Near-infrared studies of the satellites of Saturn and Uranus

ICARUS 41, 246-258 (1980)

Near-Infrared Studies of the Satellites of Saturn and Uranus I DALE P. CRUIKSHANK 2 Institute for Astronomy, University o f Hawaii, Honolulu, Hawaii 96822 Received August 2, 1979; revised September 28, 1979 New JHK photometry and spectrophotometry (1.4-2.6/~m) are presented for Enceladus, Hyperion, Phoebe, Umbriel, Titania, and Oberon. From spectral signatures, mainly in the 2-p.m region, water ice is verified on Enceladus and identified on Hyperion and the three Uranian satellites. The JHK photometry shows that Phoebe is different from all other satellites and asteroids observed thus far. The new photometry corroborates the earlier conclusion by Cruikshank et al. [(1977) Astrophys. J. 217, 1006-1010] that the Uranian satellites, as a class, have overall surface reflectances different from other water-ice-covered satellites, and the reason for the difference remains unclear. The diameters and the masses of the Uranian satellites are reviewed in light of the probable high albedo representative of ice-covered surfaces and the new dynamical studies by Greenberg [(1975) Icarus 24, 325-332; (1976)Icarus 29, 427-433; (1978)Bull. Amer. Astron. Soc. 10, 585].

lites of Saturn was summarized in a review by Cruikshank (1979a), who pointed out This paper reports on new observations that the brightest satellites which can be of four satellites of Saturn (Enceladus, studied easily are apparently dominated in Hyperion, Iapetus, and Phoebe) and three surface composition and bulk composition satellites of Uranus (Umbriel, Titania, and by frozen volatiles of which water frost has Oberon). Little direct information exists on been positively identified. Titan is the exthe physical properties of these objects ception to this, for its methane atmosphere because of their intrinsic faintness and/or and large dimensions imply a history differproximity to the bright primary planets. ent from the smaller objects. Iapetus, Both groups are of interest in terms of pos- Hyperion, and Phoebe, the outermost three sible radial gradations of physical proper- bodies, are all special cases and are rather ties that might give clues to the origin and difficult to study for reasons of intrinsic evolution of the respective planet and its faintness or for geometrical considerations. system of rings and satellites. Furthermore, These three bodies, plus Enceladus, are they are bodies which have condensed in considered in this paper. The Uranian satellite system has been litthe outer regions of the solar nebula, and knowledge of their masses, bulk densities, tle studied because of the faintness of the and surface compositions gives information objects and their projected proximity to on the physical and chemical conditions in Uranus. What little is known about them was summarized in the review by Morrison the solar nebula during planet formation. Our understanding of the system of satel- et al. (1977). The five satellites are presumed to be low-density objects com1This paper is dedicated to the memory of Leif posed largely of frozen volatiles, but the Erland Andersson, 1943-1979. Dr. Andersson adsurface compositions are unknown. The vanced the study of the objects in the outer solar apparent fact that the rings of Uranus are system by his observational work. composed of dark particles (Sinton, 1977) Visiting Scientist, Kitt Peak National Observatory, may have some bearing on the surface which is operated by the Association of Universities compositions of the innermost satellites, for Research in Astronomy under contract with the but direct observations are necessary in any National Science Foundation. INTRODUCTION

246 0019-1035/80/020246-13502.00/0 Copyright(~) 1980by AcademicPress,Inc. All rightsof reproductionin any form reserved.

SATELLITES OF SATURN AND URANUS

case. Spectrophotometric observations in the photovisual and near-infrared regions are presently possible for the four brightest satellites, Ariel, Umbriel, Titania, and Oberon, and this paper presents new results for the latter three. THE OBSERVATIONS The spectrophotometric and photometric data presented here were obtained with the K i t t P e a k N a t i o n a l O b s e r v a t o r y 4-m M a y a l l t e l e s c o p e on t h r e e nights (11-13 A p r i l 1979), e x c e p t f o r t h e E n c e l a d u s o b s e r v a tions w h i c h w e r e m a d e a y e a r earlier. T h e B l u e T o a d p h o t o m e t e r w a s u s e d w i t h a circ u l a r v a r i a b l e i n t e r f e r e n c e filter ( C V F ) , which provided the wavelength separation. T h e r e s o l u t i o n in t h e s p e c t r u m is l i m i t e d by t h e C V F to A / A h ~ 100, t h o u g h in all c a s e s t h e s p e c t r u m w a s u n d e r s a m p l e d in o r d e r to c o v e r t h e r e q u i r e d w a v e l e n g t h r a n g e in t h e t i m e a v a i l a b l e an d t h e r e s o l u t i o n a c h i e v e d w a s c o n s i d e r a b l y less t h a n t h e m a x i m u m . In a d d i t i o n , s t a n d a r d J H K filters w e r e u s e d .

