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The Ionosphere of Europa from Galileo Radio Occultations

A. J. Kliore, D. P. Hinson, F. M. Flasar, A. F. Nagy, and T. E. CravensAuthors Info & Affiliations
Science
18 Jul 1997
Vol 277, Issue 5324
pp. 355-358
DOI: 10.1126/science.277.5324.355
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Abstract

The Galileo spacecraft performed six radio occultation observations of Jupiter's Galilean satellite Europa during its tour of the jovian system. In five of the six instances, these occultations revealed the presence of a tenuous ionosphere on Europa, with an average maximum electron density of nearly 104 per cubic centimeter near the surface and a plasma scale height of about 240 ± 40 kilometers from the surface to 300 kilometers and of 440 ± 60 kilometers above 300 kilometers. Such an ionosphere could be produced by solar photoionization and jovian magnetospheric particle impact in an atmosphere having a surface density of about 108 electrons per cubic centimeter. If this atmosphere is composed primarily of O2, then the principal ion is O2 + and the neutral atmosphere temperature implied by the 240-kilometer scale height is about 600 kelvin. If it is composed of H2O, the principal ion is H3O+ and the neutral temperature is about 340 kelvin. In either case, these temperatures are much higher than those observed on Europa's surface, and an external heating source from the jovian magnetosphere is required.

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REFERENCES AND NOTES

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Radio occultations, in which a spacecraft appears to go behind a planetary body as viewed from Earth, allow the spacecraft-Earth radio link to traverse the ionosphere and atmosphere of the occulting body. Interpretation of the effects on the phase and amplitude of the signal received on Earth of refraction and defocusing in the planetary atmosphere and ionosphere allows one to infer the electron density structure in the ionosphere and the temperature-pressure profiles and absorption characteristics of the neutral atmosphere. This technique has been used with much success to measure the characteristics of the ionospheres and atmospheres of Venus, Mars, Jupiter, Saturn, Uranus, and Neptune, as well as Saturn's satellite Titan, Neptune's Triton, and Jupiter's Io and now Europa (12).
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We wish to acknowledge the contributions of the staff of the Galileo Project, who have carried out a highly successful mission under difficult circumstances; the Galileo Navigation team, with W. E. Kirhofer and J. Johannesen, without whose precise orbits this work would not have been possible; and the personnel of the Jet Propulsion Laboratory Multimission Radio Science team, especially S. Asmar, R. Herrera, D. Chong, P. Eshe, P. Priest, J. Caetta, T. Rebold, and S. Abbate, who planned and successfully executed the data acquisition process. Special thanks are due to J. Twicken and P. Schinder for their assistance in data analysis at Stanford and Goddard Space Flight Center, and to D. M. Hunten, W. H. Ip, and two anonymous referees for helpful suggestions and comments. This work was supported by NASA contracts and grants.
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Information & Authors

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Published In

Science
Volume 277 | Issue 5324
18 July 1997

Submission history

Received: 28 April 1997
Accepted: 17 June 1997
Published in print: 18 July 1997

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Authors

Affiliations

A. J. Kliore, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.
D. P. Hinson, Center for Radar Astronomy, Stanford University, Stanford, CA 94305, USA.
F. M. Flasar, Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA.
A. F. Nagy, Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, USA.
T. E. Cravens, Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA.

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  • A. J. Kliore et al.
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The Ionosphere of Europa from Galileo Radio Occultations.Science277,355-358(1997).DOI:10.1126/science.277.5324.355

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