Eight-color photometry of Hyperion, Iapetus, and Phoebe

ICARUS ~3, 341--347 (1983)

Eight-Color Photometry of Hyperion, lapetus, and Phoebe 1 DAVID J. THOLEN AND B. Z E L L N E R Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721 Received June 22, 1982; revised October 18, 1982 Eight-color spectrophotometry was obtained of Phoebe, Hyperion, and the dark side of Iapetus. Our observed V magnitudes and Voyager-derived diameters yield geometric albedos of 0.07 for Iapetus (with some bright-side contamination), 0.06 for Phoebe, and limits of 0.19 to 0.25 for Hyperion (using the satellite's maximum and minimum dimensions, respectively). Hyperion and Iapetus have quite reddish spectra similar to each other and the spectra of D-type asteroids. Hyperion, however, has a much higher albedo than the dark side of Iapetus or any D-type asteroid measured to date. The mean spectrum of Phoebe is much flatter, with a broad absorption feature near 1 wm. Therefore the surface materials of Phoebe and the dark side of Iapetus are optically quite different, a result that constrains the possible modes of interaction between Phoebe and the other two satellites.

INTRODUCTION

The origin of the hemispherical brightness asymmetry of Iapetus is one of the more interesting puzzles of the Saturn system. Zellner (1972) and Murphy et al. (1972) proved that the lightcurve was due to albedo, not shape. Morrison et al. (1975) analyzed the extensive photometry of Iapetus and deduced a nearly symmetric distribution of dark material on the leading hemisphere, which was recently confirmed by Voyager imagery (Smith et al., 1982). Cook and Franklin (1970) proposed an exogenous origin for the dark material, and Soter (1974) suggested Phoebe as a source for the infalling material. Some of the material could bypass Iapetus and fall on Hyperion as well. Ground-based observations can be used to compare the spectral reflectivities of the surface materials and to perform one test of these hypotheses. UBV data exist for all three satellites (Andersson, 1974; Degewij et al., 1980a; Millis, 1973, 1977). The satellites all show somewhat similar colors with a drop-off to the ultraviolet. The data at longer wavet Paper presented at the "Saturn Conference," Tucson, Arizona, May 11-15, 1982.

lengths are less homogeneous. Degewij et al. review the data available for Hyperion and Phoebe; Cruikshank et al. (1982) do the same for the dark side of Iapetus. Extensive photometry of the dark side of Iapetus was recently obtained by Cruikshank et al., including UBVJHK and narrow-band spectrophotometry at 21 wavelengths using McCord's 25-color system. The data seem to indicate that Phoebe and the dark side of Iapetus have different surface materials, but because of the discordance in the existing VJHK observations of Phoebe reported by Degewij et al. (1980a, 1980b) the results were in need of confirmation. The measurements of Hyperion at longer wavelengths show peculiar colors, again a result in need of confirmation. The Voyager spectrophotometry extends to only 0.6 wm (see Fig. 19 of Smith et al., 1982), so confirmation of the ground-based red data is not possible, and only limited spectral reflectivity comparisons can be made. We report here new multicolor photometric observations of these three satellites in the eight-color system. OBSERVATIONS

Two observations of Hyperion and four each of Phoebe and the dark side of Iapetus were obtained with the eight-color photom341 0019-1035/83 $3.00 Copyright© 1983by AcademicPress,Inc. All rightsof reproductionin any formreserved.

342

THOLEN AND ZELLNER TABLE I NIGHTS OF OBSERVATIONS

Date

Tel."

Dia. b

No. r

Conditions d

1981 May 9 1982 Mar 21 1982 Mar 23

C C C

12.6" 10.0 10.0

19 13 13

1982 Mar 24 1982 Mar 31

C SO

10.0 8.3

13 8

Clear. Seeing 2". Clear. Seeing 1". Intermittent clouds. Seeing 1". Clear. Seeing 1". Cirrus first half. Seeing 1".