247

Stars of spectral type A (0 Leo, y Gem) and GO (13 Com, h Ser), and near solar type (/z Ser, ¢ Leo) were observed for comparisons. Table I gives the log of the observations that are presented in this paper. The observations were made in the sky-subtract mode using the oscillating Cassegrain secondary mirror of the 4-m telescope and computer-controlled beam switching in the standard way for infrared photometry. Apertures of 5.4 and 6.8 arcsec were used on Enceladus and the satellites of Uranus, while an aperture of 10.1 arcsec was used on all the other objects. The spectral observations required typically 120 sec of integration for each spectral point for the faint objects, and substantially less for the comparison stars. Successive runs through the spectrum of a given object were coadded and divided by the comparison star indicated in the figure captions in order to provide reflectance spectra with the color of the Sun and atmospheric extinction effects removed. Adequate stan-

TABLE I LOG OF OBSERVATIONS

Photometry Date

UT

Airmass

Date

Object

UT

Airmass

0 Leo Oberon

0630 0915

1.09

22 April 1978

Enceladus Enceladus 0 Leo

0358 0415 0448

1.06 1.05 1.10

12 April 1979

0 Leo /3 Corn 0 Leo Titania /z Ser

0603 0724 0740 1125 !155

1.06 1.00 1.23 1.88 1.27

12 April 1979

0 Leo Oberon Titania /~ Ser h Ser 16 Cyg B

0615 0850 1005 1140 1155 1230

1.07 1.57 1.56 1.38 1.27 1.09

13 April 1979

HR3881 0 Leo Hyperion Phoebe 0 Leo Titania Umbriel Titania Oberon h Ser

0300 0340 0608 0626 0702 0856 0906 0936 0950 1215

1.05 1.13 1.14 1.18 1.15 1.54 1.54 1.55 1.56 1.35

13 April 1979

y Gem HR3881 Leo 0 Leo Hyperion 0 Leo Oberon Oberon Ser /,~ Ser 16 Cyg B

0230 0244 0308 0325 0505 0710 0740 1006 1058 1119 1150

1.15 1.06 1.02 1.13 1.08 1.16 1.69 1.59 1.15 1.32 1.13

11 April 1979

Object

Spectrophotometry

1.54

248

DALE P. CRUIKSHANK

The satellites of Saturn which are especially faint, lie very close to the ring system, or both have not been studied thoroughly in the infrared. In this paper I present spectra of Enceladus ($2) and Hyperion ($7), and J H K photometry of Iapetus ($8) and Phoebe ($9). Enceladus can be observed only at eastern or western elongation, at which time the angular separation from the ring ansae is about 20 arcsec. Hyperion is normally wellseparated from the rings, but is faint, requiring a large telescope for infrared spectroscopy, and Phoebe is too faint for present spectrometers, permitting only broadband photometry.

ently related to an incomplete covering of the satellite surface or an admixture of other neutral reflecting material in the frost matrix, (Pilcher et al., 1972). Temperature and scattering geometry within the frost also affects the strengths and shapes of the absorptions (Fink and Larson, 1975; Clark, 1980); the difference in temperature of the rings and Ganymede affects the strength of the 1.65-~m absorption in the sense that it is stronger in the rings. The spectral data for Enceladus show a profile characteristic of water frost seen in diffuse reflection, where the intensity peak centered near 2.2 /xm is flanked by two strong absorptions. This profile is evident in spectra of Europa and Ganymede, and the rings and intermediate satellites of Saturn, and the coincidence of the spectral data given here with the reflectance spectra of Saturn's rings and Ganymede is thus regarded as confirmation of the presence of water frost on Enceladus (Cruikshank et al., 1977).

