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The 2019 January 01 flypast of Ultima Thule by the New Horizons spacecrafthas provided the author with a new opportunity to test his gas ring model ofplanetary origin (Prentice, 1978, Moon Planets 19 341). The model proposes thatUltima Thule condensed from the first gas ring shed by the gravitationallycontracting protosolar cloud. I use the fully quantified gas ring model tocompute the thermal properties of the gas ring in which Ultima condensed andthence to predict the initial bulk chemical composition of the condensate. Itis predicted that all KBOs initially contained large stores of CO2 ice and CH4ices. These make up fractions 0.2210 and 0.0513 of the condensate mass,respectively. Water ice makes up a mass fraction 0.1845, nearly-dry rock hasfraction 0.5269 and graphite has 0.0163. Next, I compute the thermal evolutionof Ultima, taking into account the radiogenic heat released by the decay of26Al. Stellar occultation data suggest that Ultima Thule may consist of 2 lobesof radius about 10 km and 7.5 km. The thermal evolution model shows that within0.2 Myr, the peak internal temperatures are sufficient for a fraction ~0.7 ofthe CH4 ice in the larger lobe to melt and for a fraction ~0.4 of the CO2 iceto sublime. For the smaller lobe, these fractions are less. Liquid CH4 quicklymigrates upwards to the surface and refreezes to form a thick outer shell ofCH4 ice. The sublimation of CO2 takes place after the melting of CH4. Thepossibility now exists for rising CO2 vapour to become trapped beneath the CH4shell. This may lead to explosive eruptions of the outer shell and destructionof the primordial surface of Ultima and loss of the CO2. If 60% of CO2 is lost,the lobe radii each shrinks by ~5%. Even so, the intensity of 26Al radiogenicheating may not be sufficient to render the surface of Ultima Thule globallysmooth, unless the lobe sizes are of order ~15 km.

Ultima Thule: a Prediction for the Origin, Bulk Chemical Composition,  and Physical Structure, submitted prior to the New Horizons Spacecraft 100  Pixel LORRI Data Return