COUSINS TWICE REMOVED: THE PLUTO V. TRITON KUIPER BELT RIVALRY REVISITED

New Horizons exploration of Pluto revealed a complex and diverse world. Pluto may be the only isolated icy Kuiper Bet object visited to date but it was not the first. Voyager 2 encountered Neptune's large moon Triton in 1989, an object likely captured by Neptune and severely altered since. Triton and Pluto are near twins in internal density, both ~2.0 g/cm3, and surface composition, rich in methane, nitrogen, carbon monoxide and water ices, among others. Yet here they diverge. Pre-Pluto speculation centered on whether the two bodies might have geologic features in common, but post-Pluto reexamination indicates the two could scarcely be different. The primary difference between the two is age: Pluto has a wide variety of geologic units of very different age, from very ancient to crater-free and extremely young. Triton has no observable ancient surfaces, and is likely active in some way with a global ocean of liquid ices beneath the surface. Dimpled terrains betray extensive diapiric overturn of the crust and extensive smooth plains and pit chains betray protracted volcanism. Spotted terrains in the south have no known analog, even on Pluto. Pluto is dominated by extensive etched and pitted plains (perhaps analogous to some areas on Triton). Large quasicircular massifs may be volcanic but they are very different from the Triton examples, implying different materials or thermal/rheological conditions. No atmospheric plumes of the type seen on Triton are observed, and the vast convecting ice sheet on Pluto has no analog on Triton. Lastly, Pluto has much more relief, locally as high as 5 km, whereas local relief on Triton is <1 km. Overall, Pluto has the diversity of an ancient world undergoing limited but focused geologic activity, but Triton is convulsing and turning itself inside out, as might be expected for an ice-rich body subjected to extreme tidal heating since its capture by a much large planet.