The nature and origin of Charon's smooth plains

[Intrigued]

We are just three months past the New Horizons' passage of Ultima Thule, but work from the earlier encounter at the Pluto-Charon continues. This article in Icarus is an example.

"Charon displays extensive plains that cover the equatorial area and south to the terminator on the sub-Pluto hemisphere observed by New Horizons. We hypothesize that these plains are a result of Charon's global extension and early subsurface ocean yielding a large cryoflow that completely resurfaced this area leaving the plains and other features that we observe today. The cryoflow consisted of ammonia-rich material, and could have resurfaced this area either by cryovolcanic effusion similar to lunar maria emplacement or a mechanism similar to magmatic stoping where lithospheric blocks foundered. Geological observations, modeling of possible flow rheology, and an analysis of rille orientations support these hypotheses."

After six decades of solar system exploration by robotic craft we have seen remarkable diversity across the sytem and yet also intriguing similarities such as the surface shots from Viking on Mars, Venera on Venus and Huygens on Titan. How do other members imagine the diversity that may exist in the systems of exoplanets we have discovered? Different, but not by much, or orders of magnitude more diverse? (And how do you quantify diversity?)