Enceladus

Hyperion

The spectrum of Enceladus in the region - 1 . 85 - 2 .6 /zm was obtained at eastern elongation, and is shown plotted twice in Fig. 1. The upper plot shows the Enceladus data normalized to a low-resolution spectrum of Saturn's rings (heavy line), and the lower plot shows the same data normalized to a spectrum of Ganymede. The spectra of the rings and Ganymede with high photometric precision (1-2%) were provided by R. Clark (Clark and McCord, (1980a,b). Saturn' s rings are known to be covered with water frost at a temperature representing equilibrium with the solar insolation, and compared with other solar system bodies exhibiting water frost absorption, the absorption in the ring spectrum are the deepest, suggesting that the frost is more " p u r e , " or that it completely covers the particles comprising the ring mass. Ganymede's spectrum shows the characteristic absorptions attributed to water frost, but they are less deep, a fact appar-

More complete spectral coverage exists for Hyperion than for Enceladus, the data in Fig. 2 extending from 1.4 to 2.6/~m. The 2-~m spectral region shows the characteristic peak at 2.2 ~m indicative of water frost, but the spectrum in the region of 1.7 to 1.8 ~m departs from that of Ganymede or Saturn's rings. The Hyperion observations represent a single run through the spectrum and there is the possibility of observational error that does not appear in the formal error calculation represented by the error bars on the individual points. The effect of the drop in intensity in the 1.8-/xm region is to suggest an absorption that is uncharacteristic of either water frost or frosts of NHa, H2S, CO2, CH4, NH4SH, or other common volatiles which have been examined in the laboratory. While the spectrum of Hyperion in Fig. 2 is a ratio to h Ser, the drop in intensity starting at 1.73 /xm follows closely the telluric water vapor absorption in the raw spectrum

dards at airmasses comparable to the data for the Uranian satellites were not obtained, and certain problems in the ratio spectra may result, as noted below. They do not, however, affect the conclusions of this paper. THE SATELLITESOF SATURN

SATELLITES OF SATURN AND URANUS I

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FIG. 1. Spectrum of Enceladus (points with one-sigma error bars, including calibrations) compared with the spectrum of Saturn's rings (top) and the spectrum of Ganymede (bottom). of the star. Though the airmasses of star and satellite were quite similar and the spectra were obtained the s a m e night, variations in the local a t m o s p h e r i c w a t e r v a p o r concentration m a y have o c c u r r e d in the unsettled (but photometric) w e a t h e r at Kitt Peak during the observing period.

A s p e c t r u m o f H y p e r i o n obtained by Cruikshank and Pilcher (Cruikshank, 1978) presents a confusing picture when compared with the data in Fig. 2. The earlier data, obtained with lower instrumental sensitivity than the new data, suggest absorptions in the satellite s p e c t r u m that are not

250

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FIG. 2. Spectrum of Hyperion (points with one-sigma error bars) compared with the spectrum of Saturn's rings (top) and the spectrum of Ganymede (bottom).

easily recognized as due to water ice or frost, though no obvious alternative origin of the spectral profile exists. In the 2-/~m region the two data sets are similar; the general profile of this region shows a peak centered near 2.2 /~m, consistent with a water frost interpretation.

While more data are clearly needed to resolve the questions of possible anomalous absorption features at 1.8 and 2.20/~m, the best data set extant, shown in Fig. 2, suggests that water frost is at least a major component of the surface of Hyperion. We note that Cruikshank (1979b) has derived

SATELLITES OF SATURN AND URANUS the geometric albedo of Hyperion from a m e a s u r e m e n t of the thermal flux, and finds Pv = 0.47 ___ 0.11, a value consistent with an interpretation of a surface c o v e r e d with frozen volatiles. P h o e b e a n d the D a r k Side o f Iapetus

