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Saturn's Icy Moon Rhea: A Prediction for its Bulk Chemical Composition and Physical Structure at the Time of the Cassini Spacecraft First Flyby

Published online by Cambridge University Press:  05 March 2013

Andrew J. R. Prentice
Andrew J. R. Prentice*
Affiliation:
School of Mathematical Sciences, Monash University, Melbourne VIC 3800, Australia. Email: andrew.prentice@sci.monash.edu.au
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Abstract

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I report a model for the formation of Saturn's family of mid-sized icy moons to coincide with the first flypast of Rhea by the Cassini spacecraft on 2005 November 26. It is proposed that the moons had condensed from a concentric family of orbiting gas rings that were shed some 4.6 × 109 yr ago by the proto-Saturnian (hereafter p-Sat) cloud. The p-Sat cloud is made up of gas and residual grains of the gas ring that was shed by the proto-Solar cloud (hereafter PSC) at Saturn's orbit. The bulk of the condensate within this proto-Solar ring accumulates to form Saturn's central core of mass ∼10–20 M⊕ (M⊕ = Earth mass). The process of formation of Saturn's solid core thus provides an opportunity for the p-Sat cloud to become depleted in rock and water ice relative to the usual solar abundances of these materials. Nitrogen, which exists as uncondensing N2 in the PSC and as NH3 in the p-Sat cloud, retains its solar abundance relative to H2. If the depletion factor of solids relative to gas is ζdep = 0.25, as suggested by the low mass of Rhea relative to solar abundance expectations, the mass-percent ratio of NH3 to H2O in the dense p-Sat cloud is 36:64. Numerical and structural models for Rhea are constructed on the basis of a ‘cosmogonic’ bulk chemical composition of hydrated rock (mass fraction 0.385), H2O ice (0.395), and NH3 ice (0.220). It is difficult to construct a chemically differentiated model of Rhea whose mean density matches the observed value ρRhea = 1.23 ± 0.02 g cm−3 for reasonable bounds of the controlling parameters. Chemically homogeneous models can, however, be constrained to match the observed Rhea density provided that the mass fraction of NH3 is permitted to exceed the cosmogonic value by a factor ζNH3 = 1.20–1.35. A large proportion of NH3 in the ice mass inhibits the formation of the dense crystalline phase II of H2O ice at high pressure. This may explain the lack of compressional features on the surface of the satellite that are expected as a result of ice II formation in the cooling core. The favoured model of Rhea is chemically uniform and has mass proportions of rock (0.369), H2O ice (0.378), and NH3 ice (0.253). The enhancement factor of NH3 lies within the measured uncertainties of the solar abundance of nitrogen. The satellite is very cold and nearly isodense. The predicted axial moment-of-inertia coefficient is [C/MR2]Rhea = 0.399 ± 0.004.

Keywords

Solar system: formationSun: Saturn: Rheaphysical data and processes: convection
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 23 , Issue 1 , 2006 , pp. 1 - 11
Copyright
Copyright © Astronomical Society of Australia 2006

