The Pluto System: New Results from the New Horizons Flyby

The Pluto-Charon system provides a broad variety of constraints on planetary formation, structure, composition, chemistry, and evolution. Origin. The emerging view of planetesimal formation via gravitational instability in the protoplanetary gas \& particle disk aligns with the requirements imposed by Charon-forming giant impact models. Initial planetesimals (50-100 km scale) form between $\sim$20-30 AU. Accretion timing appears consistent with subsequent slow and/or stalled pebble accretion followed by hierarchical coagulation after nebular gas dispersal ($\sim$few Myr). Partially differentiated proto-Proto precursors imply slow and/or "pebbly" (buried heat gets radiated) and late (little $ ^{26}$Al) accretion in the trans-Neptunian planetesimal disk. The dynamic environment (number density, velocity dispersion) in the disk is favorable for Charon formation, and the subsequent giant planet instability emplaces Pluto-Charon system in its present 3:2 mean-motion resonance with Neptune. Interior. Partially differentiated precursors plus the Charon-forming impact should have driven both Pluto and Charon toward full ice-from-rock differentiation, and concomitantly toward early ocean formation. The latter is consistent with the general absence of compressional tectonics on both bodies. While evidence for an ocean on Pluto (and former ocean on Charon) remains circumstantial, evidence continues to accrue from detailed tectonic modeling of the Sputnik basin as a mascon and geologically young eruptions of NH$ _{3}$-rich cyroclastics (presumably ultimately sourced from a deep, pressurized ocean). Preservation of Pluto's ocean and maintenance of an uplifted ocean beneath Sputnik (the hypothesized source of the mascon) would have been strongly aided by clathrate formation within or at the base of the floating ice shell. Tectonics and Heat Flow. Contradictory estimates for Pluto's lithospheric heat flow exist, but the preponderance of evidence is for a low, steady-state radiogenic value (a few mW/m$ ^{2}$). The block tectonics of Charon's Oz Terra bear no relation to eccentricity tidal stresses. Eccentricity tides are not a given for Charon during its post-formation tidal evolution away from Pluto, however, which puts the onus on ocean freezing and, possibly, tidal bulge collapse to explain the extensive disruption of Charon's ancient crust. Composition and Chemistry. Pluto's low surface CO/N$ _{2}$ has been variously explained by burial in the Sputnik Planitia N$ _{2}$-ice sheet, destruction by aqueous chemistry in the ocean, or preferential sequestration in subsurface clathrate. There is no problem explaining the global nitrogen abundance. Within Pluto's chemically reducing core, thermochemistry favors the production of metastable organics, graphite, CH$ _{4}$ and N$ _{2}$. Atmosphere and Climate. On Pluto the global nitrogen ice distribution and the induced nitrogen condensation-sublimation flows strongly control the atmospheric circulation. GCMs predict a western boundary current within Sputnik basin, which in turn drives Pluto's general retrograde atmospheric circulation and can account for many of the geological features (including wind streaks and dunes) and longitudinal asymmetries in ice distribution observed on Pluto. Sputnik. So much of Pluto's geophysical, geological, and atmospheric behavior has been and is controlled or strongly influenced by Sputnik, raising the question of how other dwarf planets in the Kuiper Belt behave in the absence (or in the presence of more than one) giant impact basin. There were once $\sim$1000-4000 Pluto-mass bodies in the trans-Neptunian planetesimal disk. Dozens of them are still out there, in the Kuiper Belt's scattered disk and its extended (detached) component.