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| Titel |
Implications of the Fe-snow regime on inner core growth and thermal buoyancy in Ganymede's core |
| VerfasserIn |
Tina Rückriemen, Doris Breuer, Tilman Spohn |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057537
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| Zusammenfassung |
| The detection of a magnetic field at the Galilean moon Ganymede was one of the
most surprising discoveries of the Galileo mission. Ganymede’s magnetic field is
believed to be caused by an active dynamo in the core (1). A purely thermally driven
dynamo is assumed to be active only during the first few hundred million years after
accretion and core formation. Consequently, a compositionally driven dynamo was
suggested for Ganymede’s core by (2,3,4). Such a dynamo is caused by freezing of an
iron-rich core and the flow due to the associated density variations. The most recent
experimental studies for eutectic Fe-FeS-alloys show a decrease of the eutectic
temperature with increasing pressure for pressures less than 14 GPa (5,6). The negative
slope of the eutectic melting temperature can lead to crystallization regimes in
cores of small planetary bodies, which are very different from the one known from
Earth’s core. Assuming core compositions on the Fe-rich side of the eutectic, Fe
precipitates at the core-mantle boundary rather than at the core center. Due to gravity
these solid Fe particles sink towards the core center and eventually remelt–as a
consequence a chemical gradient develops in the fluid core before an inner core can
form.
In this study, we develop a detailed thermodynamic model for Ganymede’s core and
investigate the chemical gradient forming in the core. We concentrate on the questions
whether the chemical gradient can stabilize the core flow against thermal buoyancy
and how long it takes for an inner core to form. The reference model of 10Â wt.%
initial sulfur concentration and a thermal expansion of α=9.2-
10-5Â K-1 requires a
reduction of core temperature by ΔT=50Â K to form a chemical gradient across
the entire core region. Thermal evolution models of Ganymede (3) then suggest a
timescale of 1 billion years for the Fe-snow regime until an inner core grows. The
average sulfur gradient for the reference model is Δxs/Δr=0.0047Â wt.%/km. That
implies a density difference of ΔÏ=266Â kg/m3, which in turn results in a required
temperature difference between the core center and the core-mantle boundary of
ΔT=493Â K necessary for thermal motions to overcome the compositional density
difference. The actual adiabatic temperature difference between the core-mantle
boundary and the core center is only about 95Â K. Accordingly, a superadiabatic
temperature gradient is required to overcome the stable chemical one. Thus, for a core
composition on the Fe-rich side of the eutectic, thermal bouyancy most likely does not
contribute to the dynamo generation in Ganymede’s core. Whether the described
sedimentation process of Fe-snow is sufficient for driving a dynamo and, if so, for
reproducing a magnetic field of the required strength, is not known and needs to be
studied.
References:
(1) G. Schubert, K. Zhang, M.G. Kivelson, and J.D. Anderson.The magnetic
field and internal structure of Ganymede. Nature, 384(6609):544–545, December
1996.
(2) W.B. McKinnon and S.S. Desai. Internal structures of the Galilean satellites: What
can we really tell? In 34 th Lunar and Planetary Science Conference; Abstracts of Papers,
2003.
(3) S.A. Hauck II, J.M. Aurnou, and A.J. Dombard. Sulfur’s impact on core evolution and
magnetic field generation on Ganymede. Journal of Geophysical Research, 111(E9):E09008,
2006.
(4) M.T. Bland, A.P. Showman, and G. Tobie. The production of Ganymede’s magnetic
field. Icarus, 198(2):384–399, 2008.
(5) Y. Fei, C.M. Bertka, and L.W. Finger. High-pressure iron-sulfur compound, Fe3S2,
and melting relations in the Fe-FeS system. Science, 275(5306):1621, 1997.
(6) Y. Fei, J. Li, C.M. Bertka, and C.T. Prewitt. Structure type and bulk modulus
of Fe3S, a new iron-sulfur compound. American Mineralogist, 85(11-12):1830,
2000. |
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