Telescope: C = 1.54-m Catalina reflector; SO = 2.29-m Steward Observatory reflector. All observations made by D. Tholen. b Size of focal plane diaphragm in arcseconds. c Number of standard star observations. d Typical seeing conditions indicated. Seeing infrequently ballooned to 3 arcsec more than shown.

eter of the Lunar and Planetary Laboratory. The instrumentation and photometric system are described by Tedesco et al. (1982). The 1.54-m Catalina reflector and 2.29-m Steward Observatory reflector of the University of Arizona Observatories were used for all observations. The nights of observation are listed in Table I and the results of the photometry are shown in Table II. Uncertainties in the photometry are shown directly above each of the values in the table in units of 0.01 magnitude. The uncertainties represent the quadratic sum of the standard deviation of the mean for the photometry of that object and the scatter of the standard stars for the whole night. Table II also shows the mean colors for each object, weighted according to the reciprocal square of the uncertainties.

lapetus All four observations were made near eastern elongation (90° orbital phase) to minimize the contamination from the bright side. Ground-based techniques, unfortunately, are incapable of avoiding all bright material (Smith et al., 1982). The May 1981 observation fell the closest to eastern elongation (1.83 days before or 82° orbital phase), yielded the faintest absolute magnitude, and therefore suffered the least from bright-side contamination. The March 1982

observations fell, respectively, at 60, 74, and 106° orbital phase.

Hyperion Observations were made near elongation in order to minimize the sky subtraction problem associated with observations made near a bright planet. The March 21 observation, made 1.7 days after eastern elongation (119° orbital phase), was obtained with the sky subtracted 32 arcsec to the north. This position was a nearly ideal location: at the same radial distance from Saturn as Hyperion and symmetrically placed with respect to the major axis of the rings and the diffraction spike of the telescope. The March 31 observation, made 2.4 days after western elongation (311° orbital phase), was obtained through an 8.3-arcsec focal plane diaphragm; the sky was again measured 32 arcsec to the north. Assuming the sky at the position of Phoebe represented the true sky brightness (zero scattered light from Saturn), the scattered light at Hyperion amounted to only 9% of the signal from Hyperion itself at 0.55 Ixm. Hence the photometry of Hyperion is not sensitive to the accuracy of the sky subtraction on March 31. Note that even though the brightness gradient around Saturn is substantial, the color gradient is less severe, so any errors introduced in the colors will be much less than the errors in the magnitudes. The good agreement between the two observations supports the belief that sky subtraction was not a problem for these observations of Hyperion. Although the two measurements were made on opposite sides of the orbit, they do not necessarily represent opposite sides of Hyperion; the existing lightcurve data favor nonsynchronous rotation (Wisniewski, private communication), but the rotation period remains undetermined.

Phoebe The satellite was acquired using the differential coordinates provided by The Astronomical Almanac. Phoebe's true posi-

343

PHOTOMETRY OF SATURNIAN SATELLITES T A B L E II PHOTOMETRY OF IAPETUS, HYPERION, AND PHOEBE Object

Iapetus

Date (UT)

