L149 CASSINI VISUAL AND INFRARED MAPPING ... - IOPscience

The Astrophysical Journal, 622:L149–L152, 2005 April 1 䉷 2005. The American Astronomical Society. All rights reserved. Printed in U.S.A.

CASSINI VISUAL AND INFRARED MAPPING SPECTROMETER OBSERVATIONS OF IAPETUS: DETECTION OF CO2 B. J. Buratti,1 D. P. Cruikshank,2 R. H. Brown,3 R. N. Clark,4 J. M. Bauer,1 R. Jaumann,5 T. B. McCord,6 D. P. Simonelli,1 C. A. Hibbitts,7 G. B. Hansen,7 T. C. Owen,8 K. H. Baines,1 G. Bellucci,9 J.-P. Bibring,10 F. Capaccioni,9 P. Cerroni,9 A. Coradini,9 P. Drossart,11 V. Formisano,9 Y. Langevin,10 D. L. Matson,1 V. Mennella,9 R. M. Nelson,1 P. D. Nicholson,12 B. Sicardy,11 C. Sotin,13 T. L. Roush,2 K. Soderlund,1 and A. Muradyan1 Received 2004 December 17; accepted 2005 February 21; published 2005 March 8

ABSTRACT The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus in which new absorption bands are seen in the spectra of both the lowalbedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The highalbedo hemisphere is modeled with H2O ice slightly darkened with tholin. The detection of CO2 in the lowalbedo material on the leading hemisphere supports the contention that it is carbon-bearing material from an external source that has been swept up by the satellite’s orbital motion. Subject headings: planets and satellites: individual (Iapetus) — solar system: formation complete explanation of the special characteristics of Iapetus must also account for the absence of such properties on any of the many other satellites of the outer planets. Observations of the low-albedo hemisphere from Earth and from the Voyager spacecraft have shown that the dark material is arranged such that at the poles there is a significant amount of high-albedo ice exposed, and in the equatorial regions the dark material wraps around slightly onto the icy hemisphere, similar to the cover on a baseball. The centering of the lowalbedo material on the leading hemisphere in the sense of the orbital motion has led to the supposition that material from an external source (possibly a more distant satellite of Saturn such as Phoebe) was swept up as Iapetus orbits Saturn, although this scenario is by no means proven (Soter 1974; Cruikshank et al. 1983; Buratti & Mosher 1995; Vilas et al. 1996; Owen et al. 2001). Another suggestion is that the material is endogenous (Smith et al. 1981), although no completely satisfactory explanation of its geographical distribution has been put forth. Earth-based spectroscopy of sunlight diffusely reflected from the surface of Iapetus shows that the low-albedo material has a very red color (increasing reflectance toward longer wavelength), a weak absorption band at 0.67 mm, and a strong absorption band at 3 mm (Cruikshank et al. 1983; Bell et al. 1985; Vilas et al. 1996; Owen et al. 2001; Buratti et al. 2002). The spectrum of the leading hemisphere over the wavelength interval 0.3–3.8 mm has been modeled with an intimate mixture of a nitrogen-rich tholin, amorphous carbon, and a small amount of H2O ice, suggesting that a complex refractory organic solid is the principal component of the low-albedo material covering that hemisphere of the satellite (Owen et al. 2001). The tholin itself exhibits a 3 mm absorption band attributed to N i H bonding in one of its component materials, and this shape fits the band observed in the spectrum of Iapetus very well.

