Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994icar..108..255l&link_type=abstract
Icarus, vol. 108, no. 2, pt. 1, p. 255-264
Astronomy and Astrophysics
Astronomy
16
Astronomical Models, Atmospheric Composition, Atmospheric Models, Atmospheric Temperature, Light Curve, Occultation, Pluto Atmosphere, Thermal Analysis, Atmospheric Chemistry, Carbon Dioxide, Carbon Monoxide, Methane, Nitrogen, Triton
Scientific paper
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 N2 and CO ices, along with CH4, on Pluto's surface and of recent surface temperature measurements. Firm inferences from these observations are that N2 must be the major gas of Pluto's atmosphere and that the temperature at the microbar level is about 105 K. We estimate that CH4 and CO are present at the 0.1% level of N2 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 (1) if CO cooling is omitted, heating by CH4 can explain the temperature rise between the surface and the microbar level, but not the drop-off observed in several occultation lightcurves below 1215 km, unless the CH4 mixing ratio is larger than 10%; (2) if, as expected, CO is present at the approximately 0.1% level, and CH4 is lower than 10%, then cooling by CO rotational lines is so large that CH4 heating cannot explain a 105 K temperature at 1-2 microbar. possible way to explain the temperature rise between the surface and 1 microbar and the lightcurve drop-off is to invoke heating by a Titan-like haze layer with extinction optical depth approximately (0.4-4) x 10-3 for particle sizes in the range 0.01-2 microns. Such a haze can plausibly result from methane photolysis and subsequent chemistry, provided that the CH4 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) x 10-5), which is consistent with the fact that Triton's atmosphere near the microbar level is much colder than Pluto's. Estimating production and fallout time constants suggests that haze particles in Pluto's atmosphere must be small (less than or equal to 0.1 microns).
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