The apparently bimodal distribution of bright and dark material on the surface of Iapetus has been the subject o f n u m e r o u s recent studies (Cruikshank, 1978, 1979a), the results o f which have shown that the trailing hemisphere o f the satellite has a substantial covering of water frost, with some frost extending o v e r the south pole to the leading hemisphere. The leading hemisphere is characterized by a very low geometric albedo similar to that of the C - t y p e asteroids ( C h a p m a n et al., 1975). Because the dark hemisphere can be observed "full f a c e " without the contamination o f the frost f r o m the bright side at relatively infrequent intervals, reflectance spectra diagnostic of surface composition are difficult to obtain. N e w data on the reflectance s p e c t r u m from 0.32 to 0.95 ~ m , as well as infrared p h o t o m e t r y in the J H K bands obtained b y Cruikshank et al. (1980), show that the similarity between the satellite and C - t y p e asteroids is quite strong. The reason for the stunning a s y m m e t r y in the distribution of frost, and hence the brightness, on the leading and trailing faces o f l a p e t u s remains a puzzle b e c a u s e none of the other satellites of Saturn show the same properties. One recent hypothesis suggests that dark material r e m o v e d from the surface o f Phoebe, which is exterior to Iapetus, preferentially accumulates on the leading hemisphere o f Iapetus as the particles spiral toward Saturn. While marginal evidence (Degewij et al., 1977) suggests that the surface of Phoebe is dark, a detailed comparison o f the spectra o f the two objects would be desirable to ascertain if there are compositional similarities which might confirm or reject the hypothesis of a c o m m o n origin o f the surface materials. The faintness (my = 16.5) o f Phoebe m a k e s spectral stud-

251

ies v e r y difficult, especially in the infrared, and presently only relatively b r o a d b a n d p h o t o m e t r y is possible. U B V p h o t o m e t r y has been published by Andersson (1974), Degewij et al. (1977), and Degewij (1979), and in this p a p e r we present the first J H K p h o t o m e t r y o f Phoebe. The most recent evaluation of the photometric data for Phoebe gives V(1,0) = 6.91 and B-V = 0.70 (Degewig and Van Houten, 1980). The value o f B-V o f the dark side of Iapetus varies with the axial tilt of the satellite as viewed from Earth (Millis, 1977), p r e s u m a b l y because of the south polar cap of water frost (Morrison et al., 1975), but appears to lie in the range 0.69-0.82. The new observations by Cruikshank et al. (1980a) give B-V = 0.78 at an axial tilt of + 3°, a p p r o x i m a t e l y the same as when their J H K photometric observations were obtained. Thus, the B-V colors of the two objects are similar. The new data for J - H and H - K for Phoebe and Iapetus, as well as several other objects are shown in Fig. 3. Iapetus lies within the field defined by C- and S-type asteroids [the asteroid data in Fig. 3 were taken from L e a k e et al. (1978)] while Phoebe is different from Iapetus at the two-sigma level as well as all the other objects shown. 0.6

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252

D A L E P. C R U I K S H A N K

The new data for Phoebe were obtained on a single night during about 1 hr of observing. Andersson (1974) has shown observational evidence for a lightcurve amplitude of about 0.3 mag with a period of either 11.25 or 21.2 hr. Although it is possible that the lightcurve results from an asymmetric distribution of albedo on the surface of spherical Phoebe, it is equally probable that the lightcurve represents a geometric shape factor and that the surface is uniform in composition. In this latter case, the colors shown in Fig. 3 are representative of the entire surface, and the obvious difference between Phoebe and the dark side of Iapetus argues strongly for rejection of the hypothesis wherein the former object supplies material to the surface of the latter. SATELLITES OF URANUS

Photometric studies of the satellites of Uranus were made by Harris (1961), Andersson (1974), and Cruikshank et al. (1977), while spectrophotometric observations have been reported by Johnson et al. (1978). The Johnson et al. data are inconsistent with the UBV photometry, showing a steep drop in the reflectances of Titania and Oberon. New spectrophotometry of Oberon by Cruikshank et al. (1980b) is more consistent with the broadband photometry, and indicates only a small drop in reflectance toward the violet. Titania

