Influence of Viscosity on Magnetic Field Generation in Super-Earths
Abstract
The Earth's magnetic field is generated by convective flows of an electrically conducting fluid in the outer core of the planet, which is thought to consist of liquid iron, containing a few percent of light elements like oxygen. To maintain convection in the outer core, convective heat transport has to be more effective than conduction. This is ensured as long as the mantle is extracting heat from the core to a sufficient degree, but not too efficient, which would cool the Earth's core too quickly and leave a completely solid core, unable to generate a magnetic field. Mantle convection provides the Earth with the mechanism to ensure a cooling rate that is adequate to maintain a convective core, which is obvious from the presence of the Earth's magnetic field over geological time. Although the viscosity of the mantle material is many orders of magnitude larger than that of liquid iron in the core, it is low enough for mantle material to exhibit flow behaviour over geologic timescales. In view of the increasing number of detected exoplanets with masses from one to ten times the mass of the Earth, so-called Super-Earths, the question is whether mantles in these planets are also able to cool their cores sufficiently via mantle convection to sustain convective cores. By using a simple model that combines the results from the interior structure model by Wagner et al. (2011) with a parameterized mantle convection approach by Nimmo et al. (2004) the heat flow across the core-mantle boundary of Super-Earths with 5 and 10 times the mass of the Earth has been calculated. It turns out that with increasing planetary mass the dominance of conduction over convection in the cores increases due to the increasing viscosity of the lower mantle in these planets. Therefore it is very likely that the existence of self-sustaining dynamos decreases with increasing planetary mass.
- Publication:
-
Workshop on Extraterrestrial life - Beyond our expectations?
- Pub Date:
- May 2012
- Bibcode:
- 2012elbe.workE..30G