Seasonal Nitrogen Cycles on Triton and Pluto

Details Seasonal Nitrogen Cycles on Triton and Pluto Seasonal Nitrogen Cycles on Triton and Pluto

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........28h&link_type=abstract

PhD Dissertation, California Univ. Los Angeles, CA United States

Computer Science

1

Pluto (Planet), Triton, Nitrogen, Annual Variations, Cycles, Atmospheric Composition, Carbon Dioxide, Charon, Diurnal Variations, Flyby Missions, Frost, Ice, Mars Atmosphere, Neptune (Planet), Polar Caps

Scientific paper

Over the last decade a wealth of new data has been collected for Triton and Pluto. The Voyager flyby of Neptune in 1989 revealed Triton's atmospheric pressure and composition and allowed mapping of its surface. From 1985 to 1990 the series of mutual occultations and eclipses of Pluto by its moon Charon, allowed earth-based mapping of this distant planet, and a stellar occultation in 1988 revealed the presence of a tenuous atmosphere. Nitrogen has been identified as the dominant constituent of the surface ices on Triton and Pluto. Observations suggest that the atmospheres of Triton and Pluto are in vapor pressure equilibrium with the nitrogen ice on their surfaces, thus the seasonal nitrogen cycle will control their climates, analogous to the seasonal CO2 cycle on Mars. I have developed a thermal model, based on conservation of energy, which calculates diurnal and seasonal temperatures in the frost deposit and substrate, and predicts rates of condensation and sublimation of nitrogen frost as a function of time and latitude, for a given input set of frost properties, assuming vapor pressure equilibrium. Atmospheric pressure, polar cap boundaries, disk-integrated albedo, and north-south seasonal mass flux are predicted, and can be compared to observations. The results show that albedo boundaries on Triton are best reproduced by a relatively dark or transparent frost. I did not find any combination of parameters that would predict a bright seasonal nitrogen deposit which would remain unsublimed in the late spring. Thermal inertia of the substrate was shown to prevent Triton's atmosphere from condensing out seasonally. Aeolian features have been catalogued and compared to the output of the model as an additional set of constraints on volatile properties and transport. A bright nitrogen frost fits Pluto observables best, in contrast to Triton. Pluto's high obliquity was found to have a strong influence on the locations of its frost deposits, leading to configurations such as permanent latitudinal bands of frost, and cases in which seasonal caps form but sublime from the pole outward, creating a 'polar bald spot' . Pluto's atmospheric pressure exhibits two maxima per year, controlled by subsolar latitude, and under some conditions may condense out seasonally.

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