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High-Resolution 0.33–0.92 μm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related?
Icarus 155, 375–381 (2002) doi:10.1006/icar.2001.6730, available online at http://www.idealibrary.com on
High-Resolution 0.33–0.92 µm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related? Bonnie J. Buratti, Michael D. Hicks, Kimberly A. Tryka, Micah S. Sittig, and Ray L. Newburn Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mailstop 183-501, Pasadena, California 91109 E-mail: [email protected] Received January 3, 2000; revised July 6, 2001
New high-resolution spectra in the 0.33 to 0.92 µm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature 412, 163–166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences. c 2002 Elsevier Science (USA)
I. INTRODUCTION
The dichotomous placement of high- and low-albedo material on the surface of the saturnian satellite Iapetus was realized almost as soon as it was discovered by Cassini in 1671. Measurements by ground-based observers and the Voyager spacecraft show that the morphology, high albedo (∼0.6), and spectrum of the bright side is similar to that of the other medium-sized, icy satellites of Saturn (particularly Tethys, Dione, and Rhea), while the markedly redder, dark hemisphere reflects only a few percent of incident radiation (McCord et al. 1971, Murphy et al. 1972, Zellner 1972, Smith et al. 1982). The amplitude of the lightcurve of Iapetus is nearly two magnitudes (Noland et al. 1974, Morrison et al. 1975, 1976, Millis 1977) with water ice, either in pure form or bound to minerals, dominating the higher albedo trailing side (Fink et al. 1976, Lebofsky et al. 1982, Clark et al. 1984), and a reddish low-albedo contaminant dominating the dark leading hemisphere (Cruikshank 1980, Cruikshank et al. 1983, 1984, Bell et al. 1985). The dark material is centered on the leading side of Iapetus (Smith et al. 1982) and its albedo
increases, and its color becomes less red, in a sinusoidal fashion as a function of distance from its apex of motion (Squyres et al. 1983, Buratti and Mosher 1995). Over 300 years after the discovery of Iapetus, scientists still cannot agree on the cause of its unique appearance. The models for the origin of the dark side of Iapetus follow two basic scenarios: One involves the endogenous placement of material, and the other invokes deposition of, or alteration by, exogenous particles. In the endogenous model the dark material is a vast deposit of material that erupted from the interior of Iapetus (Smith et al. 1982). The simplest of the second group of models involves the accretion of exogenous material onto Iapetus’s leading side. The first version of this model proposed that particles from Phoebe, the outer retrograde satellite of Saturn, were moved inward by Poynting–Robertson drag and collided with Iapetus (Soter 1974). This model’s major problem is that Phoebe’s visible spectrum is very flat, similar to C-type asteroids (Degewij et al. 1980), unlike the reddish visible spectrum of Iapetus’s leading side. A modification of this model that explains the redder color of the dark material on Iapetus involves the volatilization of ice by the impacting particles and subsequent enrichment of an organic-rich lag deposit (Cruikshank et al. 1983, Bell et al. 1985). In another modification, methanerich native particles are darkened and reddened by UV photolysis and ballistically redistributed (Squyres and Sagan 1984). A group of hybrid models contends that the erosion of surficial ice by micrometeorites excavated a dark underlying mantle (Cook and Franklin 1970, Wilson and Sagan 1995). The model of Peterson (1975) espouses the preferential accretion of bright particles on the trailing side of the satellite. The connection between Hyperion and the dark side of Iapetus was first suggested by results of the eight-color asteroid survey (Tholen and Zellner 1983) and later by mineralogical similarities between the two bodies. Vilas et al. (1996) identified absorption bands on Iapetus that are characteristic of the ferric charge transfer transitions in iron alteration minerals such as goethite and hematite, while Jarvis et al. (2000) identified similar bands on Hyperion. As part of a study to clarify the compositional relationships among the dark side of Iapetus and other members of the family
375 0019-1035/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved.
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of saturnian satellites, we obtained new high-resolution visible spectra of Iapetus, Hyperion, Phoebe, Dione, and Rhea with the 200-inch telescope on Palomar Mountain. We also obtained three spectra representative of D-type asteroids to serve as an analogue to the dark, organic-rich contaminant that is common in the outer Solar System. Our results show that Hyperion and the dark hemisphere of Iapetus can both be closely represented in terms of a simple two-component model comprised of a “pristine” icy satellite + D-type material. The process responsible for the dark side of Iapetus may not be unique: it may be operating on a more modest scale on the lower albedo leading side of the Galilean satellite Callisto, as originally suggested by Bell (1984) and on the Uranian satellites (Buratti and Mosher 1991). II. OBSERVATIONS AND DATA ANALYSIS
All observations were obtained over a period of three nights in 1997 and 1998 with the double spectrometer (Oke and Gunn 1982) attached to the 200-inch Hale telescope on Palomar ˚ in 1997 and at Mountain. A dichroic filter (placed at 5500 A ˚ 6800 A in 1998) directed the light onto two separate 1024 square CCD detectors, covering the red and blue regions of the visible spectrum. The total spectral range is 0.33 to 0.92 µm, with reso˚ per pixel. In general a different integration time lution of 3.8 A was used for the red and blue channels (Table I). All observations were obtained within airmasses of 1.0 and 1.2, with slit widths of 6 or 8 arcseconds. The objects and standard stars were centered in the spectrometer slit by means of a crosshair on the guide telescope, and offset tracking was done for moving planetary targets. Spectral calibrations were performed with He-Ne-Ar standard lamps. The solar analogue star HD 29461 was measured at the
same airmasses as all sets of observations. Bias frames at the start and end of the night, and flatfield measurements were obtained for each run. The flatfields were obtained by two methods: on the sky at twilight, and on the telescope dome, which was illuminated by an incandescent lamp. Table I summarizes the observations obtained for this study. The spectra were reduced in a standard fashion. First, the need for flatfielding was evaluated by checking for vignetting in each spectrum, and in the location on the flatfield image where the spectra lie. Since none of the spectra or flatfields exhibited apparent vignetting (to a level of ∼1%), and because all spectra were divided by the spectrum of the standard star, we did not perform flatfield corrections. The raw spectrum was computed by coadding seven rows located above and below the center of each spectrum. The sky + bias contribution was computed by coadding seven additional rows on each side of each spectrum. Each sky + bias was modeled by a linear equation. For Dione and Rhea, the scattered light from Saturn was sufficiently bright that the sky background was modeled with a quadratic equation. The sky + bias models were then subtracted from each target’s spectrum. Each spectrum was then divided by the spectrum of our solar analogue star, HD 29461, which had been processed in a similar fashion. Finally, two or three spectra for each object were coadded to improve the signal. The results for all five satellites and three asteroids are shown in Fig. 1. III. RESULTS
For Rhea, Dione, and 911 Agamemnon, our results are the first high-resolution visual spectra published. Two spectra each of 624 Hektor and 1578 Kirkwood were published in Vilas et al.
