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| Titel |
Do Super-Earths Have Convective Cores? |
| VerfasserIn |
M. W. Gold, J. J. Leitner, M. G. Firneis |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061619
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| Zusammenfassung |
| The processes within the Earth responsible for the generation of the magnetic field are known
fairly well by now and highly sophisticated 3D dynamo models are already capable of
simulating magnetic field reversals (Glatzmaier and Roberts, 1995). But do the assumptions
about the Earth’s interior, like vigorous convection in the outer core, also hold for
Super-Earths, terrestrial exoplanets more massive than the Earth? To shed light on this matter,
as a first step an interior structure model (e.g., Wagner et al., 2011) has to be applied in order
to determine density, pressure and temperature gradients of the planetary structure
under consideration, in conjunction with an adequate equation of state that accounts
for high-pressure conditions in the deeper regions of the planet. By coupling a
structure model with a parameterized convection approach for the mantle (e.g., Nimmo
et al., 2004), the heat flow across the core-mantle boundary can be determined.
This heat flow is crucial in assessing the ability of the planet’s core for vigorous
convection, one prerequisite for dynamo action. If the heat flow is lower than the heat
conducted down the adiabat, conduction will dominate, and thus convection will be
restrained. By using the model curves calculated by Wagner et al. (2011), and employing
the parameterized approach demonstrated by Nimmo et al. (2004), the CMB heat
flow has been estimated for the case of 5 ME and 10 ME planets, assuming 205
ppm 40K in the core . It turns out that the heat conducted down the adiabat Qk
exceeds the heat flow across the core-mantle boundary QC in both cases: Qk=33.37
TW and 93.6 TW for 5 ME and 10 ME planets, respectively, using the results for
the Keane-EOS from Wagner et al. (2011), while QC=5.61 TW and 12.13 TW,
respectively.
The difficulties involved with the parameterized approach (Labrosse, 2003) and
the poorly constrained values of core quantities, like the thermal expansivity, or
thermal conductivity at the base of the mantle, can lead to substantial inaccuracies in
the estimated heat flows, but it seems to be very likely that the qualitative trend is
correct: the increasing dominance of conduction in planetary cores and mantles
with increasing planetary mass. Although radiogenic heat has been included in the
energy budget of the core in this preliminary model, its influence on viscosity hasn’t
been scrutinized so far. Moreover, the possibility of compositional convection at
pressures prevalent in massive terrestrial exoplanets should be subject of further
studies.
References:
Glatzmaier, G., Roberts., P. (1995): A three-dimensional self-consistent computer simulation
of a geomagnetic field reversal, Nature, 337, 203
Wagner, F.W., Sohl, F., Hussmann, H., Grott, M., Rauer, H. (2011): Interior structure models
of solid exoplanets using material laws in the infinite pressure limit, Icarus, 214,
366
Nimmo, F., Price, G.D., Brodholt, J., Gubbins, D. (2004): The influence of potassium on core
and geodynamo evolution, Geophys. J. Int., 156, 363
Labrosse, S. (2003): Thermal and magnetic evolution of the Earth’s core, Physics of the Earth
and Planetary Interiors, 140, 127 |
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