1. US Geological Survey, MS964, Box 25046, Federal Center, Denver, Colorado 80225, USA
2. Lunar and Planetary Laboratory and Stewart Observatory, University of Arizona, Tucson, Arizona 85721, USA
3. German Aerospace Center (DLR), Institute of Space Sensor Technology and Planetary Exploration, Rutherfordstrasse 2, D-12489 Berlin, Germany
4. NASA Ames Research Center, Moffett Field, California 94035, USA
5. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
6. University of Hawaii at Manoa, HIGP/SOEST, 1680 East-West Road, Honolulu, Hawaii 96822, USA
7. Istituto di Fisica dello Spazio Interplanetario, CNR, Rome 00133, Italy
8. Institut d'Astrophysique Spatiale, Université de Paris-Sud, F-91405 Orsay Cedex France
9. Cornell University, Astronomy Department, Ithaca, New York 14853, USA
10. Observatoire de Paris, 92195 Meudon, France
11. Laboratoire de Planétologie et Géodynamique, UMR CNRS 6112, Université de Nantes, 44322 Nantes, France
12. Planetary Science Institute NW, Corporate Center Pasadena, 255 S. Lake Avenue, Suite 300, Pasadena, California 91101, USA
13. Department of Earth and Space Sciences, University of Seattle, Washington 98195-1310, USA

Correspondence to: Roger N. Clark¹ Correspondence and requests for materials should be addressed to R.N.C. (Email: rclark@usgs.gov).

Abstract

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.

1. US Geological Survey, MS964, Box 25046, Federal Center, Denver, Colorado 80225, USA
2. Lunar and Planetary Laboratory and Stewart Observatory, University of Arizona, Tucson, Arizona 85721, USA
3. German Aerospace Center (DLR), Institute of Space Sensor Technology and Planetary Exploration, Rutherfordstrasse 2, D-12489 Berlin, Germany
4. NASA Ames Research Center, Moffett Field, California 94035, USA
5. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
6. University of Hawaii at Manoa, HIGP/SOEST, 1680 East-West Road, Honolulu, Hawaii 96822, USA
7. Istituto di Fisica dello Spazio Interplanetario, CNR, Rome 00133, Italy
8. Institut d'Astrophysique Spatiale, Université de Paris-Sud, F-91405 Orsay Cedex France
9. Cornell University, Astronomy Department, Ithaca, New York 14853, USA
10. Observatoire de Paris, 92195 Meudon, France
11. Laboratoire de Planétologie et Géodynamique, UMR CNRS 6112, Université de Nantes, 44322 Nantes, France
12. Planetary Science Institute NW, Corporate Center Pasadena, 255 S. Lake Avenue, Suite 300, Pasadena, California 91101, USA
13. Department of Earth and Space Sciences, University of Seattle, Washington 98195-1310, USA

Correspondence to: Roger N. Clark¹ Correspondence and requests for materials should be addressed to R.N.C. (Email: rclark@usgs.gov).

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