A new category for designating surface features in extraterrestrial worlds have been approved last week. IAU WGPSN has approved the use of a new descriptor term for “sinuous features with segments of positive and negative relief along its length”. There is no feature having the term Serpens yet, but a feature in Aeolis Dorsa (Mars) is expected to be approved to be designated as Serpens. It is a rule that new descriptor terms are approved only if there is a real need for that; one may ask: what is so special in these Martian serpents that made them not fit any of the previous 54 (!) categories? We have dorsa for positive features (for example for possible eskers or sinuous rigdes on Mars) and we have vallis for negative sinuous features. However, Mars hosts sinuous features that are both negative and positive along their path. When they formed, serpentes were either positive or negative features (probably negative: fluvial valleys), and their current peculiarity was caused by differential erosion and cementation. Since the geographical tradition is that features are classified by their current shape, or morphological characteristic, these paleochannels are now negative to positive landforms, and as such, deserve their own descriptor term.
A nice example of a martian sedimentary serpent (serpens landform) is visible in the following image (courtesy of Rebecca Williams)
The first paper discussing these features using the new names will soon appear in Icarus by Williams et al.