1. INTRODUCTION

Saturn’s third largest satellite, Iapetus (radius 720 km), occupies a near-circular orbit beyond Titan and Hyperion. It is in locked synchronous rotation such that it keeps one hemisphere permanently directed toward Saturn throughout its 80 day orbital period. From approximately the time of its discovery in 1671, it has been known to have the unique property among planetary satellites that the hemisphere centered on the apex of its orbital motion (the “leading” hemisphere) has a very low surface reflectance (geometric albedo) of about 2%– 6%, while the remainder of the satellite’s surface is 10 times more reflective and exhibits the characteristic spectroscopic absorption bands of H2O ice. The mean density of Iapetus is 1.1 g cm⫺3, indicative of a bulk composition dominated by ice. The composition of the low-albedo material, its origin, and the peculiarity of its geographic distribution have all made Iapetus an object of special interest not only in Saturn’s family of satellites but in the entire solar system. No other known object has such a contrast in albedo on a global scale, and so any 1 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 183-501, Pasadena, CA 91109. 2 NASA/Ames Research Center, Moffett Field, CA 94035. 3 Lunar and Planetary Laboratory and Steward Observatory, University of Arizona, Tuscon, AZ 85721-0092. 4 US Geological Survey, Mail Stop 964, Box 25046, Federal Center, Denver, CO 80225. 5 Institute of Planetary Exploration, DLR, D-12484 Berlin, Germany. 6 Planetary Geoscience Division, Institute of Geophysics and Planetology, 1680 East-West Road, Honolulu, HI 96822. 7 Planetary Science Institute, 225 South Lake Avenue, Pasadena, CA 91101. 8 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822. 9 Institute di Fisica dello Spazio Interplanetario, CNR, 100 Via del Fosso del Cavaliere, I-00133 Rome, Italy. 10 Institut d’Astrophysique Spatiale, Universite´ Paris-Sud, Baˆtiment 121, F91405 Orsay, France. 11 Observatoire de Paris, 5 Place Jules Janssen, F-92195 Meudon Cedex, France. 12 Cornell University, Department of Astronomy, 610 Space Sciences Building, Ithaca, NY 14853-6801. 13 Laboratoire de Planetologie et Geodynamique, UMR CNRS 6112, Universite de Nantes, 2 rue de la Houssinie`re, BP 92208, 44322 Nantes Cedex 3, France.

2. OBSERVATIONS AND ANALYSIS

Although the Cassini spacecraft is not scheduled to conduct a targeted flyby of Iapetus until 2007 September 10, the spacecraft observed the satellite during several serendipitous periods L149

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TABLE 1 Summary of Observations Subobserver Observation Title

Date

Time (UT)

SouthhemB001 . . . . . . GlobcolaA001 . . . . . . .

2004 July 13 2004 Oct 7

00:18–00:51 16:36–17:24

Exposure (ms)

Number of Cubes

Longitude (deg)

Latitude (deg)

Solar Phase Angle (deg)

Range (106 km)

640 640

12 7

104 133

⫺44 ⫺21

54 47

2.50 3.14

in 2004 July and October (see Table 1) when it approached the satellite to within 2.49 and 1.11 million kilometers, respectively. We report here on observations made with the Visual and Infrared Mapping Spectrometer (VIMS) in both time periods. For each set of observations, three spatial resolution elements (pixels) fell on Iapetus. Each VIMS pixel in the IR channel (which is the only one discussed here) has 256 spectral elements covering the wavelength region 0.85–5.1 mm. At a wavelength of 3 mm, the spectral resolution R p l/dl p 180. The observations obtained in July are particularly significant, because one of the spatial pixels fell directly on a region of the low-albedo hemisphere, one on the bright material of the other hemisphere, and one in an intermediate region (spacecraft pointing knowledge is known to better than a degree in latitude and longitude on Iapetus). A fourth pixel fell mainly on unilluminated terrain and contained no useful signal. Figure 1 shows an image extracted from one of the observations obtained in July. Figure 2 shows six spectra, three each from the two observational periods representing the three terrain types. CO2 band.—Each of the three regions on Iapetus (Fig. 1) has a distinctly different spectrum. The high-albedo material has the characteristic spectrum of H2O ice, with strong absorption bands at 1.5, 2.0, and 3 mm, and strong absorption beyond 4 mm. In the mixed-albedo region, the H2O ice bands are weaker but still distinct, and the absorption beyond 4 mm