TABLE I Summary of Observations Date (UT) (Start)
Object
V mag
# Spectra
α
Latitude, longitude
Airmass
Int. time (s) (blue, red channel)
23 October 1997 09:05 09:38 09:52 10:12
HD 29461 911 Agamemnon 624 Hektor 1578 Kirkwood
7.96 8.32 14.71 15.34
3 1 2 2
5.4 5.0 16.0
unknown unknown unknown
1.10 1.10 1.06 1.04
0.5; 1; 5 180 480; 240 180; 360
14 November 1998 05:35 05:52 06:04 06:39 07:28
Rhea Iapetus Hyperion Phoebe HD 29461
9.55 10.95 14.08 16.35 7.96
3 3 3 2 3
2.41 2.39 2.42 2.34
−15, 340 −13, 113 N/A −31, 306
1.11 1.10 1.10 1.11 1.12
15 90; 120 240; 420 600; 900 3
15 November 1998 05:43 05:53 06:05 06:25 07:28
Dione Iapetus Hyperion Phoebe HD 29961
10.25 10.95 14.08 16.35 7.96
3 3 2 1 3
2.51 2.50 2.52 2.44
−15, 46 −13, 117 N/A −31, 177
1.11 1.10 1.10 1.11 1.17
15; 10 90; 120 420 600; 900 3; 5
Note. On 23 October 1997 the slit width was 6 ; on the remaining two nights it was 8 . The subobserver longitude is unknown on Hyperion because of its chaotic rotation state.
SATURNIAN SATELLITE AND D-TYPE ASTEROID SPECTRA
FIG. 1. Spectra of (from top to bottom) three D-type asteroids, Hyperion, Iapetus, Rhea, Dione, and Phoebe. All spectra were obtained on the 200-inch telescope on Palomar Mountain and the double spectrograph. The noise in the asteroid spectra between 0.5 and 0.6 µm is due to the placement there of the dichroic, which disperses the light into the red and blue channels. The spike at 0.76 µm is due to the O2 A atmospheric emission line. Less prominent spikes between 0.6 and 0.7 µm and 0.8 and 0.9 µm are due to O2 and OH emission lines in the terrestrial atmosphere.
(1993) and Fitzsimmons et al. (1994) respectively. Our results are in reasonable agreement with these previous publications, and our spectra of Iapetus, Hyperion, and Phoebe are in good agreement with those that have appeared earlier (Jarvis et al. 1997, 1998, 2000), with broad-band spectroscopic studies of the saturnian satellites (Noland et al. 1974, Millis 1977, Bell et al. 1979) and with results derived from the imaging camera on Voyager (Thomas et al. 1983, Thomas and Veverka 1985, Buratti 1984). The published Voyager spectra of Phoebe and Hyperion (Thomas et al. 1983, Thomas and Veverka 1985) and Dione (Buratti 1984) are redder than those derived at Palomar, but this discrepancy is removed if additional calibrations to the Voyager camera measurements are applied (Buratti 1984, p. 400). A medium-resolution spectrum of Rhea shows a slight
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(few percent) decrease in reflectivity between 0.65 and 0.92 µm that we did not observe (Clark and Owensby 1981). We believe that this discrepancy may be due to the different subobserver longitudes between the two sets of observations. During the period of the Clark and Owensby observations, the longitude range was 91–114◦ , while our subobserver longitude was 340◦ . Diskresolved Voyager spectra of high- and low-albedo regions of Rhea show a significant difference in the 0.35 to 0.6 µm region (Buratti 1984, Buratti et al. 1990). Because the low-albedo terrain is located primarily on the trailing side of Rhea (Smith et al. 1982), it is reasonable to expect that Rhea exhibits longitudinal differences in its spectrum. Our observations of the D-type asteroids are in agreement with the broad-band colors published as part of the eight-color asteroid survey (Tholen and Zellner 1983). All three are typical of the D-class of asteroids (Fitzsimmons et al. 1984, Vilas et al. 1993). Because Phoebe is in simple rotation with a period of 9.4 ± 0.2 h (Thomas et al. 1983), our observations on two consecutive nights are well suited to detecting global compositional heterogeneity on it. An albedo map of Phoebe constructed from Voyager images shows significant changes in reflectivity on its surface (Simonelli et al. 1999). The pattern is organized roughly in a hemispherical fashion, with a large higher albedo region centered at 0◦ showing normal reflectances in the 0.10 to 0.13 range, and a lower albedo region with reflectances of ∼0.06 located about 180◦ away. Our second night’s spectrum was obtained with a subobserver longitude of 177◦ : the ideal location for isolating the spectrum of the low-albedo region. Although the subobserver longitude for the first night was not located precisely in the center of the high-albedo region, the viewing geometry included this area almost in its entirety. Thus, any spectral changes between the high- and low-albedo regions should appear in the disk-integrated spectra obtained on the two nights. Figure 2 shows the two spatially separated spectra we obtained for Phoebe. They are in good agreement, indicating that the disk-integrated surface of the satellite is spectrally homogeneous (at least in the visual region of the spectrum). However, we cannot assume this result indicates that Phoebe is compositionally homogeneous; the albedo map of Simonelli et al. (1999) has shown that it is not. Although Phoebe’s spectrum is flat, and similar to C-type (carbon-rich) bodies, its albedo is higher than carbon, particularly in the brighter areas. The presence of water ice on its surface has recently been detected (Owen et al. 1999). Because water ice is spectrally flat in the region we are studying and bright (Wagner et al. 1987) its addition to a C-type planetary surface could be expected to raise its albedo but not appreciably change its spectral shape. Our spatially resolved spectra are thus consistent with a surface consisting of patches of water ice and a low-albedo, flat material, as suggested by Simonelli et al.’s map. It is also important to remember that due to an imprecise knowledge of Phoebe’s rotation period, the placement of surface features cannot be predicted over the 15 or so years that elapsed between the Voyager encounter and the acquisition of our spectra. Thus, we cannot be sure of the current placement of Simonelli et al.’s albedo features on a geographical
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FIG. 2. Rotationally resolved spectra of Phoebe. In the top cell, spectra are shown for longitudes of 177◦ and 306◦ ; the large dots are 10-point running averages. In the bottom cell, the averaged spectra are plotted on top of each other to show their similarity.
