Thermal Simulation of a Magma Ocean on Asteroid 4 Vesta

Introduction: Recent work on chronology seem to point to older ages for eucrites: which favor a magma ocean model for Vesta. [1] indicate a Pb-Pb age for the eucrite Asuka 881394 of 4.566 + 0.3 Ga. Whole rock isochrons of eucrites based on Hf-W, Al-Mg and Mn-Cr [2-4] indicate somewhat younger ages at around 4.564 Ga (or about 3.5 Myrs after CAI formation). From the perspective of thermal modeling, the old ages of noncumulate eucrites require fast accretion followed by silicate melt generation. If 26 Al is assumed to be the heat source, a scenario of early melting (necessary for the formation of eucrites), requires a magma ocean to form on Vesta. Magma Ocean Models of Vesta: There are two variations of proposed magma ocean model: the distinguishing characteristic is the use of equilibrium crystallization [5] or fractional crys-tallization models [6], respectively, to explain the petrologic trends observed in HEDs. [5] proposed a multistage evolution of the HED parent body that satisfies several petrologic constraints except for the low volatile (Ca, Na) content of eucrites. [5] envisioned an early formed magma ocean with a core and molten mantle. Equilibrium crystallization in the magma ocean produced crystals of olivine, opx and spinel. The crystals remained suspended until the crystal fraction exceeded 0.8, when gravitational segregation of silicates occurred. Crystal settling formed a layer of dunite followed by a layer of diogenite. The residual liquids generated the Main Group Eucrites, whereas fractionation of evolved liquids within the crust generated the Nuevo Laredo trend [5]. Methodology: [7,8] presented the first thermal model of Asteroid 4 Vesta using 26 Al as a heat source. In the present abstract, we modify the thermal model to simulate a scenario of a magma ocean. The heat transfer equation is solved by the finite element method for a spherical asteroid using 26 Al and 60 Fe as heat sources. Accretion is assumed to be incremental and a radiation boundary condition is used to approximate heat loss from the body. Thermal diffusivity and specific heat are recalculated for each rock type. For clarity in modeling, the calculation is divided into five temporal domains: Stage 1: Accretion; Stage 2: Heating of homogenous asteroid until core separation; Stage 3: Further heating of the mantle until magma ocean forms; Stage 4: Cooling of the magma ocean; Stage 5: Cooling of the core, mantle and crust. Parameterized convection is implemented using [9]. Results: …