The infrared spectrum of Titania is shown relative to the rings of Saturn and Ganymede in Fig. 4. The characteristic peak in the spectrum at 2.2 /zm gives a strong indication of the presence of water frost on the surface of this satellite, though, as with the case of Hyperion, there are some inconsistencies in the 1.6-/zm region which are due at least in part to extinction. The strengths of the water frost absorptions are comparable to those of Ganymede, and because these absorptions do not appear to be strongly dependent on temperature (Fink and Larson, 1975) we may conclude as a

first approximation that the surfaces of Ganymede and Titania are compositionally similar. The strengths of the ice absorptions in this spectral region are related to the texture of the ice more than the temperature (Fink and Larson, 1975), but the relatively low photometric precision of the Titania spectral data precludes detection of subtleties of this kind. Oberon

Spectrophotometric data closely similar to those for Titania were obtained for Oberon and are shown in Fig. 5. To the level of photometric precision of the two data sets, the spectra of Titania and Oberon are identical, both showing strong similarities to the spectrum of Ganymede. The distinctive profile in the 2-/~m region indicates the presence of water ice on the surface of Oberon. Umbriel

It was possible to obtain only JH K photometric observations of Umbriel because of a telescope time constraint, but similar data were obtained for Titania and Oberon so that a direct comparison could be made. The results for Umbriel are shown in Fig. 3 with the other satellite and asteroid data. It can be seen that the JHK colors, which are strongly sensitive to absorptions by volatiles in the relevant passbands, are virtually identical for all three of the Uranian satellites studied. The sensitivity of J HK colors to absorptions in surface materials on planetary satellites has been discussed by Morrison et al. (1976), Cruikshank et al. (1976), and other authors. While broadband colors are insufficient to reveal small differences in surface areal coverage of certain materials on satellite surfaces, the spectral signatures of the condensed volatiles most probable on cosmochemical grounds can be distinguished from their infrared colors, and certainly volatiles can be distinguished from silicate rocks. On the basis of the JHK colors shown in Fig. 3, it is concluded that water ice or frost

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FiG. 4. S p e c t r u m o f Titania (points w i t h o n e - s i g m a e r r o r bars) c o m p a r e d w i t h the s p e c t r u m o f S a t u r n ' s rings (top) a n d the s p e c t r u m o f G a n y m e d e (bottom).

is a significant constituent of the surface of Umbriel. Johnson et al. (1978) found that the reflectance of Titania and Oberon were similar in the region 0.33 to 0.98 p.m, but that Umbriel was greatly different with a distinct upturn shortward of 0.7 /~m. Cruikshank et al.

(1979) have pointed out the discrepancy in published U B V photometry and other spectrophotometry of Triton with the results of Johnson et al. (1978), and a similar discrepancy exists with the Uranian satellites. The U B V photometry of Titania and Oberon by Andersson (1974) indicates a depression at

254

DALE P. CRUIKSHANK [

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FIG. 5. Spectrum of Oberon (points with one-sigma error bars) compared with the spectrum of Saturn's rings (top) and the spectrum of Ganymede (bottom).

U (0.36/zm) relative to V (0.56 tzm) of 15 to 25% while the data presented by Johnson et al. (1978) indicate a much steeper drop in reflectance toward the violet. New spectrophotometry of Oberon by Cruikshank et al. (1980b) are more consistent with the UBV photometry, showing only a weak

decrease in reflectance toward the violet. Reitsema et al. (1978) noted the spectral similarity of Umbriel, Titania, and Miranda in the region 0.56-1.0 /zm, and as noted above, the JHK colors reported here indicate a strong similarity of Umbriel to Titania and Oberon. Reitsema et al. also

