Ultraviolet Spectrometer Observations of Neptune and Triton

Abstract

Results from the occultation of the sun by Neptune imply a temperature of 750 +/- 150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman alpha (340 +/- 20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95 +/- 5 kelvins, and the surface pressure is roughly 14 microbars.

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

“…During the Voyager 2 encounter with Triton in 1989, Triton's atmosphere was determined to be primarily N 2 (Broadfoot et al 1989). Our results show that Triton did not change from a nearly pure N 2 atmosphere to a nearly pure CO one between 1989 and 1996.…”

“…This leads to gaseous CO mole fractions of 7-10% for Pluto and 6-9% for Triton. The previously published upper limits on Triton's atmospheric CO mixing ratio (Broadfoot et al 1989, Stern et al 1995 rule out an intimate mixture of CO and N 2 on the surface of Triton. Similarly, for the tropopause model of Pluto's atmosphere (PL3), we find that the non-detection of CO absorption presented here rules out the possibility of an intimate mixture of CO and N 2 on the surface of Pluto.…”

“…Even for a pure CO (dashed), and our observed 3-σ upper limit to the equivalent width is 0.028 cm −1 . Previous upper limits for Triton are also plotted: B89 (Broadfoot et al 1989) and S95 (Stern et al 1995). In both panels, shaded regions denote predicted mole fractions for different theories of surface-atmosphere interaction.…”

“…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).…”

“…For Triton, reported observed upper limits on the CO atmospheric mole fraction include 1% from Voyager 2 UVS observations (Broadfoot et al 1989) and 1.5% from Hubble Space Telescope UV spectroscopy (Stern et al 1995). For Pluto, there exists only a non-constraining upper limit from the nondetection of the pure rotational transitions of CO from radio observations (Barnes 1993).…”