1981 M a y

Color index °

9.195

1982 M a r 21.363 1982 M a r 24.456 1982 M a r 31.419 Weighted m e a n

Hyperion

1982 M a r 21.327 1982 M a r 31.408 Weighted m e a n

Phoebe

1982 M a r 21.386 1982 M a r 23.364 1982 M a r 24.432 1982 M a r 31.377 b Weighted m e a n

Mag

Color index ° v-x

v-p

v-z

1 0.14 1 0.12 2 0.16 2 0.13 1 0.14

2 0.22 2 0.20 2 0.25 2 0.19 1 0.21

2 0.27 2 0.21 2 0.28 2 0.22 1 0.24

2 0.28 1 0.27 2 0.33 3 0.27 1 0.28

2 14.47 2 14.24

2 0.12 1 0.18 1 0.16

3 0.20 2 0.22 2 0.21

3 0.26 2 0.26 2 0.26

2 0.31 2 0.30 2 0.30

5 16.66 5 16.65 4 16.45 2 16.38

6 0.05 8 -0.12 6 -0.04 2 0.02 2 0.01

12 0.02 13 -0.22 10 0.00 4 -0.03 4 -0.04

8 -0.24 9 -0.20 7 -0.30 3 -0.06 3 -0.12

12 -0.25 18 -0.45 14 -0.36 7 -0.14 5 -0.22

s-v

u-v

b-v

V

1 0.29 3 0.24 4 0.23 2 0.20 1 0.27

1 0.18 2 0.17 4 0.14 3 0.11 1 0.17

1 0.07 1 0.05 3 0.07 3 0.03 1 0.06

2 12.36 2 12.07 3 12.14 2 12.05

4 0.26 2 0.25 2 0.25

2 0.14 2 0.15 2 0.15

2 0.04 2 0.09 ! 0.06

12 0.18 15 0.16 9 0.07 5 0.22 4 0.18

6 0.00 6 0.07 5 0.05 3 0.04 2 0.04

5 -0.08 5 -0.03 5 0.05 3 -0.01 2 -0.02

v-w

Color indices are in magnitudes, more positive for redder colors, with zero color index representing a neutral reflector of sunlight (Tedesco et al., 1982). Effective wavelengths are 0.34, 0.36, 0.44, 0.55, 0.70, 0.85, 0.95, a n d 1.04 ~ m for the s, u, b, v, w, x, p, and z filter p a s s b a n d s , respectively. U B V colors can be derived using the following transformations: B-V = 0.97(b-v) + 0.67; U-B = 1.07[(u-v) - (b-v)] + 0.20. b High voltage was interrupted after the Phoebe observation and before the Hyperion and Iapetus observations leading to a discontinuity in the sensitivity of the red photomultiptier. A correction of - 0 . 1 6 m a g was applied to the Phoebe red data to splice the two parts of the night together.

tion fell somewhat less than an arcminute to the northwest of the predicted position, because the Almanac uses the orbit of Ross (1905) rather than the improved orbit by Zadunaisky (1954), who noted errors on the order of an arcminute in Ross' orbit. The expected motion was observed, providing positive identification. The four observations represent three different rotational phases of Phoebe. The rotational period found from Voyager data is 9.4 ± 0.2 hr

(Thomas et al., 1982). Table III shows the rotational circumstances of the observations based on this period with the March 21 observation taken as the zero point. ALBEDO DETERMINATION

Visual geometric albedos can be derived using the observed V magnitudes converted to absolute magnitudes, the Voyager-derived diameters (Smith et al., 1982), and the formula of Zellner (1979, p. 789). Anders-

344

THOLEN AND ZELLNER TABLE III ROTATIONS OF PHOEBE

UT (1982)

No. rotations

Mar 21.386 23.364 24.432 31.377

0.00 5.05 -+ 0.11 7.78 - 0.17 25.51 -+ 0.54

son (1974) investigated the size of the error incurred by using heliocentric and geocentric distances and phase angles for Saturn rather than the satellites themselves when deriving absolute magnitudes. The effect is small for Hyperion and Iapetus but not insignificant for Phoebe. A first-order correction to the distances and phase angles of all the observations was applied for the purpose of deriving accurate absolute magnitudes. The empirical opposition effect of Gehrels and Tedesco (1979) was used to scale the magnitudes to a phase angle of 7° and then a linear phase coefficient was used to extrapolate back to zero phase. Voyagerderived phase coefficients of 0.036 and 0.033 mag deg -1 were used for the darker and lighter areas of Phoebe (Thomas et al., 1982), respectively, and 0.026 mag deg n was used for Hyperion (Smith et al., 1982).

A value of 0.03 mag deg i was assumed for Iapetus. Diameters of 1460 and 220 km were used for Iapetus and Phoebe, respectively. Hyperion is a more difficult case because of its irregular shape, given roughly as 410 by 260 by 220 km. The cross-sectional area presented at the time of the p h o t o m e t r y is unknown. Hence only upper and lower limits to the disk-integrated geometric albedo can be estimated using approximations for the maximum and minimum cross-sectional areas possible. A circular disk of diameter 325 km gives the same cross-sectional area as an ellipse with axes of 410 and 260 km, which roughly corresponds to the maximum area Hyperion could project; this effective diameter was used to estimate the lower limit for the albedo of Hyperion. Similarly, a disk of diameter 270 km was employed to approximate the area of an endon rectangular cross section 260 by 220 km for the other extreme. Table IV shows the corrected heliocentric and geocentric distances and phase angles, along with the derived absolute magnitudes and geometric albedos. As expected, the disk-integrated albedo of Iapetus increased as the bright-side contamination increased, but the overall level of contamination is small as evidenced by the minimum albedo of 0.070. This value is only slightly

TABLE IV ABSOLUTE MAGNITUDES AND ALBEDOS

Object

Date (UT)

V

r (AU)

A (AU)

V(1 ,cx)

ct

Iapetus

1981 May 9.195 1982 Mar 21.363 1982 Mar 24.456 1982 Mar 31.419

12.36 12.07 12.14 12.05

9.574 9.673 9.668 9.658

8.831 8.732 8.710 8.672

2.73 2.44 2.51 2.43

4?26 2.04 1.73 1.01

2.68 2.54 2.64 2.63

0.070 0.079 0.072 0.073

Hyperion

1982 Mar 21.327 1982 Mar 31.408

14.47 14.24

9.657 9.671

8.716 8.684

4.85 4.62

2.04 0.97

4.98 4.84

0.17-0.25 0.19-0.28

Phoebe

1982 Mar 1982 Mar 1982 Mar 1982 Mar

16.66 16.65 16.45 16.38

9.706 9.706 9.704 9.699

8.761 8.750 8.743 8.711

7.01 7.01 6.81 6.75

1.96 1.75 1.65 0.92

7.08 7.09 6.93 6.93

0.054 0.053 0.061 0.061

21.386 23.364 24.432 31.377

V(1,0)

