On the solar nebula origin of (486958) 2014 MU69, a primordial contact binary in the Kuiper belt
Abstract
MU69 is a contact binary, and all the data returned from New Horizons are consistent with it being a planetesimal. It is not a product of heliocentric, high-speed collisional evolution. There is no evidence of it having suffered a catastrophic or even a subcatastrophic impact during its lifetime. Nor is there evidence of hierarchical accretion of independent, heliocentric planetesimals, as slow as those collisions may have been in the beginning. Rather, there is strong evidence that its two lobes ("Ultima" and "Thule") came together at an extremely low velocity, on the order of no more than a couple of m/s and possibly much more slowly. Binary formation is a theoretically predicted common outcome in protoplanetary disks when swarms of locally concentrated solids ("pebble clouds") collapse under their own gravity, and plausibly explains the high fraction of binaries among cold classical Kuiper belt objects (KBOs). Cold classical KBO binaries exhibit a range of binary orbital separations, down to the observable limit, so there is no physical reason that tight or even contact binaries could not form in a collapsing pebble cloud. The prominence of bilobate shapes among the short-period comets, which are derived from the scattered disk component of the Kuiper belt, suggests (but does not require) that there is a process that collapses or hardens Kuiper belt binaries. The alignment of the principal axes of the Ultima and Thule lobes is also consistent with tidal coupling between two co-orbiting bodies, prior to a final merger. Our examination of various mechanisms to drive binary mergers in the cold classical Kuiper belt (Kozai-Lidov, BYORP, tides, collisions, gas drag) highlights the potential importance of gas drag while the protosolar nebula is still present. We find the process to be surprisingly effective, because in a gas nebula with a radial pressure gradient the velocity of the gas deviates from the heliocentric Keplerian velocity of the binary. The headwind that the binary feels couples to the motion of the binary pair about its own center of mass. The resulting viscous, Stokes-regime gas drag can collapse MU69-scale co-orbiting binaries—as well as smaller, cometary-scale binaries—within the few-Myr lifetime of the protosolar gas nebula.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.P42C..05M
- Keywords:
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- 6040 Origin and evolution;
- PLANETARY SCIENCES: COMETS AND SMALL BODIES;
- 6224 Kuiper belt objects;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 6270 Pluto and satellites;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5455 Origin and evolution;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS