CH₄ and haze in Triton's lower atmosphere

Abstract

Voyager 2 ultraviolet spectrometer occultation observations at Triton have revealed two constituents of the troposphere: CH4 and another absorber visible between 1400 and 1600 Å below about 20 km altitude. The CH4appears 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 N2hypothesis 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 (≤ 0.03 μm) 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.

The Thermal Structure of Triton's Middle Atmosphere

“…2). Combining both CO rotational line cooling with CO mixing ratio of 0.002 and full CH 4 heating and cooling with the Voyager UVS solar occultation egress CH 4 profile (Herbert and Sandel 1991) divided by a factor of 2, we obtain a nearly isothermal atmosphere ∼51 K from 25 to 50 km, but above 50 km a temperature gradient that is too small by a factor of 2. The latter effect results from the downward ionospheric heat flux being radiated away above 120 km by CO rotational line cooling.…”

“…The immersion profile is the cooler one at higher altitudes, but below ∼80 km the profiles are virtually indistinguishable. The model profiles are as follows: (i) "conduction," (---) the most basic model, includes only ionospheric heating and chemical recombination heating above 145 km (Strobel and Summers 1995) and downward heat conduction to the surface; (ii) "add CO" (-, to left) adds to the "conduction" model CO rotational line cooling with a CO mixing ratio of 0.0002; (iii) "add CH4" (•••) adds to the "conduction" model recombination heating below 145 km, UV CH 4 photolysis heating, near-IR CH 4 heating, and thermal-IR cooling with a CH 4 profile based on Voyager UVS solar egress occultation (Herbert and Sandel 1991); (iv) "add CH4 and CO" (---) has CO mixing ratio of 0.002 and the "add CH4" CH 4 profile divided by a factor of 2; and (v) "no heat to surface" (-, to right) is the same as the "add CO" model, but with the constraint that no heat is conducted to the surface. Only the "add CO" model has the proper temperature profile above 50 km, but not below.…”

Photochemistry of Triton's atmosphere and ionosphere

“…The condensation rate coefficient is therefore rc "l = vfll4 where vr is thermal velocity of a molecule, S is the haze specific surface, i.e., a sum of surfaces of all particles per cubic centimeter. The properties of the haze as a function of height were determined by Krasnopolsky et al [1992] and Krasnopolsky [1993b] up to 30 km based on the Voyager 2 UVS and TV observations [Herbert and Sandel, 1991;Pollack et al, 1990]. From those data, "re-1 = 9x 10-Sexp(-h/H) s -l. Here H = 14 km is the atmospheric scale height.…”

“…The idea of explaining the measured CH4 profiles by these processes was suggested by Strobel et al [ 1990a], who applied it to preliminary data for CH 4 and obtained K = 6000 -I-2000 cmZ/s for a quantum yield of CH4 photolysis ¥ = 0.6. SS95 repeated this procedure with the improved methane profiles from Herbert and Sandel [ 1991] and gave K = Ko(noln) Ir2cm2/s, where K0 = 1600 and 1200 cm2/s at the egress and ingress occultation points, respectively, and n is the atmospheric gas number density. We modified their method to our model with y = 1.1.…”

“…Methane profiles from the UVS solar occultation measurements [Herbert and Sandel, 1991] are used to obtain the eddy diffusion coefficient K. There are two competing processes which determine the CH4 profile below 100 km on Triton: upward transport by eddy and molecular diffusion and photolysis by solar 1000-1400 A and Lyman-alpha background radiation. The idea of explaining the measured CH4 profiles by these processes was suggested by Strobel et al [ 1990a], who applied it to preliminary data for CH 4 and obtained K = 6000 -I-2000 cmZ/s for a quantum yield of CH4 photolysis ¥ = 0.6.…”