Global warming on Triton

Triton, Neptune's largest moon, has been predicted to undergo significant seasonal changes that would reveal themselves as changes in its mean frost temperature(1-3). But whether this temperature should at the present time be increasing, decreasing or constant depends on a number of parameters (such as the thermal properties of the surface, and frost migration patterns) that are unknown. Here we report observations of a recent stellar occultation by Triton which, when combined with earlier results, show that Triton has undergone a period of global warming since 1989, Our most conservative estimates of the rate of temperature and surface-pressure increase during this period imply that the atmosphere is doubling in bulk every 10 years-significantly faster than predicted by any published frost model for Triton(2,3) Our result suggests that permanent polar caps on Triton plap a dominant role in regulating seasonal atmospheric changes. Similar processes should also be active on Pluto.