Pv

PHOTOMETRY OF SATURNIAN SATELLITES higher than the value of 0.067 found by Cruikshank et al. (1982) only hours from eastern elongation. Lebofsky et al. (1982) reported an albedo of 0.11 at 2.2 p.m and a reflectance relative to 0.6 ~xm of 1.6, which scales to an albedo of 0.066 at 0.55 ~m. Still, as pointed out earlier, none of these albedos represents the dark material exclusively. The absolute magnitudes of Phoebe in Table IV indicate a lightcurve amplitude of about 0.2 mag, a lower limit due to the incomplete sampling of the lightcurve. Because of the nearly spherical shape, the lightcurve is due primarily to albedo variations over the surface; Voyager found variations of up to 50% in albedo and a lightcurve amplitude of about 0.3 mag (Thomas et al., 1982). In order to satisfy both observations, the lower limit for the albedo of Hyperion is about 0.19; the upper limit is about 0.25. Note that this range is due to the unknown cross-sectional area seen and does not refer to albedo variation over the surface of the satellite; Voyager found only small (10 to 20%) variation in albedo over the surface of Hyperion (Smith et al., 1982). DISCUSSION

lapetus

All four observations of Iapetus show a moderately sloping reddish spectrum (see Fig. 1). The four spectra are all normalized to unity reflectance at 0.55 ixm [see Tedesco et al. (1982) for a discussion of the solar calibration]. Only a slight change in color was found as the bright-side contamination decreased (by about 10%, as evidenced by the albedo decrease from 0.079 to 0.070), but the trend is toward redder colors as the ratio of dark to bright material increases, as expected. The eight-color spectrum agrees well with the spectrophotometry Cruikshank et al. (1982) obtained on 21 May 1979, a day before eastern elongation; they made a second measurement the following day only hours from elongation and obtained a slightly redder spec-

345

trum. The closest asteroidal spectral analog to the dark side of Iapetus is the D type, found predominantly in the outer regions of the asteroid population. Hyperion

The two observations of Hyperion are in agreement. They both show a moderately sloping reddish spectrum as well, as indicated in Fig. I. The similarity with Iapetus is striking despite the factor of 3 difference in albedo. The same curve has been drawn through both the Iapetus and Hyperion spectra in Fig. I to emphasize the similarity. Whether or not the similarity continues at JHK wavelengths would be interesting to see; the data already exist for the dark side of Iapetus, but not to our knowledge for Hyperion. Although the eight-color spectrum of Hyperion is also similar to those of some Dtype asteroids, the satellite has a much higher albedo than any D type measured to date (Tedesco and Gradie, 1982). Water frost is known to exist on the surface of Hyperion (Cruikshank, 1980; Lebofsky et al., 1981; Cruikshank and Brown, 1982). We suggest that the surface of Hyperion and the dark side of Iapetus might consist of basically the same material, with the distribution of the water frost or ice affecting the disk-integrated albedo of the satellites differently. In both cases the more neutral reflectivity of the frost or ice may be diluting the redness of the spectrum. In the case of Iapetus, the ice is concentrated in regions near the limb, apparently affecting the albedo only slightly; on Hyperion, however, the frost is more uniformly distributed over the surface, perhaps having a much stronger effect on the albedo. A somewhat analogous effect was found by Feierberg (1981) in his laboratory measurements of the spectrum and albedo of pyroxene when metallic iron was added in different ways. Phoebe

Because of the very faint mean opposition magnitude of Phoebe (V0 - 16.5), high

346

THOLEN AND ZELLNER i

i

i

i

i

i

i

r

1.4

o

1.2

1.2

1.0 0.~

i I" I Ii

d.O

lopetus

0.8

v May9 (0:82°1 o Mar 21 (a =60 ) a Mar 24 (8=74)

0.6

Phoebe

a Mar 31 1e:1061

8

o Mar 21 v Mar 2~ o Mar24

tx Mar 31

1.4 1.2

° /

1.0

zx o /

/6 /

o/

1.2 I.O

Hyperion

_2/

0.8

/

a Mor 31

0S

'

