The heating history of Vesta and the onset of differentiation

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Abstract

In this work, we study the link between the evolution of the internal structure of Vesta and thermal heating due to 26Al and 60Fe and long-lived radionuclides, taking into account the chemical differentiation of the body and the affinity of 26Al with silicates. We explored several thermal and structural scenarios differing in the available strength of energy due to the radiogenic heating and in the postsintering macroporosity. By comparing them with the data supplied by the HEDs and the Dawn NASA mission, we use our results to constrain the accretion and differentiation time as well as the physical properties of the core. Differentiation takes place in all scenarios in which Vesta completes its accretion in <1.4 Ma after the injection of 26Al into the solar nebula. In all those scenarios where Vesta completes its formation in <1 Ma from the injection of 26Al, the degree of silicate melting reaches 100 vol% throughout the whole asteroid. If Vesta completed its formation between 1 and 1.4 Ma after 26Al injection, the degree of silicate melting exceeds 50 vol% over the whole asteroid, but reaches 100 vol% only in the hottest, outermost part of the mantle in all scenarios where the porosity is lower than 5 vol%. If the formation of Vesta occurred later than 1.5 Ma after the injection of 26Al, the degree of silicate melting is always lower than 50 vol% and is limited only to a small region of the asteroid. The radiation at the surface dominates the evolution of the crust, which ranges in thickness from 8 to about 30 km after 5 Ma: a layer about 3–20 km thick is composed of primitive unmelted chondritic material, while a layer of about 5–10 km is eucritic.

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