reproduce the spectrum of the dark side of Iapetus, places the origin of the dark material on Titan. Rather than combining the spectra of pure substances that are likely to be found on the surfaces of the saturnian satellites, we adopt a new approach of coadding the spectra of astronomical bodies that represent compositional end members. This approach avoids the problem of dissimilarities between laboratory samples and planetary surfaces, and it helps to establish relationships among the bodies themselves. Our end members are a typical icy satellite, represented by the coadded and three-point averaged spectra of Dione and Rhea (Fig. 3 bottom), D-type material, represented by the coadded and three-point averaged spectrum of the three observed D asteroids (Fig. 3, top), and spectrally flat, carbon-rich material, represented by Phoebe. The albedo of the bright side of Iapetus is as well as its morphology, visual spectrum, and photometric properties are all similar to those of Dione and Rhea (Smith et al. 1982, Buratti and Veverka 1984, Squyres and Sagan 1984). Tholen and Zellner (1983), Bell et al. (1985), and Vilas and Smith (1985) noted the similarity between the low-albedo side of Iapetus and the D-type asteroids, which are believed to be rich in primitive, possibly prebiotic material. The model we thus adopt represents the integral spectrum of Iapetus as the superposition of D-type material on top of an icy satellite substrate, FIap (λ) = AFIcy (λ) + (1 − A)FD (λ),
◦
grid. Because our spectra were obtained 129 apart, they are, nevertheless, well suited for seeking global compositional variegations. IV. A TWO-COMPONENT MODEL FOR IAPETUS AND HYPERION
Previous models of the spectrum of Iapetus have assumed that it can be represented by the coaddition of discrete compositional units representing the dark and bright regions of Iapetus (Bell et al. 1985, Vilas et al. 1996, Owen et al. 2001). This “checkerboard” model is reasonable, because spectra obtained of the low-albedo hemisphere of Iapetus show prominent water ice absorption bands that are produced by high-albedo icy areas—especially the polar caps—that protrude onto that hemisphere. Bell et al. (1985) separated the icy component of the spectrum between 0.3 and 2.6 µm of Iapetus’s dark side and modeled the dark component as an intimate mixture of 90% hydrated silicates and 10% simulated meteorite organic polymers. In a similar approach, Wilson and Sagan (1995) modeled the dark material as an admixture of polymerized HCN, Murchison meteoritic material, water ice, and tholins. Vilas et al. (1996) detected spectral features in the visual suggesting the presence of a mineal such as goethite or hematite that is the product of aqueous alteration of anhydrous silicates. Later work by Jarvis et al. (1997, 1998, 2000) extended the identification of this mineral to Hyperion. The most recent model by Owen et al. (2001), which combines water ice, carbon, and Triton tholin to
(1)
where λ is the wavelength, FIap , FIcy , and FD are the normalized fluxes of Iapetus, an icy satellite, and D-type material, respectively, and A is a coefficient that expresses the fraction of the surface occupied by icy material. To derive the best fit for A, we employed a computer program that varied A and sought the minimum difference between a synthetic spectrum produced by Eq. (1) and our measured spectrum of Iapetus. The result is shown in Fig. 4 (top) and listed in Table II. Phoebe was quickly eliminated as one of the end members because its spectrum is entirely unlike that of Iapetus
FIG. 3. The composite spectra for D-type material and icy satellite material. The D-type spectrum consists of the coadded spectra of the three asteroids shown in Fig. 1. After coaddition, a three-point running average was applied to the data. The icy satellite spectrum comprises data for Rhea and Dione, which were coadded and averaged in a similar fashion.
SATURNIAN SATELLITE AND D-TYPE ASTEROID SPECTRA
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and Table II). Our results suggest that the surface contaminant on the dark hemisphere of Iapetus may be similar to a main component of Hyperion’s surface. The surfaces of both satellites can be closely represented by the admixture of the surface of a typical icy saturnian satellite and D-type material. Furthermore, our results suggest that the alteration mechanism for the dark side of Iapetus may have also operated on Hyperion: No other known satellite in the saturnian system shares its similarity to the dark side of Iapetus (see the discussion section). Our two-component superposition model can be tested by seeing whether it yields consistent results for the geometric albedos of Iapetus and Hyperion. The geometric albedo ( p) for a body represented by our model is given by p = ApIcy + (1 − A) pD ,
FIG. 4. The results for the two-component model for both Iapetus and Hyperion. The scatter plots are our measured spectra, while the solid lines represent the model summarized in Table II. The spike near 0.76 µm is due to the O2 A emission line in the Earth’s atmosphere.
(Fig. 1), and because it was not necessary to use a third component to construct the spectrum of Iapetus. We also found that the spectrum of Dione alone produced a slightly better synthetic spectrum than the coadded icy satellite spectrum did (these are the results listed in Table II). We believe that the spectrum of Rhea contains small amounts of an opaque contaminant, causing a slight downturn below ∼0.45 µm (Fig. 1). Our observations of Rhea were obtained at a subobserver longitude of 340◦ , which includes much of the dark terrain that dominates Rhea’s trailing hemisphere (Smith et al. 1982). Separate visual spectra of the dark and bright regions of Rhea and Dione do in fact show that the dark areas exhibit a downturn below 0.5 µm (Buratti 1984). Our spectrum of Dione was obtained at a location that included less of its low-albedo terrain and thus it is probably more representative of a pristine icy saturnian satellite. We applied our two-component superposition model to Hyperion and obtained similarly satisfactory results (Fig. 4, bottom,
TABLE II Summary of the Two-Component Model
Iapetus Hyperion a b
A (±0.05)
Model p
Observed p
0.42 0.28
0.27 ± 0.05 0.19 ± 0.05
0.21 ± 0.05a 0.30 ± 0.05b
Based on Noland et al. (1974). Thomas et al. (1985).
(2)
where pIcy is the geometric albedo of the icy component and pD is the geometric albedo of D-type material. For the geometric albedo of the icy component, we used 0.60, which is typical of an icy satellite of Saturn, as well as the high-albedo regions of Iapetus (Buratti and Veverka 1984, Buratti and Mosher, 1991). The geometric albedo of a typical D asteroid in the visual region is 0.025 (Tedesco et al. 1989). To obtain the observed geometric albedo of Iapetus at the same subobserver longitude as that of our observations, we used the lightcurve published by Noland et al. (1974). Table II lists the results of our two-component model with the observed geometric albedos of Hyperion and Iapetus. The agreement is reasonable, given the uncertainties in both the albedos derived from observation and the albedos of the components. Our results for Hyperion are less satisfactory than those for Iapetus, perhaps because the two components exist as an intimate mixture on the satellite’s surface. Thomas and Veverka (1985) noted a mottled surface, suggesting the validity of a linear mixing model, but Wilson and Sagan (1995) suggest that the surface of Hyperion is best represented by an intimate mix. V. SUMMARY AND DISCUSSION
New high-resolution spectra in the 0.33 to 0.92 µm region of Iapetus, Phoebe, Hyperion, Dione, Rhea, and three D-type asteroids have led to the development of a two-component mixing model that provides a model for the origin of the low albedo of the leading hemisphere of Iapetus. The primary result of this model is that both the surface of the low-albedo side of Iapetus and the surface of Hyperion can be represented by a linear admixture of two components: one typical of an icy satellite in the saturnian system, and another typical of dark material that is common in the outer Solar System. This latter component is best represented by the D-class of asteroids, a primitive, low-albedo group of bodies that are believed to be rich in organic polymers, clay silicates, and an opaque material (Vilas and Smith 1985). Previous work attempted to identify the specific composition of the dark component on Iapetus. Bell et al. (1985) found that the spectrum of a mixture of 10% organic polymers, 90%
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hydrated silicates, and a small amount of elemental carbon, closely matched the spectrum of the dark material. However, they believe this material is a native component of Iapetus’s surface that has been enriched by a devolatization process caused by bombardment of particles from Phoebe. It is our view that the dark component is exogenous and that it has been placed on both the surfaces of Iapetus and Hyperion from the same source and by a similar means of transport. Iapetus exhibits a dichotomous distribution of the dark material because it is in synchronous rotation with Saturn, while Hyperion is covered on all sides because it is in chaotic rotation (Wisdom et al. 1984). (The larger icy fraction for Iapetus does not imply that its low-albedo regions have less contaminant than Hyperion. Most of the icy fraction in our observed spectrum is caused by the polar caps and regions of the bright side that “peek” over into our observations of the dark hemisphere of Iapetus). Our model does not require additional components such as that represented by Phoebe’s spectrum. Phoebe’s visual spectral properties are unlike those of either Iapetus or Hyperion: This satellite is apparently not a factor in the formation of the dark side of Iapetus. Our contention that the dark side of Iapetus and Hyperion are related is supported by the recent identification of a broad absorption band between 0.4 and 0.6 µm and a narrower band at 0.67 µm in both the spectrum of Iapetus (Vilas et al. 1996) and Hyperion (Jarvis et al. 2000). This feature has been attributed to charge transfer transitions in minerals that are the products of aqueous alteration, such as goethite and hematite (Vilas et al. 1996). Although the goal of our analysis was not to detect specific spectral features, Hyperion, the dark side of Iapetus, and our aggregate D-spectrum do exhibit slight absorption bands in the 0.4–0.6 µm region that are consistent with the results of Jarvis et al. and Vilas et al. (see Figs. 1 and 4). However, we find no evidence for the more diagnostic 0.67-µm absorption band, although our dichroic filter that dispersed the light into red and blue channels was placed near this wavelength and could have caused the distortion of that absorption band. There are two published dynamical models that offer plausible theories for the common origin of the dark material on Iapetus and Hyperion. In one, the low-albedo region of Iapetus was created by the accretion of debris from a collision between a proto-Hyperion and a large, Chiron-like comet (Matthews 1992). Given the results of the current analysis, the impacting body was more likely a D-type Centaur. The spectral similarity between Hyperion and Iapetus could be explained by the infall of debris back onto Hyperion, as well as onto Iapetus. Hyperion’s irregular shape and chaotic rotation are proof that it has undergone such a collision. In a second dynamical model, Hyperion is shown to be a substantial source of particles throughout the entire saturnian system (Banaszkiewicz and Krivov 1997). Although most of the particles are accreted by Titan, those in the 1–2 µm size range cross the orbit of Iapetus. The model of Owen et al. (2001) places the origin of the dark contaminant on Iapetus at Titan.