Fig. 1.—Example of a 12 # 12 VIMS image of Iapetus extracted from a VIMS image cube obtained on 2004 July 13. A co-aligned Imaging Science Subsystem (ISS) image obtained at the same time is shown for context; the image is offset into the region of dark sky, and it is enlarged by a factor of about 3. Each VIMS cube consists of 352 images obtained at wavelengths extending from 0.4 to 5.0 mm; in this Letter we discuss the 256 channels in the 0.84–5.1 mm spectral region. This image was obtained at 1.0 mm. The upper right pixel consists of primarily high-albedo icy material; the lower left pixel is placed on the low-albedo terrain of Iapetus; the upper left pixel covers both terrains, and the lower right pixel is largely in the unilluminated portion of Iapetus. The other 140 pixels are dark sky. Each VIMS pixel is 0.50 mrad square, while the angular diameter of Iapetus is 0.57 mrad (ISS image courtesy of NASA/JPL).

is diminished. The region centered on the low-albedo hemisphere shows the least ice absorption, a weaker and narrower 3 mm band, and relatively high reflectance beyond 4 mm. This spectrum is most directly comparable to previous ground-based spectra of the leading hemisphere of Iapetus. The most prominent newly recognized feature in the spectra of Iapetus is the absorption band at 4.255 mm, which we identify as the n3 band of CO2 adsorbed on or included in another material, probably H2O ice, but possibly a mineral, or an organic solid substance. This band is clearly seen in the lowalbedo and mixed-albedo regions, but it is not apparent in the H2O ice-rich region. Carbon dioxide is unlikely to be a component of the original volatile inventory from which Iapetus formed. It is more likely to have been produced in situ by ultraviolet irradiation and cosmic-ray bombardment (Moore et al. 1983) of a surface containing H2O and a source of carbon. Cosmic rays can penetrate some 10 m into the surface, while solar UV penetrates only ∼0.1 mm. The formation of CO2 by cosmic rays in a surface containing H2O will ensure that the two molecular materials are in intimate contact. At T p 90 K, the residence time of CO2 formed on the surface by UV is only a few seconds, but CO2 formed deeper by cosmic rays will be retained much longer. The apparent association of greater CO2 abundance with low-albedo surface regions may result from the geographic distribution of the source of the carbon needed to produce the CO2. That source could be carbonaceous micrometeoroids impacting the surface or native material. Plausible native materials would be frozen CH4 and CO, but there is no evidence of these spectroscopically active molecules on Iapetus in the Earth-based spectroscopy or from VIMS. Furthermore, a native material such as a condensable volatile should be distributed more or less uniformly over the entire surface of the satellite, and radiation-formed CO2 should be present on the trailing hemisphere where it is not seen. In contrast, if the low-albedo material blanketing Iapetus’ leading hemisphere is indeed carbonaceous dust accumulated on an otherwise H2Orich icy surface by the body’s orbital motion, both the source of the carbon to make CO2 and its association with the lowalbedo material can be understood. The process by which carbon is incorporated into CO2 could also liberate free carbon, thus further darkening the surface. The shift in wavelength (by ∼0.01 mm) of the CO2 absorption band on Iapetus compared with the wavelength of pure solid CO2 requires explanation. Sandford & Allamandola (1990) studied the spectrum of CO2 in amorphous H2O ice over a range of temperatures and concentrations. In pure solid CO2 at the ∼90 K temperature of Iapetus’ surface, the n3-stretching mode band has its maximum absorption at 4.266 mm, but in H2O ice the band shifts to longer and shorter wavelengths, depending on the CO2 concentration and the temperature. The shift to shorter wavelength (higher frequency) can be understood in terms of the trapping of the CO2 molecule in a confined space that inhibits the full range of motion of the stretching mode. The shift to shorter wavelength is clearly seen in CO2 : H2O clathrate, but can also occur in a less structured environment in which CO2 is

