BVR photometry of Hyperion near the time of the 2005 Cassini encounter

Icarus 193 (2008) 352–358 www.elsevier.com/locate/icarus

BVR photometry of Hyperion near the time of the 2005 Cassini encounter Michael D. Hicks a,∗ , Bonnie J. Buratti a , Erik N. Basilier b a Jet Propulsion Laboratory/California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA b 774 W. Chandler Blvd., Chandler, AZ 85225, USA

Received 14 December 2006; revised 5 June 2007 Available online 11 August 2007

Abstract Six nights of BVR photometry and three nights of R photometry were collected over a month-long period shortly after the Cassini encounter with Hyperion on September 24 2005. Our observations were designed to help constrain the rotational state of the chaotically rotating satellite. Fourier analysis of our lightcurve data yields three possible periods: 10.2 ± 0.2, 13.9 ± 0.2, and 19.7 ± 0.4 days. Our B–V and V–R colors agree well with previous ground-based and Voyager 2 measurements. © 2007 Elsevier Inc. All rights reserved. Keywords: Saturn, satellites; Photometry; Rotational dynamics

1. Introduction S VII Hyperion is the largest irregularly shaped satellite in the Solar System, with approximate tri-axial dimensions of 205 × 130 × 110 km in radius. With a semi-major axis of 24.6 Saturn radii (between Titan and Iapetus), the body orbits once every 21.3 days. The geometric albedo pv = 0.28 ± 0.04 was measured by Cruikshank and Brown (1982) with an effective radius of 140 ± 19 km. Voyager results showed Hyperion to have a normal reflectance of 0.214 ± 0.006 at 0.47 µm (Thomas and Veverka, 1985). Spectrophotometry reveals a moderately red featureless spectral slope at visible/near-IR wavelengths and distinctive H2 O ice absorption features at 1.5 and 2.0 µm (Cruikshank and Brown, 1986), consistent with a surface composition of dirty ice (Clark et al., 1984). Buratti et al. (2002) found that the visible spectrum of Hyperion and the low-albedo hemisphere Iapetus could both be modeled by a linear admixture of high-albedo icy saturnian satellite and D-type material and suggested that the outer retrograde satellites of Saturn (Gladman et al., 2001) may be the source of dark material for the two satellites. The connection between Hyperion and the dark side of Iapetus was first suggested by the eight-color asteroid survey (Tholen and Zellner, 1983) and through the * Corresponding author. Fax: +1 (818) 354 0966.

E-mail address: [email protected] (M.D. Hicks). 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2007.06.021

identification of absorption bands in the spectrum of Iapetus characteristic of iron alteration minerals such as goethite and hematite (Vilas et al., 1996), later identified on Hyperion (Jarvis et al., 2000). The high-resolution images by Voyager 2 revealed a global-scale spallation scar and low crater densities although the high-resolution images covered only approximately half the surface. Preliminary analysis from the June 2005 Cassini flyby show that Hyperion is approximately 60% as dense as solid water ice, indicating significant void space and an icy rubble-pile internal structure (http://saturn.jpl.nasa.gov). The orientation of Hyperion with respect to Saturn in the Voyager 2 images was not consistent with synchronous rotation (Smith et al., 1982) and the irregular shape and high orbital eccentricity of Hyperion prompted Wisdom et al. (1984) to predict that the body maintains a chaotic rotational state. Analysis of the low resolution Voyager 2 images taken over a 60 day time span indicated a 13 day spin period with the rotational axis nearly parallel to the orbital plane (Thomas et al., 1984). Klavetter (1989a) observed Hyperion at the McGraw–Hill Observatory on 37 nights over a 53 day time span that straddled Saturn’s 1987 opposition. He concluded that his photometry was not compatible with any regular periodic state and found best-fit moment ratios consistent with the satellite having a uniform density distribution (Klavetter, 1989b). A control point analysis of the high-resolution Voyager 2 images has yielded a shape model with a 5.3 ± 0.3 day period

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Table 1 Observational circumstances Date of observation

Juliana date

Observerb r  Solar (AU) (AU) phase (deg)

Angular Pos. sep. angle (arcsec) (deg)

