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Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto


The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites—Io, Europa, Ganymede and Callisto—on each orbit. Initial results from the spacecraft's magnetometer1,2 have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede3 and possibly Io4. Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto. We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment. Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons. We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence5,6 in the case of Europa.

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Figure 1: The varying magnetic fields experienced by Europa and Callisto.
Figure 2: Magnetic field observations for the E14 pass.
Figure 3: Magnetic field observations from the C3 and C9 passes.


  1. 1

    Kivelson, M. al. Europa's magnetic signature: report from Galileo's first pass on December 19, 1996. Science 276, 1239–1241 (1997).

    ADS  CAS  Article  Google Scholar 

  2. 2

    Khurana, K. K., Kivelson, M. G., Russell, C. T., Walker, R. J. & Southwood, D. J. Absence of an internal magnetic field at Callisto. Nature 387, 262–264 (1997).

    ADS  CAS  Article  Google Scholar 

  3. 3

    Kivelson, M. G. Discovery of Ganymede's magnetic field by the Galileo spacecraft. Nature 384, 537–541 (1996).

    ADS  CAS  Article  Google Scholar 

  4. 4

    Kivelson, M. al. Amagnetic signature at Io: initial report from the Gallileo magnetometer. Science 273, 337–340 (1996).

    ADS  CAS  Article  Google Scholar 

  5. 5

    Carr, M. al. Evidence for a subsurface ocean on Europa. Nature 391, 363–365 (1998).

    ADS  CAS  Article  Google Scholar 

  6. 6

    Pappalardo, R. al. Geological evidence for solid-state convection in Europa's ice shell. Nature 391, 365–368 (1998).

    ADS  CAS  Article  Google Scholar 

  7. 7

    Parkinson, W. D. Introduction to Geomagnetism 308–340 (Scottish Academic, Edinburgh, (1993)).

    Google Scholar 

  8. 8

    Neubauer, F. M. Nonlinear standing Alfvén wave current system at Io: Theory. J. Geophys. Res. 85, 1171–1178 (1980).

    ADS  Article  Google Scholar 

  9. 9

    Owen, C. J. et al. The lunar wake at 6.8 RL: WIND magnetic field observations. Geophys. Res. Lett. 23, 1263–1266 (1996).

    ADS  Article  Google Scholar 

  10. 10

    Khurana, K. K. & Kivelson, M. G. Ultra low frequency MHD waves in Jupiter's middle magnetosphere. J. Geophys. Res. 94, 5241–5254 (1989).

    ADS  Article  Google Scholar 

  11. 11

    Ip, W-H. Europa's oxygen exosphere and its magnetospheric interaction. Icarus 120, 317–325 (1996).

    ADS  CAS  Article  Google Scholar 

  12. 12

    Kliore, A. J., Hinson, D. P., Flasar, F. M., Nagy, A. F. & Cravens, T. E. The ionosphere of Europa from Galileo radio occultations. Science 277, 355–358 (1997).

    ADS  CAS  Article  Google Scholar 

  13. 13

    Neubauer, F. M. The subalfvénic interaction of the Galilean satellites with the Jovian magnetosphere. J. Geophys. Res. (Planets) 103, 19843–19866 (1998).

    ADS  Article  Google Scholar 

  14. 14

    Montgomery, R. B. in American Institute of Physics Handbook 125–127 (McGraw Hill, New York, (1963)).

    Google Scholar 

  15. 15

    Colburn, D. S. & Reynolds, R. T. Electrolytic currents in Europa. Icarus 63, 39–44 (1985).

    ADS  CAS  Article  Google Scholar 

  16. 16

    Kargel, J. S. & Consolmagno, G. J. Magnetic fields and the detectability of brine oceans in Jupiter's icy satellites. Lunar Planet. Sci. Conf. XXVII, 643–644 (1996).

    ADS  Google Scholar 

  17. 17

    Schubert, G., Spohn, T. & Reynolds, R. in Satellites (eds Burns, J. A. & Matthews, M. S.) 224–292 (Univ. Arizona Press, Tucson, (1986)).

