DOI: 10.1126/science.289.5483.1340. Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa

Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa

10.1126/science.289.5483.1340

Crossref journal-article

American Association for the Advancement of Science (AAAS)

Science (221)

Abstract

On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter's magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. This pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of Europa's internal field. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa's field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface.

Bibliography

Kivelson, M. G., Khurana, K. K., Russell, C. T., Volwerk, M., Walker, R. J., & Zimmer, C. (2000). Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa. Science, 289(5483), 1340â1343.

Authors 6

Margaret G. Kivelson (first)
Krishan K. Khurana (additional)
Christopher T. Russell (additional)
Martin Volwerk (additional)
Raymond J. Walker (additional)
Christophe Zimmer (additional)

References 25 Referenced 561

10.1006/icar.1998.5977
10.1038/scientificamerican1099-54
10.1126/science.285.5435.1899
10.1006/icar.1999.6187
10.1126/science.276.5316.1239
10.1038/27394
10.1029/1998JA900095
10.1016/0019-1035(85)90019-3
10.1029/97JE03370
. 10. The coordinate system used in this paper (referred to as Ephio) is Europa-centric with x along the co-rotating plasma flow y radially in toward Jupiter and z parallel to Jupiter's spin axis. In conventional units of A m 2 the magnetic moment is 4πR E 3 M /μ o where μ o is the magnetic permeability.
Our rationale for ignoring passes at altitude greater than 2000 km is that at 2000-km altitude and directly over the induced magnetic pole the signal from an internal magnetic moment of ∼100 nT (see E11 Table 1) is less than 8.4 nT. Away from the pole the signal amplitude becomes even smaller. Such small amplitude signals are difficult to interpret given the levels of fluctuations typical of the plasma background.
The components of the magnetospheric field are plotted with a broad line to take into account variations of the primary field that occur at other than the synodic period. Such variations are expected because Europa's radial distance and latitude vary from pass to pass as a result of the ellipticity and slight inclination of Europa's orbit and because of temporal variations in the plasma sheet.
The measured magnetic moment is obtained by fitting the difference between the measured field components and the smoothly varying background field to a dipole field over the intervals tabulated in Table 1. The components of the dipole moment are obtained by a least squares technique.
10.1029/JA085iA03p01171
10.1029/JA085iA11p05959
Web figures 1 and 2 are available at Science Online at www.sciencemag.org/feature/data/1052679.shl.
10.1029/1999JA900217
C. Zimmer K. K. Khurana M. G. Kivelson Icarus in press. This work used the electromagnetic induction solution for multiple concentric shells given in (19).
W. D. Parkinson Introduction to Geomagnetism (Elsevier New York 1983) p. 313.
R. B. Montgomery in American Institute of Physics Handbook D. E. Gray Ed. (McGraw-Hill New York 1963) pp. 125–127.
This can be understood by noting that in the inset to Fig. 3 the primary field was increasing in the hour before each of those encounters. Thus a phase lag of 25° or 0.6 hours implies that the induced currents are responding to a smaller primary field.
10.1126/science.277.5324.355
10.1086/305604
10.1029/97JE03556
We are grateful to our programmers S. Joy and J. Mafi for their devoted efforts to acquire and process the data on a short time scale and during holiday periods. Support for this work was provided in part by contract JPL 958694 from NASA's Jet Propulsion Laboratory (JPL) which manages the Galileo mission. Special thanks are due to D. Bindschadler C. Polanskey J. Erickson and others at JPL who worked to design a Europa pass optimized to provide the test that we report here.

Dates

Type	When
Created	23 years, 1 month ago (July 27, 2002, 5:48 a.m.)
Deposited	1 year, 7 months ago (Jan. 13, 2024, 4:42 a.m.)
Indexed	39 minutes ago (Aug. 30, 2025, 5:58 a.m.)
Issued	25 years ago (Aug. 25, 2000)
Published	25 years ago (Aug. 25, 2000)
Published Print	25 years ago (Aug. 25, 2000)

Funders 0

None

BibTeX

@article{Kivelson_2000, title={Galileo Magnetometer Measurements: A Stronger Case for a Subsurface Ocean at Europa}, volume={289}, ISSN={1095-9203}, url={http://dx.doi.org/10.1126/science.289.5483.1340}, DOI={10.1126/science.289.5483.1340}, number={5483}, journal={Science}, publisher={American Association for the Advancement of Science (AAAS)}, author={Kivelson, Margaret G. and Khurana, Krishan K. and Russell, Christopher T. and Volwerk, Martin and Walker, Raymond J. and Zimmer, Christophe}, year={2000}, month=aug, pages={1340–1343} }

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