10/03/05 10/05/05 10/06/05 10/07/05 10/08/05 10/11/05 10/30/05 10/31/05

2453647 2453649 2453650 2453651 2453652 2453655 2453674 2453675

H B B B, H H H H H

145 216 231 231 217 115 212 180

9.08 9.09 9.09 9.10 9.10 9.10 9.10 9.11

9.51 9.50 9.49 9.47 9.46 9.42 9.11 9.10

5.54 5.63 5.67 5.71 5.75 5.86 6.25 6.25

283 269 264 260 256 230 253 250

a Julian date at time of transit near 12 h UT. b H = Hicks, B = Basilier.

with rotation roughly aligned with the axis of minimum rotational inertia (Thomas et al., 1995). Black et al. (1995) used the shape model to develop a dynamical model in which the largest component of the lightcurve was due to the wobble (free precession) of the rotation pole and not from the rotation itself. They concluded that the rotational state of Hyperion near the time of the Voyager 2 encounter was consistent with the effects of free wobble, forced precession, and rotation about the long axis, and gave rise to lightcurves that vary greatly in amplitude and quasi-regular period over a time-scale of several months. We have reduced 127 BVR CCD images of Hyperion obtained over the course of the 6 photometric nights during a thirty day period at the JPL Table Mountain Observatory (TMO) soon after the Cassini encounter with Hyperion in late September of 2005. In addition, we have data on three nights from a dedicated amateur observer, E. Basilier. Though weather kept us from obtaining data as well sampled as we would have preferred, our photometry can still be used as constraints for any dynamical model that may be developed from the Cassini Hyperion encounter. 2. Data acquisition and reductions Bessel BVR photometry of Hyperion was obtained over the course of 6 nights in October 2005 using the Jet Propulsion Laboratory’s Table Mountain Observatory (TMO) 24-inch telescope near Wrightwood, CA. The effective wavelengths for the Bessel B, V, and R filters were 0.44, 0.55, and 0.63 µm, respectively, and the observational circumstances are tabulated in Table 1. The φ/5.4 Cassegrain telescope was equipped with the facility liquid nitrogen cooled 1024 × 1024 pixel CCD camera, with 24-µm pixels giving a plate scale of 0.52 arcsec pixel−1 . With an ability to accurately track at high (>1000 arcsec h−1 ) non-sidereal rates and easy access from Pasadena, this system has been a reliable workhorse for the study of Solar System small bodies, capable of accurate BVRI photometry for objects as faint as V ∼ 18.5. Our observations were scheduled to supplement the Cassini encounter with Hyperion on September 26 2005, several months from the satellite’s opposition in January 2006 and the object was observable for only 2 h before dawn. To better

Fig. 1. Scattered light subtraction. Panel A is a contour map of a 101 × 101 pixel sub-array of a representative R-band exposure from October 11 2005 centered on Hyperion. Panel B is the model fit to the scattered light and panel C is the sky-subtracted image. The contours are set to increments of 50 DN. Only where the scattered light from Saturn is rapidly increasing is there any significant residual scattered light.

utilize the observing time, Hyperion observations were coupled with a program to collect BVRI lightcurves of the major uranian satellites, which transited near midnight. Twilight flats and biases were taken nightly as well as a number of standard star fields (Landolt, 1992), observed at airmasses bracketing our science targets. Our images were centered on Hyperion, and the saturated image of Saturn was always in the field of view. To facilitate subtraction of scattered light from Saturn the exposure time were kept short: 60 s for the V and R filters, 180 s for the B images. Additional photometric observations of Hyperion were collected by Erik Basilier at a dark-sky site approximately 50 km south of Phoenix, Arizona with a 200-mm Schmidt–Cassegrain telescope equipped with a focal reducer and thermo-electrically cooled CCD camera with 9 µm pixels giving a 1.68 arcsec pixel−1 plate scale. He obtained sequences of 400 s exposures on the nights of October 5, 6, and 7,

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Table 2 Reduced TMO photometry Julian datea

R(9.54, 8.54, α) (mag)

Julian datea

V(9.54, 8.54, α) (mag)

Julian datea

B(9.54, 8.54, α) (mag)

