spacecraft, we should monitor Io using adaptive optics techniques. Such observations are ideally matched to the spatial resolution of the Galileo NIMS observations, and would provide a natural extension of the Galileo NIMS and PPR monitor programs. Ideally, one might like to pursue Io-in-eclipse observations for such an endeavor, since such observations allow detection of very faint volcanoes at 1–5 µm, and at the shortest wavelengths they allow detection of the highest temperatures within the volcanic areas. These high temperatures can only be observed at the shortest (1–2.5 µm) wavelengths, where on sunlit images volcanic hot spots are overpowered by reflected sunlight. These high temperatures, although they constitute only a negligible fraction of the average global heat flow, are important to characterize the nature of volcanic eruptions and composition of the erupting magma. Unfortunately, opportunities to use one of the satellites for wavefront sensing are rare, perhaps once or twice a year at best. In the future we will use Laser Guide Star techniques to create an ‘artificial’ source as wavefront reference; however, the use of such lasers next to bright Jupiter, and the requirement of a nearby star (or satellite) for tiptilt measurements, make such experiments extremely challenging.

Acknowledgments We thank Julian Christou for helpful discussions on using the IDAC deconvolution technique. We further wish to thank T. Fusco, J.-M. Conan, and L. Mugnier from ONERA for providing the MISTRAL deconvolution method. The data presented in this paper 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. This observational study of Io was partially supported by the National Science Foundation and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST-9876783. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations of ionian volcanoes from this Hawaiian volcano.

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