Can the Sublimation of Dry Ice Explain the Flattened, Bilobate Structure
and Spin Rate of 2014 MU69?
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.