646.98584 646.98737 646.99011 646.99127 646.99286 646.99438 646.99945 647.00433 647.01123 647.01343 647.01617 647.01923 647.02258 647.02478 647.02777 647.03058 647.03247 647.03577 647.03973 647.04053 647.04144 650.99408 651.00366 651.01013 651.02399 651.02966 651.03546 651.97217 651.97900 651.98523 651.99164 651.99805 651.99982 652.02240 652.02856 652.03516 652.03668 654.96637 654.97363 654.97974 654.98584 654.99237 654.99829 655.00427 655.01465 673.93774 673.94556 673.95154 673.97827 673.98260 673.98438 674.92072 674.92676 674.93304 674.93939 674.94537 674.95160 674.95312

13.981 ± 0.010 14.165 ± 0.010 14.093 ± 0.042 14.033 ± 0.010 14.043 ± 0.010 14.141 ± 0.018 14.082 ± 0.010 14.050 ± 0.016 14.024 ± 0.029 14.051 ± 0.011 14.055 ± 0.013 14.090 ± 0.010 14.029 ± 0.010 14.118 ± 0.010 14.068 ± 0.010 14.036 ± 0.010 14.048 ± 0.021 14.087 ± 0.010 14.059 ± 0.010 14.048 ± 0.010 14.013 ± 0.012 14.020 ± 0.010 14.042 ± 0.010 14.015 ± 0.010 14.020 ± 0.010 14.031 ± 0.010 14.010 ± 0.010 13.901 ± 0.010 13.822 ± 0.012 13.881 ± 0.016 13.832 ± 0.010 13.853 ± 0.010 13.869 ± 0.010 13.870 ± 0.010 13.871 ± 0.010 13.841 ± 0.010 13.841 ± 0.010 14.134 ± 0.019 14.122 ± 0.010 14.110 ± 0.022 14.102 ± 0.010 14.117 ± 0.012 14.137 ± 0.028 14.080 ± 0.018 14.092 ± 0.010 14.039 ± 0.011 14.021 ± 0.010 14.028 ± 0.010 14.065 ± 0.033 14.021 ± 0.013 14.047 ± 0.011 14.127 ± 0.010 14.082 ± 0.035 14.118 ± 0.043 14.126 ± 0.016 14.083 ± 0.018 14.084 ± 0.024 14.064 ± 0.019

647.01221 647.01758 647.02673 647.03143 650.99841 651.00513 651.01166 651.01941 651.02551 651.03107 651.03741 651.97412 651.98059 651.98706 651.99323 652.01166 652.02380 652.03015 654.96826 654.97510 654.98145 654.98743 654.99396 654.99982 655.00702 655.01678 655.01678 673.93939 673.94110 673.94727 673.95355 674.92218 674.92834 674.93457 674.94092 674.94702 674.95520

14.501 ± 0.010 14.434 ± 0.018 14.494 ± 0.010 14.440 ± 0.010 14.483 ± 0.010 14.481 ± 0.015 14.477 ± 0.010 14.480 ± 0.010 14.450 ± 0.010 14.445 ± 0.010 14.418 ± 0.010 14.327 ± 0.013 14.340 ± 0.010 14.355 ± 0.010 14.305 ± 0.010 14.342 ± 0.102 14.313 ± 0.014 14.318 ± 0.010 14.514 ± 0.050 14.502 ± 0.010 14.533 ± 0.010 14.541 ± 0.013 14.564 ± 0.010 14.531 ± 0.010 14.566 ± 0.010 14.576 ± 0.010 14.576 ± 0.010 14.477 ± 0.010 14.482 ± 0.012 14.489 ± 0.018 14.483 ± 0.010 14.662 ± 0.016 14.510 ± 0.031 14.447 ± 0.022 14.364 ± 0.020 14.289 ± 0.021 14.173 ± 0.013

647.01465 647.02045 647.02875 647.03339 651.00061 651.00677 651.01318 651.02094 651.02679 651.03265 651.03931 651.97601 651.98230 651.99463 652.01361 652.02545 652.03223 654.97046 654.97675 654.98297 654.98895 654.99536 655.00134 655.00958 673.94263 673.94873 674.92383 674.92993 674.93616 674.93616 674.94238 674.94855