015

'

I

07

I

I ~

Phoebe

0.8

meQn spectrum

0b

I

11,

X (/~m)

FIG. 1. Eight-color reflectance spectra of H3~perion and the dark side of Iapetus. All observations are normalized to 1.0 at 0.55 ~m. Error bars are generally the size of the symbols used, the largest being about 50% larger than the symbol, and therefore are not shown. The same curve has been d r a w n through both data sets to emphasize the similarity of the two objects' spectra. The orbital phases for the observations of Iapetus are also indicated.

signal-to-noise measurements are difficult to obtain. Our four measurements are shown in Fig. 2; also shown is the weighted mean spectrum. The ultraviolet drop-off seen in the earlier UBV data is also present here, but the region around 1 ixm, seen photometrically for the first time, also shows an absorption feature. The single V-R measurement reported by Degewij et al. (1980a) gave a fairly neutral color; the present work confirms this. The latest VJHK data (Degewij et al., 1980b), which appear more consistent with our measurements than the earlier Cruikshank (1980) data, also show near-unity relative reflectance, although with uncertainties of about 10%. Apparently the 1-~m absorption feature does not extend further into the infrared. Phoebe's spectrum is in sharp contrast to the spectra of Iapetus and Hyperion. Obviously the surface materials must be quite different. The highest signal-to-noise spectrum of the

0!~

101.5

i 017

r

01.9

J

I

[.J

x (~m) FIG. 2. Eight-color reflectance spectra of Phoebe obtained in 1982. All observations are normalized to 1.0 at 0.55 ~m. Only the e x t r e m e s of the error bars are shown rather than individual o n e s , except where they do not overlap. The weighted m e a n of the four individual spectra is s h o w n at the bottom; error bars are shown only if larger than the symbol.

four (March 31) resembles a C-type asteroid, both in relative reflectivity and in albedo. The other three, however, show a more pronounced 1-~m feature than is typical of C-type asteroids. The dissimilarity of the spectra rules out Phoebe dust as a simple contaminant of the leading side of Iapetus [Model A of Cruikshank et al. (1982)]. As Cruikshank et al. point out, however, there may still be some interaction between Iapetus and infalling Phoebe material. The reader is referred to their paper for a full discussion of the proposed models. The photometry suggests and the Voyager images show large albedo variations over the surface of Phoebe, so color variations might exist as well. Unfortunately, the low signal-to-noise ratios make color variations difficult to confirm, but we note the conspicuously neutral blue reflectance of the March 24 observation (and its unique rotational phase) and the more neutral infrared colors shown in the March 31 obser-

P H O T O M E T R Y OF S A T U R N I A N S A T E L L I T E S v a t i o n , in c o n t r a s t to t h e m o r e p r o n o u n c e d d r o p - o f f p r e s e n t in t h e o t h e r t h r e e o b s e r v a tions. CONCLUSIONS

347

FEIERBERG, M. A. (1981). Evidence for a Compositional Relationship between Asteroids and Meteorites from Infrared Spectral Reflectances. Ph.D. the-

sis, University of Arizona. GEHRELS,T., AND E. F. TEDESCO(1979). Minor planets and related objects XXVIII. Astron. J. 84, 10791087. LEBOFSKY, L. A., G. H. RIEKE, AND M. J. LEBOFSKY (1981). Infrared reflectance spectra of Hyperion, Titania, and Triton. Icarus 46, 169-174.

