The Thermal Structure of Triton's Middle Atmosphere

Upper Limits on Gaseous CO at Pluto and Triton from High-Resolution Near-IR Spectroscopy

“…The atmosphere of Triton was measured by Voyager 2 in 1989 (Broadfoot et al 1989, Tyler et al 1989, and by stellar occultations in the 1990's (Elliot et al 2000). The occultations indicate both an increase in pressure since the Voyager encounter and a change in the thermal profile (Elliot et al 2000).…”

“…The occultations indicate both an increase in pressure since the Voyager encounter and a change in the thermal profile (Elliot et al 2000). Again, we consider specific models for the pressures and temperatures in Triton's atmosphere.…”

“…Again, we consider specific models for the pressures and temperatures in Triton's atmosphere. Model TR1 is based on the Voyager observations (see Yelle et al 1995), while TR2 is based on a highquality stellar occultation in November 1997 (Elliot et al 2000).…”

“…The lack of observational constraints on Pluto's atmospheric CO abundance is a significant source of uncertainty in thermal models that attempt to explain Pluto's warm atmosphere at 1 µbar (∼60 K warmer than the surface), as well as the steep thermal gradient near 3 µbar, (e.g., Strobel et al 1996). Similarly, knowledge of the gaseous CO mole fraction is needed to model the observed changes in Triton's thermal structure between 1989 and 1997 (Elliot et al 2000).…”

“…Energy balance considerations in Triton's thermosphere suggest upper limits on the CO atmospheric mole fraction of 0.02 to 1% (Stevens et al 1992, but these limits depend critically on the location of the thermosphere's lower boundary and the magnetospheric power input (Strobel and Summers 1995). CO mixing ratios of 0.02-0.2% were considered in an attempt to model Triton's lower atmosphere (Elliot et al 2000), but as none of the models reproduced the temperatures observed by a 1997 stellar occultation (Elliot et al 2000), no clear conclusion on the derived CO mixing ratio can be reached. On Pluto, radiativeconvective models have been run with assumed ratios for CO (0.075% by Lellouch 1994 and 0.046% by Strobel et al 1996), but were not run with varying amounts of CO to set upper limits.…”