    Google Scholar 

  18. 18

    Anderson, J. D. Europa's differentiated internal structure: inferences from four Galileo encounters. Science(in the press).

  19. 19

    Reynolds, R. T. & Cassen, P. On the internal structure of the major satellites of the outer planets. Geophys. Res. Lett. 6, 121–124 (1979).

    ADS  CAS  Article  Google Scholar 

  20. 20

    Kirk, R. L. & Stevenson, D. J. Thermal evolution of a differentiated Ganymede and implications for surface features. Icarus 69, 91–135 (1987).

    ADS  Article  Google Scholar 

  21. 21

    Cassen, P. M., Reynolds, R. T. & Peale, S. J. Is there liquid water on Europa? Geophys. Res. Lett. 6, 731–734 (1979).

    ADS  Article  Google Scholar 

  22. 22

    Squyres, S. W., Reynolds, R. T., Cassen, P. M. & Peale, S. J. Liquid water and active resurfacing on Europa. Nature 301, 225–226 (1983).

    ADS  CAS  Article  Google Scholar 

  23. 23

    Ross, M. N. & Schubert, G. Tidal heating in an internal ocean model of Europa. Nature 325, 133–134 (1987).

    ADS  Article  Google Scholar 

  24. 24

    Ojakangas, G. W. & Stevenson, D. J. Thermal state of an ice shell on Europa. Icarus 81, 220–241 (1989).

    ADS  CAS  Article  Google Scholar 

  25. 25

    Durham, W. B., Kirby, S. H. & Stern, L. A. Creep of water ices at planetary conditions: a compilation. J. Geophys. Res. 102, 16293–16302 (1997).

    ADS  CAS  Article  Google Scholar 

  26. 26

    McCord, T. al. Salts on Europa's surface detected by Galileo's Near Infrared Mapping Spectrometer. Science 280, 1242–1245 (1998).

    ADS  CAS  Article  Google Scholar 

  27. 27

    Stevenson, D. J. in Proc. Europa Conf. 69–70 (San Juan Capistrano Research Inst., San Juan Capistrano, CA, (1996)).

    Google Scholar 

  28. 28

    Anderson, J. al. Distribution of rock, metals, and ices in Callisto. Science 280, 1573–1576 (1998).

    ADS  CAS  Article  Google Scholar 

  29. 29

    Anderson, J. D., Lau, E. L., Sjogren, W. L., Schubert, G. & Moore, W. B. Gravitational constraints on the internal structure of Ganymede. Nature 384, 541–543 (1996).

    ADS  CAS  Article  Google Scholar 

  30. 30

    Schubert, G., Zhang, K., Kivelson, M. G. & Anderson, J. D. The magnetic field and internal structure of Ganymede. Nature 384, 544–545 (1996).

    ADS  CAS  Article  Google Scholar 

  31. 31

    Jackson, J. D. Classical Electrodynamics 2nd edn 338 (Wiley, New York, (1975)).

    Google Scholar 

  32. 32

    Connerney, J. E. P. Magnetic fields of the outer planets. J. Geophys. Res. 98, 18659–18679 (1993).

    ADS  Article  Google Scholar 

  33. 33

    Khurana, K. K. Euler potential models of Jupiter's magnetospheric field. J. Geophys. Res. 102, 11295–11306 (1997).

    ADS  CAS  Article  Google Scholar 

  34. 34

    Kivelson M. al. Europa and Callisto; induced or intrinsic fields in a periodically varying plasma environment. J. Geophys. Res. (submitted).

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We thank D. Bindschadler of the Jet Propulsion Laboratory for support in all phases of data collection and J. Mafi for data processing and preparation of data plots. This work was partially supported by the Jet Propulsion Laboratory.

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Correspondence to K. K. Khurana.

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Khurana, K., Kivelson, M., Stevenson, D. et al. Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto. Nature 395, 777–780 (1998).

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