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CO2 ON IAPETUS

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Fig. 2.—Disk-resolved spectra of Iapetus obtained in (a) 2004 July and (b) 2004 October. For each set of observations, high-albedo, low-albedo, and mixedalbedo regions are averaged. Data are normalized to 1.9 mm, and the spectrum of Phoebe (Clark et al. 2005) is shown for comparison. Although the spectrum of Phoebe is similar in the 1–5 mm region, the visual spectrum is very different (Buratti et al. 2002), making Phoebe the sole source of the low-albedo material on Iapetus problematical.

trapped in H2O in more random, but still confined, configurations. The H2O ice on the trailing hemisphere of Iapetus is hexagonal crystalline (Grundy et al. 1999), and that on the leading hemisphere is expected to be the same. The entrapment configurations of CO2 in crystalline H2O are expected to be a bit different from the amorphous ice in the Sandford & Allamandola experiments, but the basic ideas are the same. Glandorf et al. (2002) found that CO2 condensed on a previously frozen substrate of H2O ice has its absorption band at 4.255 mm, but the surface binding energy is so low that at the 90 K temperature of Iapetus, the retention time is only a few seconds. However, the Sandford & Allamandola (1990) experiments showed that for CO2 in H2O ice at concentrations of 2%–5%, there was no appreciable loss of CO2 as the ice was warmed until a temperature of ∼140 K was reached. From this we may infer that CO2 created in a matrix of H2O will be stable for long periods of time at the temperature of Iapetus. On the other hand, we must also infer from the low albedo of the materials in which the CO2 and the weak H2O bands occur that the spectrum is produced by sunlight scattered from relatively opaque H2O ice particles that contain CO2 and possibly amorphous carbon. Alternatively, the carbon could occur separately as amorphous clusters or as partially hydrogenated graphitic carbon moieties intimately intermixed with the H2O ⫹ CO2 grains. CO2 is found on Jupiter’s satellites Europa, Ganymede, and Callisto (Carlson et al. 1996; McCord et al. 1997) and Saturn’s satellite Phoebe (Clark et al. 2005) with the n3 band shifted to a shorter wavelength as in the case of Iapetus. In all cases, the CO2 band is strongest in the H2O ice-poor regions, giving rise to the proposal that the molecule exists as gaseous or fluid inclusions in minerals. McCord et al. (1997) show spectra of four minerals containing CO2 inclusions with the n3 band at various wavelengths at and around 4.25 mm. Those authors note that CO2 inclusions are common in minerals, but the presence of the 4.25 mm band is not necessarily diagnostic of the host material. Other bands.—Another absorption band prominent in the VIMS spectrum of the low-albedo region lies at 2.44 mm and appears to be part of a broader absorption extending from 2.3 to 2.6 mm. The broad absorption is also seen in the mixedalbedo region, while the spectrum of the bright icy region is distinctly different. The ground-based spectrum of the lowalbedo hemisphere of Iapetus also shows this broad absorption,

which is not explained by the N-rich tholin in the model of Owen et al. (2001). We have been unable to identify this absorption band, but we note that it also appears distinctly in the spectrum of the ice-poor regions of Phoebe: Clark et al. (2005) suggest that it is an overtone of a i CIN–bearing molecule, the fundamental of which (in the Phoebe spectrum) lies at 4.8– 5.0 mm. We do not see a band clearly defined at the wavelength of the fundamental in the Iapetus data, but the signal precision in these preliminary data is perhaps not sufficient to permit detection. Data from future Cassini encounters with Iapetus should clarify this point. An unidentified absorption feature at 2.39 mm was also observed in the spectrum of comet 19P/ Borrelly during its encounter with Deep Space 1 on 2001 September 22 (Soderblom et al. 2002). In the spectrum of the ice-rich region, there is a prominent absorption band at 3.8 mm, which is also seen in the spectrum of ice-rich regions of Phoebe (Clark et al. 2005). This band lies near a filter boundary in the VIMS detector array at ∼3.8 mm, such that the exact band profile is distorted by the loss of ∼2 spectral channels. We have been unable to identify this band. It does not match bands of simple hydrocarbons, formaldehyde, methanol, or other molecules that are readily produced by photochemistry in mixed ices. Both the high-albedo and mixed-albedo regions show the Fresnel reflection peak at 3.12 mm at the bottom of the strong H2O band extending from ∼2.8 to 3.4 mm. This peak is characteristic of hexagonal crystalline H2O ice (Grundy et al. 1999). 3. DISCUSSION