SATELLITES

OF

SATURN

found that Ariel and Oberon are similar to one another, but as a pair are slightly less red than than the other three. While the issue of the violet and visible-region reflectances requires clarification, the identification of water ice on the surfaces of the three outer Uranian satellites is an independent matter. Some ice-covered satellites (Europa and Ganymede) have reflectances which drop relatively steeply toward the violet, while others (the inner satellites of Saturn) are nearly neutral (McCord et al., 1971). The violet reflectance appears to be related to the nonice constituents of the surface an d /o r the irradiation history of the ices. While these questions are of great importance in the study of the Uranian system, a fuller understanding requires additional observational data. In a study of J H K L photometry of outer planet satellites, Cruikshank et al. (1977) concluded that Triton, Titania, Oberon, and Hyperion constitute a special class of small solar system bodies, distinct in surface composition from the ice-covered bodies and other objects. The compositional similarities of these objects and their difference from others are borne out in the new photometry shown in Fig. 3, but it is now clear from the spectrophotometry in this paper that water ice occurs on Titania, Oberon, and Hyperion. Triton has been shown to have a tenuous methane atmosphere (Cruikshank and Silvaggio, 1979) and gas absorption at 2.3/xm affects the K magnitude; similarity to the Uranian satellites and Triton is regarded as fortuitous. It was recognized at the time that Cruikshank et al. (1977) discussed the separate class of objects represented by the Uranian satellites and Hyperion that they have infrared reflectances intermediate between Callisto (a rocky surface) and the water-frost-covered bodies (Ganymede, etc.), suggesting that the surfaces of these objects might have partial covers or very thin deposits of water frost. Another possibility is that some other material mixed with the water frost might affect the satellite colors, raising their over-

AND

255

URANUS

all reflectances in the region of water frost absorptions. This latter possibility cannot be eliminated with the present data. Smythe (1975) has shown, for example, that clathrate compounds of methane and carbon dioxide have infrared reflectances virtually indistinguishable from that of pure water frost--certainly at the level of precision of the satellite reflectance data given in this paper it would be impossible to make such a distinction. With the presence of at least a substantial quantity of water frost or ice, it is useful to make some estimates of the physical dimensions of the Uranian satellites. Thermal emission from these bodies has not yet been detected, and no other direct technique for determination of their dimensions has yet yielded diameters. If we assume that the geometric albedos are those representative of ice-covered surfaces, we can make some preliminary estimates of the diameters. The geometric albedo, Pv, of Ganymede is 0.43, and for Rhea and Dione 0.60 (Morrison et al., 1977). If we assume that the surfaces of the Uranian satellites are less ice-covered than Ganymede, but of higher albedo than Callisto (0.17), the range 0.2-0.5 appears acceptable. The upper end of this range is consistent with Cruikshank's (1979) determination of Pv = 0 . 4 7 + 0.11 for Hyperion. The results of the calculation, assuming the magnitude of the Sun Vo = -26.77 and using the mean opposition magnitudes of the satellites given by Morrison et al. (1977), are given in Table II. Although there is no direct eviTABLE

II

THE SATELLITES OF URANUS

Satellites

Miranda Ariel Umbriel Titania Oberon

Radius (kin) Pv = 0.2

Pv = 0.5

260 680 450 820 730

160 430 280 520 460

M a s s ( U r a n u s = 1.0y' Pv 1.7 3.1 8.9 5.3 3.7

x x × x z

0.2 10 -6 10 -~ 10 6 10 ~ I0 ~

p~ = 0.5 4.3 7.8 2.2 1.3 9.4

x x x x x

A s s u m i n g ,6 = 2.0. M a s s o f U r a n u s = 8.66 x 102~ g.

I0 7 10 ~ I0 -s 10 ~ 10 ~

256

DALE P. C R U I K S H A N K

d e n c e for the s u r f a c e c o m p o s i t i o n s o f the i n n e r m o s t k n o w n satellites, Ariel a n d M i r a n d a , t h e s e o b j e c t s are i n c l u d e d in the table for c o m p a r i s o n . G r e e n b e r g (1975, 1976, 1978) has rev i e w e d the q u e s t i o n o f the m a s s e s o f the U r a n i a n satellites, a n d on the basis o f b u l k m e a n d e n s i t y c o n s i d e r a t i o n s has s u g g e s t e d for M i r a n d a a n d Ariel that " . . . at least one o f t h e s e b o d i e s is icy, either in b u l k or s u r f a c e c o m p o s i t i o n . T h e s e satellites are v e r y different t h a n (sic) o t h e r parts o f the U r a n u s s y s t e m . T h e y are m a d e o f m u c h b r i g h t e r m a t e r i a l t h a n the rings a n d are significantly b r i g h t e r a n d / o r less d e n s e t h a n O b e r o n . " His a n a l y s i s yields the p r o d u c t s o f the m a s s e s o f pairs o f the U r a n i a n satellites, b u t not the i n d i v i d u a l m a s s e s . F r o m the m o t i o n o f M i r a n d a he finds that the p r o d u c t o f the m a s s e s o f Ariel a n d U m b r i e l is ~<10 -a that o f U r a n u s , a n d the p r o d u c t o f the m a s s e s o f M i r a n d a a n d Ariel is < 5 × 10 -'z f r o m the m o t i o n o f U m b r i e l . O f these t h r e e satellites, we k n o w o n l y that w a t e r frost o c c u r s on the s u r f a c e o f U m briel, a n d we s u s p e c t that o t h e r m a t e r i a l h a v i n g different spectral p r o p e r t i e s coexists. O n the b r o a d a s s u m p t i o n that the b u l k m e a n d e n s i t i e s , / 5 , of the satellites are all the s a m e , a n d that their s u r f a c e a l b e d o s (from w h i c h the radii are d e r i v e d ) are the s a m e , G r e e n b e r g ' s c o n d i t i o n s are met for Pv - > 0 . 4 a n d ~5 ~< 2 g c m --3, o r for pv = 0.3 a n d / 5 ~< 1 g c m - ' .

