Varuna: thermal evolution

We present results obtained using a one-dimensional thermal model of the evolution of the Edgeworth-Kuiper Belt object (20000) Varuna. Physical processes described by the model include radiogenic heating and impact heating of the accreting surface. We assumed that the non-volatile fraction of the accretion material was similar in chemical composition and physical properties to chondritic meteorites, thus containing elements of the radioactive series which are the internal sources of radiogenic heat. The small average density of Varuna (∼ 1000 kg m-3) requires a high porosity of the material (∼ 0.62). It follows that the volume power of radiogenic heat sources is only a fraction of that of chondritic material. However, this is enough to increase the temperature of the celestial body interior up to 180 K by the end of the formation process. Over the lifetime of the Solar System, the low temperature of the surface (∼ 60 K) leads to the cooling of the interior down to the temperature of about 85 K regardless of the chosen model parameters. We computed different formation models of Varuna by varying accretion time, accretion rate, and fraction of kinetic energy of impacts converted into heat. If at the act of collision the residual kinetic energy exceeds the sublimation energy of ices in the infalling material, then the outer layers of the body will be depleted in volatiles. Otherwise, due to the low temperature of subsurface layers, ices will be preserved in these layers until the present time.