Spectral mixing models that produce synthetic spectra consisting of adjustable proportions of likely surface constituents have been published during the past decade. Although the models do not in general produce unique results for the composition of a specific planetary surface, they do point to the identification of the most likely surface components present, and they can eliminate certain materials. The models can consist of the most simple “checkerboard” rendition (Buratti et al. 2002), in which the various surface components exist on the surface as discrete patches, to intimate mixing models (Roush 1994), in which the surface components are chemically mixed and the grain size is varied. We fitted an intimate mixing model based on the code developed by Roush (1994) to both the high-albedo and lowalbedo hemispheres of Iapetus. Figure 3 shows that the highalbedo hemisphere of Iapetus can be fitted fairly well with a

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Fig. 3.—Intimate mixing model (Roush 1994) fitted to the high-albedo terrain of Iapetus. The optical constants for water ice are from Grundy & Schmitt (1998), and the optical constants for Triton tholin are from Khare et al. (1994a).

composition of ∼78% water ice mixed with Triton tholin, a nitrogen-rich organic compound used in the spectral modeling of Owen et al. (2001). A good fit to the low-albedo spectrum (Fig. 4) requires only a small amount of ice, an even smaller amount of Fe2O3 to account for the ferric absorption band at ∼1.0 mm, 36% Triton tholin, and substantial amounts of the HCN polymer (“poly-HCN”). This latter compound was first invoked by Wilson & Sagan (1996) to explain the reddish color of the visible spectrum of Iapetus. Owen et al. (2001) found that polyHCN caused too much absorption in the 3–3.5 mm region. However, the extension of the spectrum of the low-albedo material of Iapetus to the 3.5–5 mm region suggests that this compound may be a significant constituent of the low-albedo material. (CO2 was not used in the mixing model because the published optical constants have been measured for its pure ice form, which is not the form existing on the surface of Iapetus.) We emphasize that our models are not unique and that they may be substantially revised as more spectra are obtained by Cassini during the targeted flyby in 2007 September. Additional observations are also being analyzed from an untargeted flyby of Iapetus that occurred on 2005 January 1 during the Huygens probe relay period. With more detailed disk-resolved spectra, and with the potential to map specific absorption bands, the nature of the low-albedo material can be explored as successively more of it is added to the water ice substrate.

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Fig. 4.—Same model as in Fig. 3, but fitted to the low-albedo terrain of Iapetus. The optical constants for poly-HCN are from Khare et al. (1994b).

As the 3–5 mm region is explored on more and more bodies that have substantial amounts of nonice low-albedo material, the CO2 band seems to be ubiquitous. So far it is has been found on three of Jupiter’s satellites and on Phoebe. In each case, it is most prominent in H2O ice-poor regions, giving rise to the supposition that it is not chemically or physically associated with the H2O. However, in the cases of Phoebe and Iapetus, there is sufficient H2O ice present to make a chemical or physical association possible. The intrinsic strengths (Avalues) of the CO2 and H2O (at 2.0 mm) bands are 1.7 # 10⫺16 and 2 # 10⫺16, respectively. Thus, the abundance of CO2 scales as the ratio of its band strength to that of H2O times the ratio of A-values (0.85). For the ice-poor low-albedo region of Iapetus, the band area ratio of CO2 : H2O is ∼0.8, corresponding to an abundance for CO2 of ∼0.6 that of H2O in a given surface grain. For the high-albedo region, the abundance of CO2 is ∼0.05 that of H2O. This research was performed while J. M. B and D. P. S. were National Research Council Postdoctoral and Senior Research Associates at JPL. K. S. was a Planetary Geology and Geophysics Intern at JPL, and A. M. was a SIRI Intern. We are grateful to Thomas Momary for assistance with Figure 1. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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