SUMMARY OF PHOTOMETRY OF THE SATELLITES OF URANUS AND SATURN A s u m m a r y o f e x i s t i n g p h o t o m e t r y o f the satellites of S a t u r n a n d U r a n u s c o n s i d e r e d in this p a p e r is g i v e n in Table III. I n addition to the n e w d a t a in this p a p e r , the o t h e r q u a n t i t i e s w e r e t a k e n from a v a r i e t y o f s o u r c e s . U B V d a t a for the U r a n i a n satellites w e r e t a k e n f r o m A n d e r s s o n (1974) except w h e r e n e w i n f o r m a t i o n (V0 a n d B-V for P h o e b e f r o m D e g e w i j a n d Van H o u t e n , 1979) h a v e s u p e r c e d e d t h e m . I t o o k V0 for U m b r i e l f r o m R e i t s m a e t al. (1978), b y adding their i n s t r u m e n t a l AV0 to A n d e r s s o n ' s V0 for T i t a n i a , a w e l l - e s t a b l i s h e d q u a n t i t y . T h e S a t u r n ' s rings d a t a are f r o m M o r r i s o n e t al. (1976), the i n f r a r e d d a t a for R h e a , T e t h y s , a n d D i o n e f r o m C r u i k s h a n k e t al. (1977), a n d the U B V d a t a for the s a m e satellites f r o m C r u i k s h a n k (1979a) who c o m p i l e d t h e m from o t h e r s o u r c e s , chiefly N o l a n d et al. (1974). T h e solar colors are from J o h n s o n (1965). T h e e r r o r s in t h e s e q u a n t i t i e s are those g i v e n in the original s o u r c e s , b u t in some c a s e s e s t i m a t e s w e r e m a d e w h e r e insufficient explicit i n f o r m a t i o n was a v a i l a b l e . T h e r e f l e c t a n c e s o f the v a r i o u s satellites, after r e m o v a l o f the solar colors, are p l o t t e d in Fig. 6 for c o m p a r i s o n a m o n g g r o u p s . T h e s e data are n o r m a l i z e d to 1.0 at the V wavelength. No clear distinction among these o b j e c t s is a p p a r e n t in the U a n d B

TABLE III SUMMARY OF PHOTOMETRY

V0 Enceladus Tethys Dione Rhea Hyperion Phoebe Umbriel Titania Oberon Sun

11.8 ± 10.3 ± 10.4 ± 9.7 ± 14.16 ± 16.47 ± 15.05 ± 14.01 ± 14.27 ± --

V-U 0.1 0.1 0.1 0.1 0.03 0.03 0.06 0.06 0.03 --

--1.07 -1.04 -1.11 -1.04 -0.99 . -0.98 -0.88 -0.70

+ ± + ± ± . ± ±

V-B -0.08 0.08 0.08 0.04 0.03 . 0.03 0.03 --

0.62 -0.74 -0.76 -0,76 -0.74 0.70 . -0.70 -0.68 -0.64

± ± ± ± ± ±

0.08 0.08 0.08 0.08 0.04 0.02

± 0.04 ± 0.03 --

V-J

V-H

V-K

1.06 ± 0.05 0.9 ± 0.1 0.8 _+ 0.2 1.06 ± 0.08 1.03 ± 0.05 0.98 ± 0.10 1.30 ± 0.06 1.30 ± 0.06 1.35 ± 0.04 1.06 --