15.296 ± 0.010 15.249 ± 0.010 15.291 ± 0.023 15.260 ± 0.010 15.222 ± 0.010 15.252 ± 0.010 15.254 ± 0.012 15.232 ± 0.010 15.251 ± 0.010 15.248 ± 0.010 15.236 ± 0.010 15.084 ± 0.010 15.104 ± 0.016 15.138 ± 0.013 15.059 ± 0.010 15.061 ± 0.010 15.027 ± 0.010 15.342 ± 0.054 15.346 ± 0.038 15.362 ± 0.032 15.353 ± 0.023 15.405 ± 0.041 15.401 ± 0.036 15.380 ± 0.035 15.263 ± 0.010 15.302 ± 0.010 15.303 ± 0.015 15.290 ± 0.017 15.285 ± 0.013 15.327 ± 0.026 15.292 ± 0.022 15.314 ± 0.010

a All times midexposure JD—2453000.0.

2005. The filter used was Schuler red, which has an effective wavelength very similar to the R filter used at Table Mountain.

The reductions of the Table Mountain imaging data began in the standard way, with bias-subtraction and flat-fielding within the IRAF environment (Tody, 1993). Instrumental magnitudes

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Table 3 Background star calibrations for Basilier photometry Guide star catalog number

(J2000) Right ascension Declination

Cataloged R (mag)

Calibrated R (mag)

1396 0294 1396 1201 1396 1311 1396 1314 1396 1535 1396 1595 1396 1880 1396 1925 1396 2402

08:45:19.41 08:47:18.25 08:45:56.12 08:47:47.73 08:45:45.17 08:46:39.72 08:46:25.72 08:46:55.11 08:47:32.89

11.5 12.3 11.8 9.8 11.6 11.0 10.9 11.7 9.9

11.59 12.12 11.78 9.71 11.51 10.95 10.64 11.59 9.84

18:40:31.70 18:35:27.50 18:29:27.00 18:33:25.10 18:39:15.10 18:27:08.10 18:43:24.10 18:34:46.30 18:23:44.00

Table 4 Reduced Basilier photometry

Fig. 2. Hyperion reduced magnitude as a function of time. The broad-band photometry has been corrected for heliocentric and geocentric distance but not solar phase angle. Plotted are the values obtained at the Table Mountain Observatory for B magnitudes (open diamonds), V magnitudes (open triangles), and R magnitudes (filled circles). Also plotted is the R filter photometry obtained by Basilier (open circles) on the nights of October 5, 6, and 7, 2005. The two datasets are in very good agreement for the night of October 7, when they where taken nearly simultaneously.

for all Hyperion and flux standard star frames were measured for 10, 11, and 12 pixel radius apertures with sky values derived using an annulus with inner and outer radii of 12 and 24 pixels, respectively. Hyperion’s galactic latitude at the time of our observations was +33◦ and contamination from background stars in our images was negligible. The calibrated observed magnitudes for the three photometric apertures were compared and averaged as a consistency check against seeing effects and spurious cosmic ray hits. Our calibration error for each filter was 0.01 mag which was approximately twice the formal error given by photon statistics, etc. We corrected the observed magnitudes to mean opposition distance using the relation M  = M − 5 log

r , rs s

(1)

where M  is the mean opposition magnitude M uncorrected for solar phase, r = heliocentric distance,  = geocentric distance, rs = 9.54 AU, Saturn’s mean heliocentric distance and s = 8.54, Saturn’s mean geocentric distance. At the epoch of our observations, Hyperion ranged from 1 to 4 arcmin in angular separation from Saturn, with the minimum angular distance for our observations of 115 arcsec occurring on October 11, 2005. The scattered light from the planet prompted us to model the background sky around Hyperion. We developed a program which isolated a 101 × 101 pixel sub-array centered on the satellite in each flat-fielded frame with vectors defined from each pixel to the center of Saturn. Sky values along every vector was used to generate a 5th order polynomial

Julian datea

R(9.54, 8.54, α) (mag)

648.97998 648.98999 649.01001 649.96002 649.96997 649.97998 649.97998 649.98999 649.98999 650.00000 650.01001 650.01001 650.02002 650.96240 650.97510 650.98419 651.00989 651.02039

