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The Primordial Disk of small icy planetesimals, once located at 15-30 AU fromthe Sun, was disrupted by giant planet migration in the early Solar System. ThePrimordial Disk thereby became the source region of objects in the current-dayKuiper Belt, Scattered Disk, and Oort Cloud. I present the thermophysics code"Numerical Icy Minor Body evolUtion Simulator", or NIMBUS, and use it to studythe thermophysical evolution of planetesimals in the Primordial Disk prior toits disruption. Such modelling is mandatory in order to understand thebehaviour of dynamically new comets from the Oort Cloud, as well as theactivity of Centaurs and short-period comets from the Scattered Disk, thatreturn pre-processed to the vicinity of the Sun. I find that bodies in themidst of the Primordial Disk with diameters ranging 4-200 km lost all their COice on time-scales of order 0.1-10 Myr depending on size, through a combinationof protosolar and long-lived radionuclide heating. CO and other hypervolatilestherefore require a less volatile host for their storage. I consider twopossible hosts: amorphous water ice and CO2 ice. Because of the high luminosityof the protosun, some Primordial Disk bodies may have sustained significantcrystallisation, CO:CO2 segregation, and CO2 sublimation in the uppermost fewtens of meters. I discuss how this may affect coma abundance ratios and distantactivity in dynamically new comets.

Thermophysical evolution of planetesimals in the Primordial Disk