The scenario we favor is one in which both Iapetus and Hyperion are being coated by particles from retrograde satellites exterior to Iapetus’s orbit, a possibility first mentioned by Cruikshank et al. (1983). The recent discovery of five new such satellites— which appear to be created from an impact with Phoebe—has bolstered this claim (Gladman et al. 2001). Bell et al. hypothesized a similar scenario for the Galilean satellite Callisto: Its lower albedo leading hemisphere was coated by particles from the outer, presumably captured retrograde satellites of Jupiter (technically it is not quite true that the leading hemisphere of Callisto is darker, because at opposition Callisto’s leading side becomes brighter than the trailing side). The Uranian satellites may also be subjected to a similar mechanism. The leading sides of the outer Uranian satellites (Umbriel, Titania, and Oberon) are darker and redder than their trailing sides (Buratti and Mosher 1991). The source of the accreting dust for the leading sides of these satellites could be several outer retrograde satellites that have recently been discovered orbiting Uranus (Holman et al. 2000). If indeed a common mechanism exists for the creation of dark hemispheres of the satellites of the outer planets, Iapetus just represents one extreme case of a mechanism that is not unique to it alone. ACKNOWLEDGMENTS This research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration. We are grateful for the detailed reviews of two anonymous referees. Funding is provided by the NASA Planetary Geology and Geophysics and Astronomy Programs and National Science Foundation Grant AST 0074555.
REFERENCES Banaszkiewicz, M., and A. Krivov 1997. Hyperion as a dust source in the saturnian system. Icarus 129, 289–303. Bell, J. F. 1984. Callisto: Jupiter’s Iapetus. Lunar Planet. Sci. 15, 44–45 (abstract). Bell, J. F., R. N. Clark, T. B. McCord, and D. P. Cruikshank 1979. Reflection spectra of Pluto and three distant satellites. Bull. Am. Astron. Soc. 11, 570. Bell, J. F., D. P. Cruikshank, and M. J. Gaffey 1985. The composition and origin of the Iapetus dark material. Icarus 61, 192–207. Buratti, B. J. 1984. Voyager disk resolved photometry of the saturnian satellites. Icarus 59, 392–405. Buratti, B. J., and J. A. Mosher 1991. Comparative color and albedo maps of the uranian satellites. Icarus 90, 1–13. Buratti, B. J., and J. A. Mosher 1995. The dark side of Iapetus: Additional evidence for an exogenous origin. Icarus 115, 219–227. Buratti, B. J., and J. Veverka 1984. Voyager photometry of Rhea, Dione, Tethys, Enceladus, and Mimas. Icarus 58, 254–264. Buratti, B. J., J. A. Mosher, and T. V. Johnson 1990. Albedo and color maps of the saturnian satellites. Icarus 87, 339–357. Clark, R. N., and P. D. Owensby 1981. The infrared spectrum of Rhea. Icarus 46, 354–360. Clark, R. N., R. H. Brown, P. D. Owensby, and A. Steele 1984. Saturn’s satellites: Near-infrared spectrophotometry and compositional implications. Icarus 58, 265–281.
SATURNIAN SATELLITE AND D-TYPE ASTEROID SPECTRA Cook, A. F., and F. A. Franklin 1970. An explanation of the light curve of Iapetus. Icarus 13, 282–291. Cruikshank, D. P. 1980. Near-infrared studies of the satellites of Saturm and Uranus. Icarus 41, 246–258. Cruikshank, D. P., J. F. Bell, M. J. Gaffey, R. H. Brown, R. Howell, C. Beerman, and M. Rognstad 1983. The dark side of Iapetus. Icarus 53, 90–104. Cruikshank, D. P., J. Veverka, and L. A. Lebofsky 1984. Satellites of Saturn: Optical properties. In Saturn (T. Gehrels and M. S. Matthews, Eds). pp. 640– 667. Univ. of Arizona Press, Tucson. Degewij, J., L. E. Andersson, and B. Zellner 1980. Photometric properties of outer planetary satellites. Icarus 44, 520–540. Fink, U., H. P. Larson, T. N. Gautier, and R. Treffers 1976. Infrared spectra of the satellites of Saturn: Identification of water ice on Iapetus, Rhea, Dione, and Tethys. Astrophys. J. 207, L63–67. Fitzsimmons, A., M. Dahlgren, C.-I. Lagerkvist, P. Magnusson, and I. P. Williams 1994. A spectroscopic survey of D-type asteroids. Astron. Astrophys. 282, 634–642. Gladman, B., and 10 colleagues 2001. Orbital clustering for twelve newlydiscovered saturnian satellites: Clues to their origin. Nature 412, 163–166. Holman, M., B. Gladman, J. Kavelaars, J.-M. Petit, H. Scholl, P. Nicholson, and J. A. Burns 2000. The discovery and recovery of Uranus XVIII–XX. Bull. Amer. Astron. Soc. 32, 1077. Jarvis, K. S., F. Vilas, S. M. Larsen, and M. J. Gaffey 1997. S4 Hyperion and S9 Phoebe—Testing a link to Iapetus. Lunar Planet. Sci. 97, 661–662. Jarvis, K. S., F. Vilas, S. M. Larsen, M. J. Gaffey, and D. Domingue 1998. Hyperion, Phoebe and Iapetus: Relationships of the Saturnian satellites. Lunar Planet. Sci. 98, 1453 (abstract). Jarvis, K. S., F. Vilas, S. M. Larsen, and M. J. Gaffey 2000. Are Hyperion and Phoebe linked to Iapetus? Icarus 146, 125–132. Lebofsky, L. A., M. A. Feierberg, and A. T. Tokunaga 1982. Infrared observations of the dark side of Iapetus. Icarus 49, 382–396. Matthews, R. A. J. 1992. The darkening of Iapetus and the origin of Hyperion. Q. J. R. Astron. Soc. 33, 253–258. McCord, T. B., T. V. Johnson, and J. H. Elias 1971. Saturn and its satellites: Narrow-band spectrophotometry (0.3–1.1 µm). Astrophys. J. 165, 413–424. Millis, R. L. 1977. UBV photometry of Iapetus. Icarus 31, 81–88. Morrison, D., T. J. Jones, D. P. Cruikshank, and R. E. Murphy 1975. The two faces of Iapetus. Icarus 24, 157–171. Morrison, D., D. P. Cruikshank, C. B. Pilcher, and G. H. Rieke 1976. Surface compositions of the satellites of Saturn from infrared photometry. Astrophys. J. 207, L213–216. Morrison, D., T. V. Johnson, E. M. Shoemaker, L. A. Soderblom, P. Thomas, J. Veverka, and B. A. Smith 1984. Satellites of Saturn: Geological perspective. In Saturn (T. Gehrels and M. S. Matthews, Eds). pp. 609–639. Univ. of Arizona Press, Tucson. Murphy, R. E., D. P. Cruikshank, and D. Morrison 1972. Radii, albedos and 20-micron brightness temperatures of Iapetus and Rhea. Astrophys. J. 177, L93–96. Noland, M., J. Veverka, D. Morrison, D. P. Cruikshank, A. Lazarewicz, N. D.
