Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusm.p41a..02p&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #P41A-02
Physics
5410 Composition (1060, 3672), 5417 Gravitational Fields (1221), 5430 Interiors (8147), 5455 Origin And Evolution, 5460 Physical Properties Of Materials
Scientific paper
The confirmation by the NASA MESSENGER spacecraft that Mercury has an internal magnetic field that is well described by a dipole nearly aligned with the spin axis strongly suggests that the planet may have an outer core of molten metal (S. Solomon, MESSENGER news release of 30 January 2008). The existence of an internal layer of liquid has also been invoked to explain radar measurements of the large amplitude of the longitudinal libration of Mercury relative to the value expected for a wholly solid planet (J.L. Margot et al 2007 Science 316 710). The existence of molten metal in the planet`s interior is surprising since previous numerical models for the thermal evolution of the planet, calculated on the basis of the heat released by the decay of the radioactive isotopes of U and Th, indicated that the present temperature at the edge of the metal core is only ~ 1200 K (cf. Siegfried & Solomon 1974 Icarus 23 192) . This value is well below the melting temperature Tm = 2030 K of Fe-Ni alloy at the core/mantle boundary (CMB) pressure of ~ 70 kbar. Those earlier thermal calculations were, however, based on low abundances of U and Th found in lunar samples. Prentice (2008 LPSC 2008 abs. # 1945.pdf - see URL below) has put forward a new model for the bulk chemical composition of Mercury. It is based on the idea that this planet condensed from a gas ring that was cast off by the protosolar cloud close to the planet`s present orbit. The temperature of the gas ring Tn at the moment of detachment from the cloud is 1628 K and the pressure on the mean orbit of the ring is 0.168 bar. Because Tn is so high, the condensate contains a much reduced proportion of magnesium silicates relative to metals. This is because metals have a much lower vapour pressure than those silicates. The condensate consists mostly of Fe-Ni-Cr-Co-V (mass fraction 0.671), gehlenite (0.190) and Mg-silicates (0.081). What is really important in the gas ring model of solar system origin, however, is that the abundances of U and Th in the Mercury condensate are a factor of 4.3 times those of the Earth. The UO2 mass fraction is 6.4 × 10- 8. All bulk compositional mass fractions are computed using the protosolar elemental abundance compilation of K. Lodders (2003 Ap.J. 591 1220). The cold-start thermal evolutionary model for Mercury presented in Prentice (2008) is based on the U & Th abundances given above. The temperature at the CMB rises quickly from 350 K to 1630 K in 1 byr. Further increase was then stopped since it was assumed that efficient solid state convection set in once the temperature exceeds the value 0.7 Tm, where Tm is the local melt temperature of the rock. But a creep factor Fcreep = 0.6 - 0.7 is true just for metals, not rocks. For silicates, a higher factor ~ 1 is indicated (J-P Poirier 1985 Creep of Crystals, CUP, p. 163). Adopting Fcreep = 0.9, the CMB temperature now rises to 2100 K and the outer portion of the core become molten. If a hot-start is made, corresponding to an initial fully- differentiated body with central temperature Tc = 2500 K, the present Tc is 2150 K and the outer 13.2% of the core`s mass remains molten at solar age. This layer is still cooling at its inner edge. The estimated axial moment-of-inertia factor of this new hot model for Mercury is C/MR2 = 0.330 +/- 0.003. I thank George W. Null [JPL] for much hospitality in Pasadena and Steve Morton [Monash] for technical support.
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