14.351 ± 0.014 14.348 ± 0.018 14.390 ± 0.012 14.188 ± 0.019 14.187 ± 0.021 14.171 ± 0.019 14.186 ± 0.020 14.172 ± 0.032 14.186 ± 0.026 14.194 ± 0.027 14.161 ± 0.033 14.099 ± 0.039 14.099 ± 0.029 14.033 ± 0.026 14.012 ± 0.029 14.042 ± 0.032 14.003 ± 0.029 14.033 ± 0.028

a Date = JD—2453000.0.

fit to determine the background value at each pixel. As shown in Fig. 1 for the worst-case night of October 11, this technique was extremely effective at flattening the sky background to well within the outer boundary of the sky annulus. The background subtraction typically gave only small corrections to the raw calibrated aperture photometry of ≈0.02–0.03 mag except on the night of October 11 when Hyperion was at minimum distance to Saturn’s disk, in which the correction was on the order of 0.4 mag. After the background fitting and subtraction the scatter in the photometry between individual frames for each filter was significantly reduced. The opposition magnitudes for data collected at Table Mountain are listed in Table 2 and plotted as a function of Julian date in Fig. 2. The Hyperion R-band observations taken by Erik Basilier were reduced in a similar manner. All CCD images were acquired using the CCDops package from the Santa Barbara Instrument Group and reduced using Maxim DL software from Diffraction Limited. After dark frames of matching detector temperature and exposure time where subtracted from the satellite and flat-field frames the imaging data could then be properly flat-fielded. The instrumental magnitudes were measured using a 4 pixel (6.7 arcsec) photometric aperture and 7–14 pixel sky annulus for Hyperion and a number of GSC background stars

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in each frame and this data was forwarded to the JPL team for analysis. Rough photometry could be found using the listed R magnitudes in the Guide Star Catalog to compute the zeropoint magnitudes but we obtained significant improvement after calibrating the background stars used in the Basilier photometry by observing and remeasuring the Hyperion fields at the Table Mountain Observatory on the night of October 21, 2006, as listed in Table 3. We are confident that the Basilier photometry is calibrated correctly to within approximately 0.01 mag. We list the reduced mean opposition magnitude from this data in Table 4.

of B–V = 0.798 ± 0.030 mag and V–R = 0.438 ± 0.031 mag agree well with ground-based (Klavetter, 1989a) and Voyager (Thomas and Veverka, 1985) observations. When converted to

3. Analysis and discussion Table 5 summarizes our color measurements of Hyperion. Some evidence for color variations was reported by Harris (1961) and Thomas and Veverka (1985), however no significant rotational variability was found in our data. Our averaged colors Table 5 Daily mean opposition magnitudes and colors UT date B (mag) 10/03/05 10/07/05 10/08/05 10/11/05 10/30/05 10/31/05

15.27 ± 0.02 15.24 ± 0.01 15.08 ± 0.04 15.37 ± 0.03 15.28 ± 0.03 15.30 ± 0.02

V (mag)

R (mag)

B–V (mag) V–R (mag)

14.47 ± 0.04 14.06 ± 0.04 14.46 ± 0.02 14.02 ± 0.01 14.33 ± 0.02 13.86 ± 0.02 14.54 ± 0.03 14.11 ± 0.02 14.48 ± 0.01 14.04 ± 0.02 14.41 ± 0.17 14.10 ± 0.03 Weighted average

0.81 ± 0.04 0.78 ± 0.03 0.75 ± 0.04 0.83 ± 0.04 0.80 ± 0.02 0.89 ± 0.16 0.80 ± 0.03

0.40 ± 0.05 0.44 ± 0.03 0.47 ± 0.03 0.43 ± 0.03 0.45 ± 0.02 0.31 ± 0.16 0.44 ± 0.03

Fig. 3. Spectral reflectance of Hyperion. A long-slit CCD spectrum of Hyperion taken at the 200-inch telescope on Palomar Mountain (Buratti et al., 2002) is plotted normalized at 0.55 µm. Also plotted are the BVR colors measured at Table Mountain (open circles), with very good agreement.

Fig. 4. Short-term variability of R-band Hyperion photometry.