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Morrison, J. L. Elliot, J. Goguen, and J. A. Burns 1974. Six-color photometry of Iapetus, Titan, Rhea, Dione, and Tethys. Icarus 23, 334–354. Oke, B., and J. Gunn 1982. An efficient low- and moderate-resolution spectrograph for the Hale Telescope. Publ. Astron. Soc. Pac. 94, 586–595. Owen, T. C., D. P. Cruikshank, C. M. Dalle Ore, T. R. Geballe, T. L. Roush, and C. de Bergh 1999. The detection of water ice on Saturn’s satellite Phoebe. Icarus 139, 379–382. Owen, T. C., D. P. Cruikshank, C. M. Dalle Ore, T. R. Geballe, T. L. Roush, C. de Bergh, R. Meier, Y. J. Pendleton, and B. N. Khare 2001. Decoding the domino: The dark side of Iapetus. Icarus 149, 160–172. Peterson, C. 1975. An explanation for Iapetus’ asymmetric reflectance. Icarus 24, 499–503. Simonelli, D., J. Kay, D. Adinolfi, J. Veverka, P. C. Thomas, and P. Helfenstein 1999. Phoebe: Albedo map and photometric properties. Icarus 138, 249–258. Smith, B. A., and 28 colleagues 1982. A new look at the Saturn System: The Voyager 2 images. Science 215, 504–536. Soter, S. 1974. Brightness of Iapetus. Paper presented at IAU Colloq. 28, Cornell University, August 1974. Squyres, S. W., and C. Sagan 1984. The albedo asymmetry of Iapetus. Nature 303, 782–785. Squyres, S. W., B. J. Buratti, J. Veverka, and C. Sagan 1983. Voyager photometry of Iapetus. Icarus 59, 426–435. Tedesco, E. F., J. G. Williams, D. L. Matson, G. L. Veeder, J. C. Gradie, and L. A. Lebofsky 1989. A three parameter asteroid taxonomy. Astron. J. 97, 580–606. Tholen, D. J., and B. Zellner 1983. Eight-color photometry of Hyperion, Iapetus, and Phoebe. Icarus 53, 341–347. Thomas, P., and J. Veverka, 1985. Hyperion: Analysis of Voyager observations. Icarus 64, 414–424. Thomas, P., J. Veverka, D. Morrison, M. Davies, and T. V. Johnson 1983. Phoebe: Voyager 2 observations. J. Geophys. Res. 88, 8736–8742. Vilas, F., and B. Smith 1985. Reflectance spectrophotometry (about 0.5 to 1.0 micron) of outer-belt asteroids: Implications for primitive, organic Solar System material. Icarus 64, 503–516. Vilas, F., S. M. Larsen, E. C. Hatch, and K. S. Jarvis 1993. CCD spectra of selected asteroids. II. Low-albedo asteroid spectra and data extraction techniques. Icarus 105, 67–78. Vilas, F., S. M. Larsen, K. R. Stockstill, and M. J. Gaffery 1996. Unraveling the zebra: Clues to the Iapetus dark material composition. Icarus 124, 262– 267. Wagner, J. K., B. W. Hapke, and E. N. Wells 1987. Atlas of reflectance spectra of terrestrial, lunar and meteoritic powders and frosts from 92 to 1800 nm. Icarus 69, 14–28. Wilson, P., and C. Sagan 1995. Spectrophotometry and organic matter on Iapetus. 1. Composition models. J. Geophys. Res. 100, 7531–7537. Wisdom, J., S. J. Peale, and F. Mignard 1984. The chaotic rotation of Hyperion. Icarus 58, 137–146. Zellner, B. 1972. On the nature of Iapetus. Astrophys. J. 174, L107–109.
High-Resolution 0.33–0.92 µm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related? Bonnie J. Buratti, Michael D. Hicks, Kimberly A. Tryka, Micah S. Sittig, and Ray L. Newburn Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mailstop 183-501, Pasadena, California 91109 E-mail: [email protected] Received January 3, 2000; revised July 6, 2001
New high-resolution spectra in the 0.33 to 0.92 µm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature 412, 163–166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences. c 2002 Elsevier Science (USA)
I. INTRODUCTION
The dichotomous placement of high- and low-albedo material on the surface of the saturnian satellite Iapetus was realized almost as soon as it was discovered by Cassini in 1671. Measurements by ground-based observers and the Voyager spacecraft show that the morphology, high albedo (∼0.6), and spectrum of the bright side is similar to that of the other medium-sized, icy satellites of Saturn (particularly Tethys, Dione, and Rhea), while the markedly redder, dark hemisphere reflects only a few percent of incident radiation (McCord et al. 1971, Murphy et al. 1972, Zellner 1972, Smith et al. 1982). The amplitude of the lightcurve of Iapetus is nearly two magnitudes (Noland et al. 1974, Morrison et al. 1975, 1976, Millis 1977) with water ice, either in pure form or bound to minerals, dominating the higher albedo trailing side (Fink et al. 1976, Lebofsky et al. 1982, Clark et al. 1984), and a reddish low-albedo contaminant dominating the dark leading hemisphere (Cruikshank 1980, Cruikshank et al. 1983, 1984, Bell et al. 1985). The dark material is centered on the leading side of Iapetus (Smith et al. 1982) and its albedo
increases, and its color becomes less red, in a sinusoidal fashion as a function of distance from its apex of motion (Squyres et al. 1983, Buratti and Mosher 1995). Over 300 years after the discovery of Iapetus, scientists still cannot agree on the cause of its unique appearance. The models for the origin of the dark side of Iapetus follow two basic scenarios: One involves the endogenous placement of material, and the other invokes deposition of, or alteration by, exogenous particles. In the endogenous model the dark material is a vast deposit of material that erupted from the interior of Iapetus (Smith et al. 1982). The simplest of the second group of models involves the accretion of exogenous material onto Iapetus’s leading side. The first version of this model proposed that particles from Phoebe, the outer retrograde satellite of Saturn, were moved inward by Poynting–Robertson drag and collided with Iapetus (Soter 1974). This model’s major problem is that Phoebe’s visible spectrum is very flat, similar to C-type asteroids (Degewij et al. 1980), unlike the reddish visible spectrum of Iapetus’s leading side. A modification of this model that explains the redder color of the dark material on Iapetus involves the volatilization of ice by the impacting particles and subsequent enrichment of an organic-rich lag deposit (Cruikshank et al. 1983, Bell et al. 1985). In another modification, methanerich native particles are darkened and reddened by UV photolysis and ballistically redistributed (Squyres and Sagan 1984). A group of hybrid models contends that the erosion of surficial ice by micrometeorites excavated a dark underlying mantle (Cook and Franklin 1970, Wilson and Sagan 1995). The model of Peterson (1975) espouses the preferential accretion of bright particles on the trailing side of the satellite. The connection between Hyperion and the dark side of Iapetus was first suggested by results of the eight-color asteroid survey (Tholen and Zellner 1983) and later by mineralogical similarities between the two bodies. Vilas et al. (1996) identified absorption bands on Iapetus that are characteristic of the ferric charge transfer transitions in iron alteration minerals such as goethite and hematite, while Jarvis et al. (2000) identified similar bands on Hyperion. As part of a study to clarify the compositional relationships among the dark side of Iapetus and other members of the family
375 0019-1035/02 $35.00 c 2002 Elsevier Science (USA) All rights reserved.