H y p e r i o n an d t h e d a r k side o f I a p e t u s have virtually identical eight-color spectra a l t h o u g h t h e i r v i s u a l g e o m e t r i c a l b e d o s diffe r by a b o u t a f a c t o r o f 3. T h e a l b e d o differLEBOFSKY, L. A., M. A. FEIERBERG, AND A. T. TOence may not necessarily imply different KUNAGA(1982). Infrared observations of the dark s u r f a c e m a t e r i a l s , h o w e v e r , as in e a c h c a s e side of Iapetus. Icarus 49, 382-386. w a t e r f r o s t o r ice c o n t a m i n a t e s p a r t o f t h e • MILLIS, R. L. (1973). UBV photometry of Iapetus. Icarus 18, 247-252. s u r f a c e . A d d i t i o n a l w o r k is n e e d e d to inMILLIS, R. L. (1977). UBV photometry of Iapetus: v e s t i g a t e this p o s s i b i l i t y . P h o e b e ' s s p e c Results from five apparitions. Icarus 31, 81-88. t r u m is t o t a l l y d i f f e r e n t f r o m b o t h H y p e r i o n MORRISON, D., T. J. JONES, D. P. CRUIKSHANK, AND a nd I a p e t u s at r e d a n d n e a r - i n f r a r e d w a v e R. E. MURPHY (1975). The two faces of Iapetus. Icarus 24~ 157-171. l e n g t h s , thus P h o e b e d u s t c a n be r u l e d o u t MURPHY, R. E., D. P. CRUIKSHANK, AND U. MORRIas a s i m p l e c o n t a m i n a n t o f t h e l e a d i n g side SON (1972). Radii, albedos, and 20-micron brightof Iapetus. A n y process for darkening the ness temperatures of Iapetus and Rhea. Astrophys. l e a din g side o f I a p e t u s that i n v o k e s infalling J. 177, L93-L96. material from Phoebe must be a process by Ross, F. E. (1905). Investigations on the orbit of Phoebe. Ann. Harvard College Obs. 53, 101-142. w h i c h P h o e b e d o e s n o t l e a v e its s p e c t r a l SMITH, B. A., L. SODERBLOM, R. BATSON, P. signature on Iapetus. ACKNOWLEDGMENTS We thank W. Binkert for his assistance at the 2.29-m telescope. The paper benefitted from comments by M. Gaffey, R. Millis, and E. Tedesco. The work was supported by NASA Grant NSG 7502. REFERENCES ANDERSSON, L. E. (1974). A Photometric Study o f Pluto and Satellites o f the Outer Planets. Ph.D. thesis, Indiana University. COOK, A. F., AND F. A. FRANKLIN(1970). An explanation of the lightcurve of Iapetus. Icarus 13, 282291. CRUIKSHANK, D. P. (1980). Near-infrared studies of the satellites of Saturn and Uranus. Icarus 41, 246258. CRUIKSHANK, D. P., J. F. BELL, M. J. GAFFEY, R. H.

BRIDGES, J. INGE, H. MASURSKY,E. SHOEMAKER, R. BEEBE, J. BOYCE, G. BRIGGS, A. BUNKER, S. A. COLLINS, C. J. HANSEN, T. V. JOHNSON, J. L. MITCHELL, R. J. TERRILE, A. F. COOK II, J. Cuzzl, J. B. POLLACK, G. E. DANIELSON, A. P. INGERSOLL, M. E. DAVIES, G. E. HUNT, D. MORRISON, T. OWEN, C. SAGAN, J. VEVERKA,R. STROM, AND V.

SUOMI(1982). A new look at the Saturn system: The Voyager 2 images. Science 215, 504-537. SOTER, S. (1974). Brightness asymmetry of Iapetus. Presented at IAU Colloquium No. 28, Cornell University, August 1974. TEDESCO, E. F., AND J. C. GRADIE (1982). Taxonomy

of outer belt asteroids and the distribution of the taxonomic types. Submitted. TEDESCO, E. F., D. J. THOLEN, AND B. ZELLNER

(1982). The eight-color asteroid survey: Standard stars. Astron. J. 87, 1585-1592.

BROWN, R. HOWELL, C. BEERMAN, AND M. ROGN-

THOMAS, P., J. VEVERKA,D. MORRISON, M. DAVIES,

STAD (1982). The dark side of lapetus. Submitted.

AND T. V. JOHNSON(1982). Phoebe: Voyager 2 observations. Submitted. ZADUNAISKY, P. E. (1954). A determination of new elements of the orbit of Phoebe, ninth satellite of Saturn. Astron. J. 59, 1-6. ZELLNER, B. (1972). On the nature of Iapetus. Astrophys. J. 174, LI07-LI09. ZELLNER, B. (1979). Asteroid taxonomy and the distribution of the compositional types. In Asteroids (T. Gehrels, Ed.), pp. 783-806. Univ. of Arizona Press, Tucson.

CRUIKSHANK, D. P., AND R. n . BROWN (1982). Surface composition and radius of Hyperion. Icarus 50,

82-87. DEGEWIJ, J., L. E. ANDERSSON, AND B. ZELLNER

(1980a). Photometric properties of outer planetary satellites. Icarus 44, 520-540. DEGEWIJ, J., D. P. CRUIKSHANK, AND W. K. HART-

MANN (1980b). Near-infrared colorimetry of J6 Himalia and $9 Phoebe: A summary of 0.3-2.2 ~m reflectances. Icarus 44, 541-547.