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| Titel |
Evolution of the Moon's core in the Fe-snow regime |
| VerfasserIn |
Tina Rückriemen, Doris Breuer, Matthieu Laneuville, Tilman Spohn |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091606
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| Publikation (Nr.) |
 EGU/EGU2014-5908.pdf |
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| Zusammenfassung |
| Since the Apollo missions it is well known that the crust shows remanent magnetization
[e.g.,1] suggesting that the Moon experienced a magnetic era between 4.2 and 3.2 Gyr ago
with a field strength of some μT [2,3,4]. The recent findings of a present-day solid
inner core [4] imply that compositional convection may be responsible for dynamo
action in the Moon’s core. The solidification of iron in the Moon’s core may either
start in the center or at the core-mantle boundary (Fe-snow regime) depending
on the sulfur content [5]. A recent study shows whenever solidification starts in
the center a dynamo would still be active today (LPSC abstract by Laneuville et
al.).
In the present study, we examine the evolution of the Moon’s core in the Fe-snow regime,
where solidification of iron first starts at the core-mantle boundary. The Fe-snow regime
occurs for initial sulfur concentrations xs ≥9 wt.% and is characterized by a snow zone, in
which crystallized iron sinks, and a fluid core layer below, in which the crystallized iron
remelts — the lower fluid layer convects due to compositional buoyancy by sinking of dense
Fe-FeS fluid enriched in remolten Fe. We propose that in the Fe-snow regime the lower fluid
core is the origin of the magnetic field [6]. Consequently, such a dynamo is restricted in time,
because it stops when the growing snow zone comprises the entire core. To study the onset of
the Fe-snow and the duration of the dynamo, we apply a 1D thermo-chemical core
evolution model treating the Fe-snow process [6], which in turn is coupled to a 1D
parameterized convection model to calculate the thermal evolution of mantle and crust
[7].
We vary four parameters: the inital sulfur concentration xs (9-19 wt.%), the thermal
conductivity kc (20-84 W/mK), the reference mantle viscosity (η -1019 Pas (wet), η -1021
Pas (dry)) and the initial mantle temperature (Tm=1560 K (cold), Tm= 1750 K (warm) ) with
our reference model being xs=13 wt.%, kc=32 W/mK, η -1019 Pas, Tm=1560 K. Within the
frame of the present parameter study we can show that a dynamo due to Fe-snow in the
Moon’s core does not match the observed data as suggested by [2,3,4]. Hence,
the present findings suggest that the limited occurence of a lunar dynamo may be
caused by a transition from the classic inner core growth to the Fe-snow regime. This
scenario considers a lunar core that starts to crystallize in the center thereby starting a
chemical dynamo. During inner core growth the outer core becomes enriched in sulfur.
Eventually, the enrichment in sulfur leads to the concurrent start of Fe-snow in the outer
core. Since we find that the duration of the Fe-snow regime is rather short (-¤ 500
Myr) it could lead to an early shut down of the dynamo, which is not observed
within the mere inner core growth regime (see LPSC abstract by Laneuville et al.).
To conclude, we reveal that the pure Fe-snow regime is unlikely to explain the
observed data of remanent magnetization. Likewise, the mere inner core growth
regime has the same caveat. Thus, we suggest that a transition from the inner core
growth to the Fe-snow regime may be the key to explaining the lunar magnetic
field.
References [1] Wieczorek, M. A. et al. (2006) RIMG, 60, 1, pp. 221–364, [2] Garrick-Bethell et
al. (2009), Science, 323(5), pp. 356-359, [3] Suavet et al., PNAS, 110, 21, pp. 8453–8458, [4]
Shea et al., Science, 335(6067), pp. 453-456, [4] Weber, R. C. et al. (2011), Science,
331(6015), 309–312, [5] Williams, Q. (2009), EPSL, 284(3-4), pp. 564–569, [6] Rückriemen
et al. (2013), EGU, Vienna, Austria, [7] Grott, M. and Breuer, D. (2008), Icarus 193, pp.
503–515 |
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