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of saturnian satellites, we obtained new high-resolution visible spectra of Iapetus, Hyperion, Phoebe, Dione, and Rhea with the 200-inch telescope on Palomar Mountain. We also obtained three spectra representative of D-type asteroids to serve as an analogue to the dark, organic-rich contaminant that is common in the outer Solar System. Our results show that Hyperion and the dark hemisphere of Iapetus can both be closely represented in terms of a simple two-component model comprised of a “pristine” icy satellite + D-type material. The process responsible for the dark side of Iapetus may not be unique: it may be operating on a more modest scale on the lower albedo leading side of the Galilean satellite Callisto, as originally suggested by Bell (1984) and on the Uranian satellites (Buratti and Mosher 1991). II. OBSERVATIONS AND DATA ANALYSIS
All observations were obtained over a period of three nights in 1997 and 1998 with the double spectrometer (Oke and Gunn 1982) attached to the 200-inch Hale telescope on Palomar ˚ in 1997 and at Mountain. A dichroic filter (placed at 5500 A ˚ 6800 A in 1998) directed the light onto two separate 1024 square CCD detectors, covering the red and blue regions of the visible spectrum. The total spectral range is 0.33 to 0.92 µm, with reso˚ per pixel. In general a different integration time lution of 3.8 A was used for the red and blue channels (Table I). All observations were obtained within airmasses of 1.0 and 1.2, with slit widths of 6 or 8 arcseconds. The objects and standard stars were centered in the spectrometer slit by means of a crosshair on the guide telescope, and offset tracking was done for moving planetary targets. Spectral calibrations were performed with He-Ne-Ar standard lamps. The solar analogue star HD 29461 was measured at the
same airmasses as all sets of observations. Bias frames at the start and end of the night, and flatfield measurements were obtained for each run. The flatfields were obtained by two methods: on the sky at twilight, and on the telescope dome, which was illuminated by an incandescent lamp. Table I summarizes the observations obtained for this study. The spectra were reduced in a standard fashion. First, the need for flatfielding was evaluated by checking for vignetting in each spectrum, and in the location on the flatfield image where the spectra lie. Since none of the spectra or flatfields exhibited apparent vignetting (to a level of ∼1%), and because all spectra were divided by the spectrum of the standard star, we did not perform flatfield corrections. The raw spectrum was computed by coadding seven rows located above and below the center of each spectrum. The sky + bias contribution was computed by coadding seven additional rows on each side of each spectrum. Each sky + bias was modeled by a linear equation. For Dione and Rhea, the scattered light from Saturn was sufficiently bright that the sky background was modeled with a quadratic equation. The sky + bias models were then subtracted from each target’s spectrum. Each spectrum was then divided by the spectrum of our solar analogue star, HD 29461, which had been processed in a similar fashion. Finally, two or three spectra for each object were coadded to improve the signal. The results for all five satellites and three asteroids are shown in Fig. 1. III. RESULTS
For Rhea, Dione, and 911 Agamemnon, our results are the first high-resolution visual spectra published. Two spectra each of 624 Hektor and 1578 Kirkwood were published in Vilas et al.
TABLE I Summary of Observations Date (UT) (Start)
Object
V mag
# Spectra
α
Latitude, longitude
Airmass
Int. time (s) (blue, red channel)
23 October 1997 09:05 09:38 09:52 10:12
HD 29461 911 Agamemnon 624 Hektor 1578 Kirkwood
7.96 8.32 14.71 15.34
3 1 2 2
5.4 5.0 16.0
unknown unknown unknown
1.10 1.10 1.06 1.04
0.5; 1; 5 180 480; 240 180; 360
14 November 1998 05:35 05:52 06:04 06:39 07:28
Rhea Iapetus Hyperion Phoebe HD 29461
9.55 10.95 14.08 16.35 7.96
3 3 3 2 3
2.41 2.39 2.42 2.34
−15, 340 −13, 113 N/A −31, 306
1.11 1.10 1.10 1.11 1.12
15 90; 120 240; 420 600; 900 3
15 November 1998 05:43 05:53 06:05 06:25 07:28
Dione Iapetus Hyperion Phoebe HD 29961
10.25 10.95 14.08 16.35 7.96
3 3 2 1 3
2.51 2.50 2.52 2.44
−15, 46 −13, 117 N/A −31, 177
1.11 1.10 1.10 1.11 1.17
15; 10 90; 120 420 600; 900 3; 5
Note. On 23 October 1997 the slit width was 6 ; on the remaining two nights it was 8 . The subobserver longitude is unknown on Hyperion because of its chaotic rotation state.
SATURNIAN SATELLITE AND D-TYPE ASTEROID SPECTRA
FIG. 1. Spectra of (from top to bottom) three D-type asteroids, Hyperion, Iapetus, Rhea, Dione, and Phoebe. All spectra were obtained on the 200-inch telescope on Palomar Mountain and the double spectrograph. The noise in the asteroid spectra between 0.5 and 0.6 µm is due to the placement there of the dichroic, which disperses the light into the red and blue channels. The spike at 0.76 µm is due to the O2 A atmospheric emission line. Less prominent spikes between 0.6 and 0.7 µm and 0.8 and 0.9 µm are due to O2 and OH emission lines in the terrestrial atmosphere.
