Nature Astronomy 1, 0016 (2017); published 4 January 2017; corrected 13 January 2017.

In the version of this News and Views originally published the sentence giving the penitent formation timescale was incorrect and should have read: ‘Moreover, in order to explain the current height and tri-modal orientation, they infer that these features must be a few tens of millions of years old.’ In addition, N2 and CH4 were mistakenly displayed with superscript numbers in the HTML version. These errors have been corrected in all versions of the News and Views.