As we've gathered more details about the other planets of the Solar System, we've largely managed to explain the geography we've found by drawing analogies to things we're familiar with from Earth. Glaciers and wind-driven erosion produce similar results both here and on Mars, for instance. But further out in the Solar System, the materials involved in the geology change—water ice becomes as hard as rock, and methane and nitrogen freeze—which raises the prospect of some entirely unfamiliar processes.
This week, scientists proposed that some weird terrain found on Pluto could be the product of large fields of nitrogen ice sublimating off into the atmosphere. While this explanation could account for some properties of Pluto's geography, it doesn't explain why the process resulted in a series of parallel ridges.
On the washboard
The strange terrain lies to the northwest of Sputnik Planitia, the heart-shaped plane that dominates the side of Pluto we have the best images of. Called "washboard" or "fluted," the area consists of large numbers of roughly parallel ridges with roughly a kilometer or two separating them. Aside from their appearance and general orientation, these ridges don't seem to have a lot in common. They're discontiguous and don't fill the entire region. They run down slopes and spread across valley floors—in some cases a single ridge will run down a slope and then flatten out. And in several cases, they create a starburst-like pattern on along the walls of craters.