(1993) and Fitzsimmons et al. (1994) respectively. Our results are in reasonable agreement with these previous publications, and our spectra of Iapetus, Hyperion, and Phoebe are in good agreement with those that have appeared earlier (Jarvis et al. 1997, 1998, 2000), with broad-band spectroscopic studies of the saturnian satellites (Noland et al. 1974, Millis 1977, Bell et al. 1979) and with results derived from the imaging camera on Voyager (Thomas et al. 1983, Thomas and Veverka 1985, Buratti 1984). The published Voyager spectra of Phoebe and Hyperion (Thomas et al. 1983, Thomas and Veverka 1985) and Dione (Buratti 1984) are redder than those derived at Palomar, but this discrepancy is removed if additional calibrations to the Voyager camera measurements are applied (Buratti 1984, p. 400). A medium-resolution spectrum of Rhea shows a slight
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(few percent) decrease in reflectivity between 0.65 and 0.92 µm that we did not observe (Clark and Owensby 1981). We believe that this discrepancy may be due to the different subobserver longitudes between the two sets of observations. During the period of the Clark and Owensby observations, the longitude range was 91–114◦ , while our subobserver longitude was 340◦ . Diskresolved Voyager spectra of high- and low-albedo regions of Rhea show a significant difference in the 0.35 to 0.6 µm region (Buratti 1984, Buratti et al. 1990). Because the low-albedo terrain is located primarily on the trailing side of Rhea (Smith et al. 1982), it is reasonable to expect that Rhea exhibits longitudinal differences in its spectrum. Our observations of the D-type asteroids are in agreement with the broad-band colors published as part of the eight-color asteroid survey (Tholen and Zellner 1983). All three are typical of the D-class of asteroids (Fitzsimmons et al. 1984, Vilas et al. 1993). Because Phoebe is in simple rotation with a period of 9.4 ± 0.2 h (Thomas et al. 1983), our observations on two consecutive nights are well suited to detecting global compositional heterogeneity on it. An albedo map of Phoebe constructed from Voyager images shows significant changes in reflectivity on its surface (Simonelli et al. 1999). The pattern is organized roughly in a hemispherical fashion, with a large higher albedo region centered at 0◦ showing normal reflectances in the 0.10 to 0.13 range, and a lower albedo region with reflectances of ∼0.06 located about 180◦ away. Our second night’s spectrum was obtained with a subobserver longitude of 177◦ : the ideal location for isolating the spectrum of the low-albedo region. Although the subobserver longitude for the first night was not located precisely in the center of the high-albedo region, the viewing geometry included this area almost in its entirety. Thus, any spectral changes between the high- and low-albedo regions should appear in the disk-integrated spectra obtained on the two nights. Figure 2 shows the two spatially separated spectra we obtained for Phoebe. They are in good agreement, indicating that the disk-integrated surface of the satellite is spectrally homogeneous (at least in the visual region of the spectrum). However, we cannot assume this result indicates that Phoebe is compositionally homogeneous; the albedo map of Simonelli et al. (1999) has shown that it is not. Although Phoebe’s spectrum is flat, and similar to C-type (carbon-rich) bodies, its albedo is higher than carbon, particularly in the brighter areas. The presence of water ice on its surface has recently been detected (Owen et al. 1999). Because water ice is spectrally flat in the region we are studying and bright (Wagner et al. 1987) its addition to a C-type planetary surface could be expected to raise its albedo but not appreciably change its spectral shape. Our spatially resolved spectra are thus consistent with a surface consisting of patches of water ice and a low-albedo, flat material, as suggested by Simonelli et al.’s map. It is also important to remember that due to an imprecise knowledge of Phoebe’s rotation period, the placement of surface features cannot be predicted over the 15 or so years that elapsed between the Voyager encounter and the acquisition of our spectra. Thus, we cannot be sure of the current placement of Simonelli et al.’s albedo features on a geographical
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FIG. 2. Rotationally resolved spectra of Phoebe. In the top cell, spectra are shown for longitudes of 177◦ and 306◦ ; the large dots are 10-point running averages. In the bottom cell, the averaged spectra are plotted on top of each other to show their similarity.
reproduce the spectrum of the dark side of Iapetus, places the origin of the dark material on Titan. Rather than combining the spectra of pure substances that are likely to be found on the surfaces of the saturnian satellites, we adopt a new approach of coadding the spectra of astronomical bodies that represent compositional end members. This approach avoids the problem of dissimilarities between laboratory samples and planetary surfaces, and it helps to establish relationships among the bodies themselves. Our end members are a typical icy satellite, represented by the coadded and three-point averaged spectra of Dione and Rhea (Fig. 3 bottom), D-type material, represented by the coadded and three-point averaged spectrum of the three observed D asteroids (Fig. 3, top), and spectrally flat, carbon-rich material, represented by Phoebe. The albedo of the bright side of Iapetus is as well as its morphology, visual spectrum, and photometric properties are all similar to those of Dione and Rhea (Smith et al. 1982, Buratti and Veverka 1984, Squyres and Sagan 1984). Tholen and Zellner (1983), Bell et al. (1985), and Vilas and Smith (1985) noted the similarity between the low-albedo side of Iapetus and the D-type asteroids, which are believed to be rich in primitive, possibly prebiotic material. The model we thus adopt represents the integral spectrum of Iapetus as the superposition of D-type material on top of an icy satellite substrate, FIap (λ) = AFIcy (λ) + (1 − A)FD (λ),
◦
grid. Because our spectra were obtained 129 apart, they are, nevertheless, well suited for seeking global compositional variegations. IV. A TWO-COMPONENT MODEL FOR IAPETUS AND HYPERION
Previous models of the spectrum of Iapetus have assumed that it can be represented by the coaddition of discrete compositional units representing the dark and bright regions of Iapetus (Bell et al. 1985, Vilas et al. 1996, Owen et al. 2001). This “checkerboard” model is reasonable, because spectra obtained of the low-albedo hemisphere of Iapetus show prominent water ice absorption bands that are produced by high-albedo icy areas—especially the polar caps—that protrude onto that hemisphere. Bell et al. (1985) separated the icy component of the spectrum between 0.3 and 2.6 µm of Iapetus’s dark side and modeled the dark component as an intimate mixture of 90% hydrated silicates and 10% simulated meteorite organic polymers. In a similar approach, Wilson and Sagan (1995) modeled the dark material as an admixture of polymerized HCN, Murchison meteoritic material, water ice, and tholins. Vilas et al. (1996) detected spectral features in the visual suggesting the presence of a mineal such as goethite or hematite that is the product of aqueous alteration of anhydrous silicates. Later work by Jarvis et al. (1997, 1998, 2000) extended the identification of this mineral to Hyperion. The most recent model by Owen et al. (2001), which combines water ice, carbon, and Triton tholin to
(1)
where λ is the wavelength, FIap , FIcy , and FD are the normalized fluxes of Iapetus, an icy satellite, and D-type material, respectively, and A is a coefficient that expresses the fraction of the surface occupied by icy material. To derive the best fit for A, we employed a computer program that varied A and sought the minimum difference between a synthetic spectrum produced by Eq. (1) and our measured spectrum of Iapetus. The result is shown in Fig. 4 (top) and listed in Table II. Phoebe was quickly eliminated as one of the end members because its spectrum is entirely unlike that of Iapetus
FIG. 3. The composite spectra for D-type material and icy satellite material. The D-type spectrum consists of the coadded spectra of the three asteroids shown in Fig. 1. After coaddition, a three-point running average was applied to the data. The icy satellite spectrum comprises data for Rhea and Dione, which were coadded and averaged in a similar fashion.
SATURNIAN SATELLITE AND D-TYPE ASTEROID SPECTRA
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and Table II). Our results suggest that the surface contaminant on the dark hemisphere of Iapetus may be similar to a main component of Hyperion’s surface. The surfaces of both satellites can be closely represented by the admixture of the surface of a typical icy saturnian satellite and D-type material. Furthermore, our results suggest that the alteration mechanism for the dark side of Iapetus may have also operated on Hyperion: No other known satellite in the saturnian system shares its similarity to the dark side of Iapetus (see the discussion section). Our two-component superposition model can be tested by seeing whether it yields consistent results for the geometric albedos of Iapetus and Hyperion. The geometric albedo ( p) for a body represented by our model is given by p = ApIcy + (1 − A) pD ,
FIG. 4. The results for the two-component model for both Iapetus and Hyperion. The scatter plots are our measured spectra, while the solid lines represent the model summarized in Table II. The spike near 0.76 µm is due to the O2 A emission line in the Earth’s atmosphere.
