Comparing the Hubble-obtained 1995 map of brightness to the latest map from New Horizons shows big differences on the surface. The only consistent feature between both maps is that the northern hemisphere is brighter -- the rest is almost completely different. That's sign of processes capable of changing the surfaces brightness on quite a rapid scale. Is it just the cycle of methane decay through UV light, or is it cryovolcanism also at work? Are there any other mechanisms that haven't been considered yet affecting the surface? Hopefully those questions will be answered "shortly" (in the following years that is).