References

Anderson, J. D., Colombo, G., Esposito, P. B., Lau, E. L., & Trager, G. B. 1987, Icar, 71, 337 Google Scholar
Asplund, M., Grevesse, N., & Sauval, A. J. 2005, in ASP Conf. Ser. 336: Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis (Eds Bash, F.N., & Barnes, T.G.), 25 (San Francisco: ASP)Google Scholar
Bahcall, J. N., Pinsonneault, M. H., & Basu, S. 2001, ApJ, 555, 990 Google Scholar
Castillo, J. C. 2005, LPI, 36, 2243 Google Scholar
Clark, R. N., Brown, R. H., Owensby, P. D., & Steele, A. 1984, Icar, 58, 265 CrossRefGoogle Scholar
Consolmagno, G. J. 1985, Icar, 64, 401 Google Scholar
Consolmagno, G. J., & Lewis, J. S. 1978, Icar, 34, 280 Google Scholar
Croft, S. K., Lunine, J. I., & Kargel, J. 1988, Icar, 73, 279 CrossRefGoogle Scholar
Ellsworth, K., & Schubert, G. 1983, Icar, 54, 490 Google Scholar
Fortes, A. D., Wood, I. G., Brodholt, J. P., & Vocadlo, L. 2003, Icar, 162, 59 CrossRefGoogle Scholar
Hogenboom, D. L., Kargel, J. S., Consolmagno, G. J., Holden, T. C., Lee, L., & Buyyounouski, M. 1997, Icar, 128, 171 CrossRefGoogle Scholar
Jacobson, R. A. 2004, AJ, 128, 492 CrossRefGoogle Scholar
Jacobson, R. A., Antreasian, P. G., Bordi, J. J., Criddle, K. E., Ionasescu, R., Jones, J. B., Mackenzie, R. A., Meek, M. C., Pelletier, F. J., Roth, D. C., Roundhill, I. M., & Stauch, J. R. 2005, BAAS, 36, 524 Google Scholar
Krupskii, I. N., Manzhely, V. G., & Koloskova, L. A., 1968, PSS, 27, 263 Google Scholar
Lebofsky, L. A. 1975, Icar, 25, 205 Google Scholar
Lewis, J. S. 1972, Icar, 16, 241 Google Scholar
Lodders, K. 2003, ApJ, 591, 1220 CrossRefGoogle Scholar
Lupo, M. J., & Lewis, J. S. 1979, Icar, 40, 157 Google Scholar
Peale, S. J. 1999, ARA&A, 37, 533 Google Scholar
Prentice, A. J. 1973, A&A, 27, 237 Google Scholar
Prentice, A. J. R. 1978a, in The Origin of the Solar System (Ed. Dermott, S. F.), p. 111 (New York: Wiley)Google Scholar
Prentice, A. J. R. 1978b, M&P, 19, 341 Google Scholar
Prentice, A. J. R. 1984, EM&P, 30, 209 Google Scholar
Prentice, A. J. R. 1996a, EM&P, 73, 237 Google Scholar
Prentice, A. J. R. 1996b, PhyLA, 213, 253 Google Scholar
Prentice, A. J. R. 2001, EM&P, 87, 11 Google Scholar
Prentice, A. J. R. 2004a, BAAS, 36, 780 Google Scholar
Prentice, A. J. R. 2004b, BAAS, 36, 1116 Google Scholar
Prentice, A. J. R. 2005a, LPI, 36, 2378 Google Scholar
Prentice, A. J. R. 2005b, BAAS, 37, 729 Google Scholar
Prentice, A. J., & Freeman, J. C. 1999, TAGU, 80, F607Google Scholar
Prentice, A. J. R., & Dyt, C. P. 2003, MNRAS, 341, 644 Google Scholar
Prinn, R. G., & Fegley, B. 1981, ApJ, 249, 308 Google Scholar
Rappaport, N.J., Iess, L., Tortora, P., Asmar, S.W., Somenzi, L., Anabtawi, A., Barbinis, E., Fleischman, D. U., & Goltz, G. L. 2005, BAAS, 37, 704 Google Scholar
Ross, J. E., & Aller, L. H. 1976, Sci, 191, 1223 Google Scholar
Smith, B.A., Soderblom, L., Beebe, R., Boyce, J., Briggs, G., Bunker, A., Collins, S. A., Hansen, C. J., Johnson, T. V., Mitchell, J. L., Terrile, R. J., Carr, M., Cook, A. F., Cuzzi, J., Pollack, J. B., Danielson, G. E., Ingersoll, A., Davies, M. E., Hunt, G.E., Masursky, H., Shoemaker, E., Morrison, D., Owen, T., Sagan, C., Veverka, J., Strom, R., & Suomi, V. 1981, Sci, 212, 163 Google Scholar
Stewart, J. W. 1960, JChPh, 33, 128 Google Scholar
Tyler, G. L., Eshleman, V. R., Anderson, J. D., Levy, G. S., Lindal, G. F., Wood, G. E., & Croft, T. A. 1981, Sci, 212, 201 Google Scholar