(Fig. 1), and because it was not necessary to use a third component to construct the spectrum of Iapetus. We also found that the spectrum of Dione alone produced a slightly better synthetic spectrum than the coadded icy satellite spectrum did (these are the results listed in Table II). We believe that the spectrum of Rhea contains small amounts of an opaque contaminant, causing a slight downturn below ∼0.45 µm (Fig. 1). Our observations of Rhea were obtained at a subobserver longitude of 340◦ , which includes much of the dark terrain that dominates Rhea’s trailing hemisphere (Smith et al. 1982). Separate visual spectra of the dark and bright regions of Rhea and Dione do in fact show that the dark areas exhibit a downturn below 0.5 µm (Buratti 1984). Our spectrum of Dione was obtained at a location that included less of its low-albedo terrain and thus it is probably more representative of a pristine icy saturnian satellite. We applied our two-component superposition model to Hyperion and obtained similarly satisfactory results (Fig. 4, bottom,
TABLE II Summary of the Two-Component Model
Iapetus Hyperion a b
A (±0.05)
Model p
Observed p
0.42 0.28
0.27 ± 0.05 0.19 ± 0.05
0.21 ± 0.05a 0.30 ± 0.05b
Based on Noland et al. (1974). Thomas et al. (1985).
(2)
where pIcy is the geometric albedo of the icy component and pD is the geometric albedo of D-type material. For the geometric albedo of the icy component, we used 0.60, which is typical of an icy satellite of Saturn, as well as the high-albedo regions of Iapetus (Buratti and Veverka 1984, Buratti and Mosher, 1991). The geometric albedo of a typical D asteroid in the visual region is 0.025 (Tedesco et al. 1989). To obtain the observed geometric albedo of Iapetus at the same subobserver longitude as that of our observations, we used the lightcurve published by Noland et al. (1974). Table II lists the results of our two-component model with the observed geometric albedos of Hyperion and Iapetus. The agreement is reasonable, given the uncertainties in both the albedos derived from observation and the albedos of the components. Our results for Hyperion are less satisfactory than those for Iapetus, perhaps because the two components exist as an intimate mixture on the satellite’s surface. Thomas and Veverka (1985) noted a mottled surface, suggesting the validity of a linear mixing model, but Wilson and Sagan (1995) suggest that the surface of Hyperion is best represented by an intimate mix. V. SUMMARY AND DISCUSSION
New high-resolution spectra in the 0.33 to 0.92 µm region of Iapetus, Phoebe, Hyperion, Dione, Rhea, and three D-type asteroids have led to the development of a two-component mixing model that provides a model for the origin of the low albedo of the leading hemisphere of Iapetus. The primary result of this model is that both the surface of the low-albedo side of Iapetus and the surface of Hyperion can be represented by a linear admixture of two components: one typical of an icy satellite in the saturnian system, and another typical of dark material that is common in the outer Solar System. This latter component is best represented by the D-class of asteroids, a primitive, low-albedo group of bodies that are believed to be rich in organic polymers, clay silicates, and an opaque material (Vilas and Smith 1985). Previous work attempted to identify the specific composition of the dark component on Iapetus. Bell et al. (1985) found that the spectrum of a mixture of 10% organic polymers, 90%
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hydrated silicates, and a small amount of elemental carbon, closely matched the spectrum of the dark material. However, they believe this material is a native component of Iapetus’s surface that has been enriched by a devolatization process caused by bombardment of particles from Phoebe. It is our view that the dark component is exogenous and that it has been placed on both the surfaces of Iapetus and Hyperion from the same source and by a similar means of transport. Iapetus exhibits a dichotomous distribution of the dark material because it is in synchronous rotation with Saturn, while Hyperion is covered on all sides because it is in chaotic rotation (Wisdom et al. 1984). (The larger icy fraction for Iapetus does not imply that its low-albedo regions have less contaminant than Hyperion. Most of the icy fraction in our observed spectrum is caused by the polar caps and regions of the bright side that “peek” over into our observations of the dark hemisphere of Iapetus). Our model does not require additional components such as that represented by Phoebe’s spectrum. Phoebe’s visual spectral properties are unlike those of either Iapetus or Hyperion: This satellite is apparently not a factor in the formation of the dark side of Iapetus. Our contention that the dark side of Iapetus and Hyperion are related is supported by the recent identification of a broad absorption band between 0.4 and 0.6 µm and a narrower band at 0.67 µm in both the spectrum of Iapetus (Vilas et al. 1996) and Hyperion (Jarvis et al. 2000). This feature has been attributed to charge transfer transitions in minerals that are the products of aqueous alteration, such as goethite and hematite (Vilas et al. 1996). Although the goal of our analysis was not to detect specific spectral features, Hyperion, the dark side of Iapetus, and our aggregate D-spectrum do exhibit slight absorption bands in the 0.4–0.6 µm region that are consistent with the results of Jarvis et al. and Vilas et al. (see Figs. 1 and 4). However, we find no evidence for the more diagnostic 0.67-µm absorption band, although our dichroic filter that dispersed the light into red and blue channels was placed near this wavelength and could have caused the distortion of that absorption band. There are two published dynamical models that offer plausible theories for the common origin of the dark material on Iapetus and Hyperion. In one, the low-albedo region of Iapetus was created by the accretion of debris from a collision between a proto-Hyperion and a large, Chiron-like comet (Matthews 1992). Given the results of the current analysis, the impacting body was more likely a D-type Centaur. The spectral similarity between Hyperion and Iapetus could be explained by the infall of debris back onto Hyperion, as well as onto Iapetus. Hyperion’s irregular shape and chaotic rotation are proof that it has undergone such a collision. In a second dynamical model, Hyperion is shown to be a substantial source of particles throughout the entire saturnian system (Banaszkiewicz and Krivov 1997). Although most of the particles are accreted by Titan, those in the 1–2 µm size range cross the orbit of Iapetus. The model of Owen et al. (2001) places the origin of the dark contaminant on Iapetus at Titan.
The scenario we favor is one in which both Iapetus and Hyperion are being coated by particles from retrograde satellites exterior to Iapetus’s orbit, a possibility first mentioned by Cruikshank et al. (1983). The recent discovery of five new such satellites— which appear to be created from an impact with Phoebe—has bolstered this claim (Gladman et al. 2001). Bell et al. hypothesized a similar scenario for the Galilean satellite Callisto: Its lower albedo leading hemisphere was coated by particles from the outer, presumably captured retrograde satellites of Jupiter (technically it is not quite true that the leading hemisphere of Callisto is darker, because at opposition Callisto’s leading side becomes brighter than the trailing side). The Uranian satellites may also be subjected to a similar mechanism. The leading sides of the outer Uranian satellites (Umbriel, Titania, and Oberon) are darker and redder than their trailing sides (Buratti and Mosher 1991). The source of the accreting dust for the leading sides of these satellites could be several outer retrograde satellites that have recently been discovered orbiting Uranus (Holman et al. 2000). If indeed a common mechanism exists for the creation of dark hemispheres of the satellites of the outer planets, Iapetus just represents one extreme case of a mechanism that is not unique to it alone. ACKNOWLEDGMENTS This research was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administration. We are grateful for the detailed reviews of two anonymous referees. Funding is provided by the NASA Planetary Geology and Geophysics and Astronomy Programs and National Science Foundation Grant AST 0074555.
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