GLOBAL WARMING ON TRITON

Triton, Neptune's largest moon, has been predicted to undergo signific ant seasonal changes that would reveal themselves as changes in its me an 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 fro st migration patterns) that are unknown. Here we report observations o f a recent stellar occultation by Triton which, when combined with ear lier results, show that Triton has undergone a period of global warmin g since 1989, Our most conservative estimates of the rate of temperatu re and surface-pressure increase during this period imply that the atm osphere is doubling in bulk every 10 years-significantly faster than p redicted by any published frost model for Triton(2,3) Our result sugge sts that permanent polar caps on Triton plap a dominant role in regula ting seasonal atmospheric changes. Similar processes should also be ac tive on Pluto.