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

The 2019 January 01 flypast of Ultima Thule by the New Horizons spacecraft has provided the author with a new opportunity to test his gas ring model of planetary origin (Prentice, 1978, Moon Planets 19 341). The model proposes that Ultima Thule condensed from the first gas ring shed by the gravitationally contracting protosolar cloud. I use the fully quantified gas ring model to compute the thermal properties of the gas ring in which Ultima condensed and thence to predict the initial bulk chemical composition of the condensate. It is predicted that all KBOs initially contained large stores of CO2 ice and CH4 ices. 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 has fraction 0.5269 and graphite has 0.0163. Next, I compute the thermal evolution of Ultima, taking into account the radiogenic heat released by the decay of 26Al. Stellar occultation data suggest that Ultima Thule may consist of 2 lobes of radius about 10 km and 7.5 km. The thermal evolution model shows that within 0.2 Myr, the peak internal temperatures are sufficient for a fraction ~0.7 of the CH4 ice in the larger lobe to melt and for a fraction ~0.4 of the CO2 ice to sublime. For the smaller lobe, these fractions are less. Liquid CH4 quickly migrates upwards to the surface and refreezes to form a thick outer shell of CH4 ice. The sublimation of CO2 takes place after the melting of CH4. The possibility now exists for rising CO2 vapour to become trapped beneath the CH4 shell. This may lead to explosive eruptions of the outer shell and destruction of 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 radiogenic heating may not be sufficient to render the surface of Ultima Thule globally smooth, unless the lobe sizes are of order ~15 km.

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