1.01 ± 0.05 0.70 ± 0.1 0.60 ± 0.15 1.01 ± 0.08 1.15 +- 0.05 1.21 _+ 0.10 1.55 ± 0.06 1.50 ± 0.06 1.55 ± 0.04 1.31 --

0.77 ± 0.05 0,54 ±0. I 0.48 ± 0.15 0.77 ± 0.08 0,95 ± 0.05 1.62 ± 0.12 1.46 ± 0.06 1.36 ± 0.06 1.41 ± 0.04 1.41

SATELLITES

1.4

I

OF SATURN

1.0 } 0.8

J

I ./~TU~.~

1.2 p e,.~. . . . . . . . . . . TO " ~ DRHTe "~. 0 "'-PT

I "~OU.~_

P

--~-Eu E\ HpR\ \

¢0

257

AND URANUS

~-

~T

0

\ Te ~''D\ \

\-.

,

H \"~ \ P

,.,

~_ 0.6 --4

- - - - - ~ _ _ res0

0.4 0.2 U B V

0

II~l 0.5

J

I

1

H

L

1.0 1.5 Wavelength (/zm)

1

K

l

I

2D

FIG. 6. Spectral reflectance of several outer planet satellites from p h o t o m e t r y listed in Table IlI. The solar colors have been removed. U B V J H K denote central wavelengths of standard photometric bands. K e y to symbols: E = Enceladus, R = Rhea, T = Titania, O = Oberon, U = Umbriel, H = Hyperion, Te = Tethys, D = Dione, S = S a t u r n ' s rings, Eu = Europa, P = Phoebe. D a s h e d and dot-dashed lines delineate fields discussed in the text. The data for Saturn's rings were taken from Morrison et al. (1976) and are normalized to 1.0 at J.

bands, but at longer wavelengths a separation occurs, as noted by Cruikshank et al. (1977). The three Uranian satellites form a group delineated by the d o t - d a s h lines, while the Saturn satellites, including the rings and ice-covered Galilean satellite Europa, define another field enclosed in the dashed lines. Hyperion is a minor exception, lying just outside this field, and Phoebe is a renegade that crosses both fields. While the photometry given in Table III and Fig. 6 is better than previously available, the basic picture given by Cruikshank et al. (1977) has not changed. That is, the Uranian satellites as a group have distinctly different infrared reflectances than do the other satellites of the outer planets. Now that we have established the presence of water ice on both groups o f bodies, the difference is all the more puzzling, as noted earlier in this paper. The difference may lie in the fractions o f the surfaces covered by the water ice, other bright material mixed in with the ice, or the presence o f bound impurities in the ice structure. Phoebe appears to be a special case, perhaps representing

the nucleus of a captured comet; it clearly does not resemble the asteroids, however, and one wonders why an extinct comet would not look like either an asteroid or a water-ice-covered body. ACKNOWLEDGMENTS I thank Richard Joyce for ensuring o p t i m u m perform a n c e of the infrared e q u i p m e n t at Kitt Peak, and Barbara Schaefer and R. H. Brown for their assistance at the telescope. I a m especially grateful to Kaare A k s n e s for providing the finding e p h e m e r i s for Phoebe. R. N. Clark kindly assisted in the data reduction. This work was supported in part by N A S A Grant N G L 12-001-057.

Note added in proof." P. D. Nicholson and T. J. Jones (Icarus, submitted) obtained J H K photometry of Titania, Oberon, and Ariel contemporaneously with the data reported here. Their data for Oberon and Titania agree satisfactorily with m y p h o t o m e t r y , and they show that Ariel h a s virtually the same J H K colors as the other Uranian satellites. This provides strong evidence that Ariel is likewise covered with water ice or frost. REFERENCES ANDEaSSON, L. E. (1974). A photometric s t u d y of Pluto and satellites of the outer planets. Ph.D. thesis, Indiana University.

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