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F. Forget, N. Decamp (Lab. de Meteorologie Dynamique, Paris, France), J. Berthier, C. Le Guyader (Inst. de Mecanique celeste, Paris, France)
The seasonal evolution of Triton's surface and atmosphere remains poorly understood. No model  has been able to fully reproduce the main characterictics of the Voyager 2 observations in 1989 in combination with the "Global warming" recently inferred from stellar occultations . Within this context, we have developped a new thermal model to study the seasonal nitrogen cycle on Triton. The model is the surface part of a Triton atmosphere General circulation model developped at LMD .
The nitrogen cycle was found to be very sensitive to Triton complex seasonal variations of the subsolar point latitude, especially during the current decade (south summer solstice). Since only pre-Voyager formulations were available for such a study, this has motivated some new calculations of Triton's motion based on more recent rotationnal elements combined with a relatively complete dynamic solution  adapted to Triton. A new analytic formulation suitable for climate modelling has been derived.
On this basis, we wish to suggest a new, realistic scenario to explain Triton's apparence and evolution based on solar-induced variation of the frost albedo. Such variations have been observed in Mars CO2 ice seasonal polar caps . Although they seem to result from complex microphysical behavior, they are likely to occur on Triton since both Triton and Mars polar caps are composed of weakly absorbing ice (N2 or CO2) in vapor pressure equilibrium with the main constituant of the atmosphere.
 e.g. Hansen and Paige, Icarus 99, 273-288 (1992); Brown and Kirk, J. Geophys. Res. 99, 1965-1981 (1994); Spencer and Moore, Icarus 99, 261-272 (1992).
 Elliot et al., Nature 393, 765-767 (1998).
 Forget, Descamp and Hourdin, in ``Pluto and Triton, comparisons and evolution over time", Lowell Observatory's fourth annual workshop, Flagstaff, Arizona. (1999)
 Le Guyader, Astron. Astrophys. 272, 687-694 (1993).
 Kieffer et al., J. Geophys. Res. 105, 9653-9700 (2000).
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