Robert Farrar sent in the link to http://astronomynow.com/2015/09/20/the-surface-of-rosettas-comet-is-chan… …. which seems to have preceded the link on PhysOrg (http://phys.org/print362124520.html). Robert asks if electrical erosion has been captured by the Rosetta camera (in the process of forming a crater).
It seems that as long ago as June, 2 months prior to perihelion, scientists noticed significant change on the surface of the comet nucleus. A general increase in the outflow of gas and dust followed by the emergence of jets and rapid outbursts in the weeks around perihelion were recorded but the surface of the nucleus also changed. A new round feature appeared in a formerly smooth grained area – and it grew in size very quickly (from June onwards). This was followed by the appearance of two similar features in July. Again, rims were formed and these grew quickly. At first sublimation of volatiles was thought to be responsible but the expansion appeared to be more rapid than sublimation rate allowed. Exposed ice was seen on the rims but the sublimation rate was so high that scientists began thinking some other mechanism was also taking place. In other words, some kind of energy is required to make the comet nucleus erode so quickly. The findings will be published in Astronomy and Astrophysics later in the year.