One...
...explanation is that the leading edge is preferentially warmed by frictional heat as minute objects impact.
The Cassini spacecraft has sent back some unexpected surface temperature data from Saturnian moon Mimas - a satellite previous dubbed the "Death Star" for its prominent Herschel Crater, but now demonstrating its allegiance may lie more with Pac-Man than Star Wars: Cassini images of the temperature on Mimas. Pic: NASA Cassini …
these scientist guys had predicted unifom temperatures, but reality proved to be vastly different than what their computerized climate models suggested.
So their computerized climate models turned out to be wrong guesses. Good guesses, made with the best minds that science has to offer, but still wrong.
Now why does that sound so familiar?