The Thermal Structure of Pluto's Atmosphere: Clear vs Hazy Models

Abstract

The problem of Pluto's atmosphere thermal structure, as determined from the June 9, 1988 occultation data, is reassessed in the light of the detection of N 2 and CO ices, along with CH 4, on Pluto's surface and of recent surface temperature measurements. Firm inferences from these observations are that N 2 must be the major gas of Pluto's atmosphere and that the temperature at the microbar level is about 105 K. We estimate that CH 4 and CO are present at the 0.1% level of N 2 in the atmosphere and we develop simple 1-D aeronomical models (solving the heat equation) for various atmospheric cases. It is found in particular that (i) if CO cooling is omitted, heating by CH 4 can explain the temperature rise between the surface and the μbar level, but not the drop-off observed in several occultation lightcurves below 1215 km, unless the CH 4 mixing ratio is larger than 10%; (ii) if, as expected, CO is present at the ≈0.1% level, and CH 4 is lower than 10%, then cooling by CO rotational lines is so large that CH 4 heating cannot explain a 105 K temperature at 1-2 μbar. A possible way to explain the temperature rise between the surface and 1 μbar and the lightcurve drop-off is to invoke heating by a Titan-like haze layer with extinction optical depth ≈(0.4-4) × 10 -3 for particle sizes in the range 0.01-2 μm. Such a haze can plausibly result from methane photolysis and subsequent chemistry, provided that the CH 4 mixing ratio is significantly larger than 10 -4. A similar haze is not expected to be present on Triton (where the methane mixing ratio is about (2-20) × 10 -5), which is consistent with the fact that Triton's atmosphere near the μbar level is much colder than Pluto's. Estimating production and fallout time constants suggests that haze particles in Pluto's atmosphere must be small (≤0.1 μm).