Physics
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmmr14a..02n&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #MR14A-02
Physics
[1507] Geomagnetism And Paleomagnetism / Core Processes, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [3919] Mineral Physics / Equations Of State
Scientific paper
How can measurements of planetary core materials improve our understanding of their geodynamical behaviour? Here I will focus on three aspects of this questions: 1) core formation; 2) the growth and rheology of solid cores; 3) dynamo activity. Core formation occurs either due to the heat generated by short-lived nuclides (for small bodies) or due to gravitational energy released during impacts (for large bodies) [1]. Core formation results in elemental fractionation; such fractionation depends on P,T and oxygen fugacity [2], and for Earth-mass bodies occurs as a succession of discrete events. Experimental measurements of siderophile element partition coefficients are necessary to infer conditions during accretion, though these inferences are non-unique [3]. Core formation may also lead to isotopic fractionation of elements such as Si [4] and Fe [5], although the latter in particular is currently uncertain and merits further experimental investigation. Core solidification depends on the slopes of the adiabat and melting curve, and on the concentration and nature of the light element(s) present [6,7]. Solidification may proceed from outside in (for small bodies) or from inside out (for larger bodies); the solid may be either lighter or heavier than the fluid, depending on the core composition. Thus, core solidification is complex and poorly understood; for instance, Ganymede and Mercury’s cores may be in a completely different solidification regime to that of the Earth [8,9]. Solidification can also vary spatially, giving rise to inner core seismological structure [10,11]. The viscosity of a solid inner core is an important and poorly constrained parameter [12] which controls core deformation, core-mantle coupling and tidal heating. Super-Earths probably lack solid inner cores [13], though further high-P experimental data are needed. Core dynamos are usually thought to be driven by compositional or thermal buoyancy [14] , with the former effect dominant for small bodies. However, forcing driven by tidal or precessional effects may also be important [e.g. 15]. As noted above, the complexities of core solidification can lead to a rich range of potential dynamo styles [e.g. 16]. The long-term evolution of dynamos is governed primarily by the mantle's ability to extract heat from the core. For the Earth, a factor of 2 uncertainty in the thermal conductivity of liquid iron is a current impediment to a better understanding of the dynamo's evolution [14]. [1] Rubie et al. Treat. Geophys. 2007 [2] Righter 2003 [3] Rudge et al. 2010 [4] Ziegler et al. 2010 [5] Polyakov 2009 [6] Chen et al. 2008 [7] Morard et al. 2007 [8] Nimmo & Alfe 2007 [9] Hauck et al. 2006 [10] Monnereau et al. 2010 [11] Alboussiere et al. 2010 [12] Mound & Buffett 2006 [13] Gaidos et al. 2010 [14] Nimmo Treat. Geophys. 2007 [15] Tilgner 2005 [16] Vilim et al. in press
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