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The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by

Saturn, having accreted outside the region of the solar nebula in which Saturn formed1. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably

accreted within the sub-nebula in which Saturn itself formed2. Here we report imaging spectroscopy of Phoebe resulting from the Cassini–Huygens spacecraft encounter on 11 June 2004. We

mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the

most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of

cometary origin.

This work was funded by the Cassini project. Authors from American institutions were funded by NASA; authors from European institutions were funded by ESA.

US Geological Survey, MS964, Federal Center, Box 25046, Colorado, 80225, Denver, USA

Lunar and Planetary Laboratory and Stewart Observatory, University of Arizona, Tucson, Arizona, 85721, USA

German Aerospace Center (DLR), Institute of Space Sensor Technology and Planetary Exploration, Rutherfordstrasse 2, D-12489, Berlin, Germany

NASA Ames Research Center, Moffett Field, California, 94035, USA

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, 91109, USA

Robert M. Nelson, Bonnie J. Buratti, K. H. Baines & D. L. Matson

University of Hawaii at Manoa, HIGP/SOEST, 1680 East-West Road, Honolulu, Hawaii, 96822, USA

Istituto di Fisica dello Spazio Interplanetario, CNR, 00133, Rome, Italy

G. Bellucci, F. Capaccioni, P. Cerroni, A. Coradini, V. Formisano & V. Mennella

Institut d'Astrophysique Spatiale, Université de Paris-Sud, F-91405, Orsay Cedex, France

Astronomy Department, Cornell University, Ithaca, New York, 14853, USA

Laboratoire de Planétologie et Géodynamique, UMR CNRS 6112, Université de Nantes, 44322, Nantes, France

Planetary Science Institute NW, Corporate Center Pasadena, 255 S. Lake Avenue, Suite 300, Pasadena, California, 91101, USA

Department of Earth and Space Sciences, University of Seattle, Washington, 8195-1310, USA

The authors declare that they have no competing financial interests.

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Phoebe, the outermost large satellite of Saturn, is of particular interest because its unusual orbit suggests that it was gravitationally captured by Saturn, having formed outside the solar

nebula where Saturn itself formed. The Cassini–Huygens spacecraft encountered Phoebe on 11 June 2004, and imaging spectroscopy from Cassini was used to detect iron, bound water, trapped CO2,

phyllosilicates, organics, nitriles and cyanide compounds on Phoebe. The presence of all these compounds makes Phoebe one of the most compositionally diverse objects in our Solar System,

consistent with a surface of cometary origin incorporating primitive materials from the outer Solar System. Further evidence on Phoebe's past comes from density measurements made by two

other instrument systems on Cassini. Phoebe's composition is distinctly different from the ice-rich material that formed the intermediate-sized saturnian satellites, and is consistent with

formation from the same material out of which Pluto and Triton (archetypical Kuiper-belt objects) formed.