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Can the Sublimation of Dry Ice Explain the Flattened, Bilobate Structure and Spin Rate of 2014 MU69?
  • Andrew Prentice
Andrew Prentice
Monash University

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The New Horizons flyby of MU69 has yielded a wealth of new data on the physical structure and surface chemistry of this cold classical KBO (CCKBO). One surprising discovery is the flattened shape of each lobe. Another is the low spin rate. This means that the lobes must have lost orbital angular momentum, prior to merging (Stern et al 2019, Science 364 649). Importantly, I note here that the MU69 ang. mom. density equals that of a single globe of period ~5.8 hrs. This matches the mean of single-lobed KBOs (Benecchi & Sheppard 2013). I now assume that initially MU69 was a rough spheroid of mean diameter 21 km. I suggest that the CCKBOs accreted from a torus of co-orbiting condensate grains that settled gravitationally on to the mean circular orbit of the 1st gas ring shed by the contracting protosolar cloud (Prentice 2018, DPS #50 id.113-03; 2019, AAS #233 id.467.01). The mean orbital radius, temperature and mean-orbit pressure of this gas ring are 35.6 AU, 26.3 K and 1.27 x 10-9 bar. The bulk chemical composition of the condensate is nearly-dry rock [mass fraction: 0.5269], graphite [0.0163], H2O ice [0.1845], CO2 ice [0.2210] and CH4 ice [0.0513]. Its mass density is 1.73 g/cm3. A thermal evolution model for proto-MU69 is constructed assuming that all heat is derived from the decay of 26Al. An initial temperature of 35 K is chosen. By 24,000 yr, ~50% of the inner CH4 ice mass has both melted and vaporized. It is proposed that although CH4 vapour rises to the surface, much of it recondenses before escaping into space, so forming an outer solid shell of thickness ~0.6 km. At time ~60,000 yr, central sublimation of CO2 ice begins. Within a further 4400 yr, all CO2 ice out to the 0.5 radius point has sublimed. This gas is prevented from escaping because of the outer CH4 shell. I conjecture that gas pressure build-up cleaved MU69 into two in its equatorial plane. Centrifugal force then caused these ‘halves’ to drift apart until stopped by the drag of the frozen shell.