CH4 AND HAZE IN TRITONS LOWER ATMOSPHERE

Voyager 2 ultraviolet spectrometer occultation observations at Triton have revealed two constituents of the troposphere: CH4 and another absorber visible between 1400 and 1600 angstrom below about 20 km altitude. The CH4 appears to be saturated at the surface at both entrance and exit occultation sites. The density scale height and wavelength dependence of the long-wavelength absorber are consistent with Rayleigh scattering in N2. However, the inferred N2 column abundance is inconsistent with the Voyager radio science (RSS) occultation measurement, and so the N2 hypothesis is discarded. The most attractive alternative hypothesis for the identity of the long-wavelength opacity is Rayleigh scattering by an aerosol haze, because the optical depth is noticeably wavelength dependent, both within the EUV range of measurement and over the range between the EUV and visual wavelengths. EUV Rayleigh scattering requires that the haze particles be very small (less-than-or-equal-to 0.03-mu-m) and abundant. These characteristics suggest formation as photochemical smog or condensing N2 with a great abundance of nucleation centers. Condensation onto ions allows rapid formation of very small particles, so possibly ions created by penetrating magnetospheric charged particles (0.1 to several MeV) are nucleating the haze. Small particle size also promotes long residence times against settling, essential for accumulation to significant abundance and suggesting that eddy transport dominates settling. If this is the case, the haze scale height likely approximates the gas scale height, and the implied temperature is around 38 K. This is lower than the 50 K estimated by the RSS and is also too low for the dust devil hypothesis for the visible atmospheric plumes.