There is a new paper in press in Icarus on Triton:
On the Negligible Surface Age of Triton
Paul M. Schenk and Kevin Zahnle
The article was published online on Sunday. I just got my hands on it. I haven't ready through all of it yet, but it is very intriguing. The authors preformed a new crater counting analysis of Voyager 2 images of Triton. The counted only 100 craters larger than 5 km across, and 21 craters larger than 10 km across. Their analysis allowed them to differentiate between craters and circular diapir features in the cantaloupe terrain.
What is particularly striking isn't so much the low number of observed impact craters. Smith et al. (1990) suggested a surface of less than 500 My based on the low number of crater they counted. What makes Schenk and Zahnle's analysis striking is the complete and utter lack of impact craters on the trailing hemisphere. Every single one of the measured impact craters are on the leading hemisphere. There are a couple of possible impact features on the trailing hemisphere, but all the definitive impact craters are on the leading hemisphere. The authors suggest that this indicates that all the craters likely formed from planetocentric sources (e.g. impact spalls from one of the other Neptunian satellites or disrupted satellites). Calculations of the impactor flux at Triton from planetocentric and heliocentric sources suggest a surface age of 50 My for the "heavily" cratered terrain and 6 My for the cantaloupe terrain. This suggest that Triton has a younger surface than everyone's darling, Europa.
Once you start looking at surface ages like this, the authors suggest that Triton (EDIT: not Neptune, silly) is likely still experiencing cryovolcanism, in the form of diapirs and cryoflows. They further suggest, as Stern and McKinnon did back in 2000, that this level of activity could be evidence that the south polar plumes are driven by internal heat rather than solar heating.