Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0030p&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0030
Astronomy and Astrophysics
Astronomy
5410 Composition (1060, 3672), 5417 Gravitational Fields (1221), 5430 Interiors (8147), 5455 Origin And Evolution, 5460 Physical Properties Of Materials
Scientific paper
The MESSENGER spacecraft has confirmed that Mercury's magnetic field is dominantly dipolar and due to an active dynamo in a molten outer core (Solomon et al, 2008 Science 321 59). An energy source is needed to maintain this dynamo. Either liquid iron is freezing at the surface of an inner solid core (as proposed here) or solid iron is precipitating within an outer sulphur-rich core (Chen et al, 2008 GRL 35 L07201). If the outer core does not contain sulphur and consists solely of pure metal (Fe, Ni, Cr,..), then an active dynamo is inconsistent with previous numerical models for the radiogenic thermal evolution of the planet. Those earlier models found that the present temperature at the core/mantle boundary (CMB) is ~ 500 K below the melting temperature of metal ~ 2030 K for a CMB pressure of 70 kbar. The earlier calculations were based on low lunar abundances of U and Th. Here I present a new model for the bulk chemical composition, thermal evolution and current internal structure of Mercury. The model is based on the modern Laplacian theory of solar system origin (Prentice, 1978 Moon Planets 19 341; 2001 Earth Moon & Planets 87 11; 2006 Publ. Astron. Soc. Aust. (PASA) 23 1; 2008 - URL below). A key feature of this theory is that the planets formed from a concentric system of gas rings (n = 0, 1, 2,..) that were shed by the contracting protosolar cloud. The temperatures Tn of the rings scale with mean orbital radius Rn closely as Tn ~ Rn-0.9. Mercury plays a crucial role in calibrating this relationship because of a condensation process of metal/silicate fractionation (Lewis, 1972 EPSL 15 286). Choosing Tn ~ 1630 K for mean orbit gas ring pressure of 0.17 bar, the condensate consists mostly of Fe-Ni-Cr (mass fraction 0.671), gehlenite (0.190) and Mg-silicates (0.081). It has mean density 5.30 g/cm3. Na, K and S are absent. The mass fractions of U and Th, namely 5.66 × 10-8 & 2.08 × 10-7, are a factor of 4.3 times greater than those of the proto-Earth condensate. The interior thermal profile of Mercury has been computed with the above bulk composition. The planet is assumed to be a 2-zone structure (core/mantle) with initial central temperature 2500 K and constant surface value 350 K. The temperature profile T(r) at time 0 is fitted smoothly against radius r so that TCMB is a fixed fraction φ of the local melting temperature of gehlenite, namely Tm(p)/K = 1863 + 6.8p/kbar for pressure p (Hirschberg, 1970). The rock is assumed to locally convect if T(r) exceeds φTm. For model 1, φ = 0.90 and the outer 13% of the cooling core is still molten today. The mean density of the model, 5.25 g/cm3, falls short of the observed value 5.43 g/cm3 (Anderson et al, 1987 Icarus 71 337). Increasing the core mass fraction to 0.701 to fix this problem, the melt mass drops to 10% of the core mass. Taking φ = 0.95 and core mass fraction 0.707, the melt mass increases to 30%. The core is everywhere cooling, except at its outer edge where TCMB is steady. The mass of the solid inner core continues to grow as molten metal freezes out at its surface. The predicted polar moment-of-inertia factor of Mercury is C/MR2 = 0.332 ± 0.002. I thank George Null (JPL) and David & Michelle Warren (Hobart) for continued support.
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