Optical observations of Hyperion

Fig. 5. χ 2 residuals of the R-band photometry as a function of rotation period, assuming a second-order Fourier fit to the data. Our data is under-sampled and several periods fit equally well.

relative reflectance, our BVR colors also match the featureless low-resolution long-slit spectrum of Hyperion obtained at the Palomar 200-inch by Buratti et al. (2002), as shown in Fig. 3, giving us confidence in our observations and reduction techniques. Quasi-rotation period estimates for Hyperion have ranged from 5 to 13 days. Klavetter (1989a) reported large scale (∼0.4 mag) night-to-night brightness variations in his photometric survey that covers 13 weeks near opposition yet found that Hyperion’s brightness remained constant at the 0.01 mag level over a time period of 6 h. We find similar behavior in our data, as plotted in Fig. 4, strongly suggesting a multi-day rotation period. The large scatter in the inter-day photometry of October 3, 2005 is likely an artifact: though the night was photometric the seeing was soft and variable and the high scattered light levels from Saturn made photometry difficult. For a multi-day lightcurve period, our Hyperion data is too sparse for phase-dispersion minimization and therefor we have used the Fourier fitting technique as described by Harris and Lupishko (1989). The weight of a given night’s photometry is generally proportional to the number of observations obtained on that night. For the lightcurve analysis the R(9.54, 8.54, α) magnitudes were reduced to zero degree phase assuming a phase coefficient G = 0.56 (Klavetter, 1989a). Fig. 5 plots the χ 2 misfit of a second-order Fourier fit to our R-band Hyperion data for rotational periods ranging from 1 to 25 days. We find three nearly equal minima, with two possible solutions illustrated in Figs. 6 and 7. Our solutions include P = 10.2 ± 0.2, 13.9 ± 0.2, and 19.7 ± 0.4 day. Our data does not allow us to definitively distinguish between the four solutions. Though numerical integrations of Hyperion by Black et al. (1995) suggest that the spin of Hyperion evolves on ∼100-

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Fig. 6. Hyperion lightcurve and second-order Fourier fit to the R-band photometry assuming rotation period of 10.2 days.

Fig. 7. Hyperion lightcurve and second-order Fourier fit to the R-band photometry assuming rotation period of 13.9 days.

day time-scales, our P = 13.9±0.2 solution is reasonably close to the 13.1 ± 0.3 day period measured using the disk integrated low-resolution clear filter (λeff ∼ 0.48 µm) Voyager 2 images taken over the two month time span June 24–August 23, 1981 (Thomas et al., 1984). The ∼0.4–0.5 mag amplitude of our lightcurves fits well the mean ellipsoidal cross-section (A × B) of 370 × 280 ± 20 km as derived from the high-resolution Voyager (Thomas and Veverka, 1985). Our lightcurves are not consistent with the extremely chaotic state as reported by Klavetter (1989a), though our observations are much less well sampled.

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4. Conclusions Our ground-based BVR photometry of Hyperion give B–V and V–R colors that are consistent with previous measurements and our R-band images support a quasi-periodic multi-day rotational period; however, our data is too sparse to uniquely determine a unique solution. We can rule out a period synchronous with the orbit but we see no firm evidence of chaotic rotation on the 20-day time scale of our observations. Acknowledgments This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We thank Jim Young for the excellent engineering support at Table Mountain. We thank Stephen Larson and one anonymous review, both of who’s comments greatly improved this paper. References Black, G.J., Nicholson, P.D., Thomas, P.C., 1995. Hyperion: Rotational dynamics. Icarus 117, 149–161. Buratti, B.J., Hicks, M.D., Tryka, K.A., Sittig, M.S., Newburn, R.L., 2002. High-resolution 0.33–0.92 µm spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-type asteroids: How are the related? Icarus 155, 375– 381. Clark, R.N., Brown, R.H., Owensby, P.D., Steele, A., 1984. Saturnian satellites: Near infrared spectrophotometry and compositional implications. Icarus 58, 265–281. Cruikshank, D.P., Brown, R.H., 1982. Surface composition and radius of Hyperion. Icarus 50, 82–87. Cruikshank, D.P., Brown, R.H., 1986. Satellites of Uranus and Neptune, and the Pluto–Charon system. In: Burns, J.A., Matthews, M.S. (Eds.), Satellites. Univ. of Arizona Press, Tucson, pp. 836–873.

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