Investigating the Thermal Effects of Water on the Differentiation of Large Proto-planetary Bodies in the Early Solar System

Arguments regarding the origin of iron meteorite parent bodies suggest that they accreted and differentiated in the inner Solar System amongst the material that would ultimately become the terrestrial planets (Bottke et al. 2006). Results from high-precision W isotopic measurements (Markowski et al. 2006) suggest that these parent bodies experienced metal-silicate segregation no later that 1 Myr after CAI formation, making them some of the earliest solid bodies to have formed. However, there is considerable uncertainty regarding the water content of proto-planetary material that originated in the terrestrial planet region of the Solar System. Planetesimal formation in an optically thick dust disk may have resulted in bodies with water contents as high as 50% by mass (Machida & Abe 2006), whereas trends in the water contents of carbonaceous, ordinary and enstatite chondrites as a function of heliocentric distance suggest that the terrestrial planets formed from anhydrous material with water mass fractions of ~ 0.001% (Raymond et al. 2004). A possible range of four orders of magnitude in the initial water content of planetesimals in the inner Solar System would clearly influence the thermal evolution of these bodies: water acts as a thermal buffer due to its high heat capacity and could cause significant exo- and endothermic water-rock reactions (Cohen & Coker 2000). We will present thermal evolution calculations to investigate the effect of water on the timescales of differentiation for large (~100-1000 km size) proto-planetary bodies. The initial temperatures of these bodies will be set to that of the ambient solar nebula (~180 K) and their initial compositions will be assumed to be a mixture of water ice and silicates. The temperature evolution will primarily be dictated by the decay of short-lived radioactive isotopes like 26Al (Grimm & McSween 1993), which will be the dominant heat source for time scales of several half-lives (~3 Myr). Particular attention will be paid to the thermal effects that hydration and dehydration reactions will have on this evolution. We will consider specific serpentinization reactions and differences in the enthalpies of reaction for the forward and reverse reactions as a function of temperature. We will also explore the thermal buffering effect of water on the overall thermal evolution of these bodies. The ultimate goal of this study is to constrain the conditions in a thermal evolution parameter space (e.g. time of accretion, water abundance, heats of reaction) that will result in thermal histories that are consistent with the differentiation timescale constraints provided by the iron meteorites.