Reality is not so simple though, as a team of the Observatoire de Paris with several collaborators showed that Pluto's atmosphere is expanding, rather than contracting, a quite surprising results published in the 10 July 2003 issue of the journal Nature.: "Large changes in Pluto's atmosphere as revealed by recent stellar occultations", by Sicardy et al.

Although solid methane and carbon monoxide on the planet surface have been revealed several years ago by spectroscopic measurements, the existence of a tenuous Pluto's atmosphere is much more difficult to ascertain.

Even using the Very Large Telescope of the European Southern Observatory with adaptive optics, the planet appears only a few pixels wide, and the atmosphere is too tenuous anyway to show in such images.

Also, Pluto is the only planet in the Solar System that has never been visited by a spacecraft (though a flyby mission is now in the construction phase, see the article by Alan Stern in the May 2002 issue of Scientific American), so that its detailed physical properties remain poorly known.

So far, the only way to study Pluto's atmosphere is to wait for the rare circumstance when the planet comes in front of a star. The latter is then used as a probe during a so-called "stellar occultation".

If there were no atmosphere, the star would merely vanish when reaching the edge of the planet. With an atmosphere, however, there will be a gradual dimming of the starlight, as the stellar rays are increasingly refracted when traversing thicker and thicker amounts of gas, see the movie at the end.

An occultation observed in 1988 revealed Pluto's tenuous nitrogen atmosphere, whose deepest layers reach pressures of no more than a few microbars; for comparison, the Earth atmosphere reaches one bar at the surface, so that Pluto's atmosphere represents a few millionths of our atmosphere. This scarcity is explained by the fact that the solid nitrogen ice on the surface, with a temperature of 40-60 K, is at thermodynamical equilibrium with the nitrogen vapor above it.

The 1988 occultation light curve showed a "knee" (see Fig. 3) that was interpreted as due to either a layer of haze, or to a sharp inversion layer 20-50 km above the surface. Numerous other attempts to observe occultations since then have failed.

The first Pluto occultation successfully observed since 1988 occurred on 20 July 2002, during a campaign in South America organized in part by a team of Observatoire de Paris. The star dubbed "P126" went behind the planet as observed from the area between the dotted lines in Fig.1.

Both large fixed telescopes and small portable instruments were used, with the participation of professional and amateur astronomers. A successful observation was made by the French team near Arica, in northern Chile (see the front picture), using a 30-cm portable telescope and a Audine digital camera.

One month later, on 21 August 2002, another occultation (of the star "P131.1") was successfully observed from large telescopes in Mauna Kea (Fig. 2), and in particular with the Canada-France-Hawaii 3.6-m Telescope (CFHT), yielding high quality data. As seen in the figure below, the "knee" observed in the 1988 data is absent from the 2002 data, revealing immediately that large changes in Pluto's atmosphere occurred during the intervening time.

Further analysis of the data reveals that the pressure in Pluto's atmosphere more than doubled between 1988 and 2002. One might naively expect an overall collapse of the atmosphere: the gases should freeze onto the surface as the planet moves farther from the Sun and cools.

Seasonal variations, however, might well overcome this tendency and explain the present expansion. Candice Hansen of the Jet Propulsion Laboratory and David Paige of UCLA proposed in 1996 (Icarus, volume 120, p 247) that there would be a period in Pluto's orbit -- shortly after perihelion -- when the south polar cap of the planet comes into sunlight after spending more than 120 years in darkness.

This happened in 1987, and the sublimation of the solid nitrogen accumulated in this region would then feed the atmosphere, while it would take some time for the north polar cap, in darkness since 1987, to re-condense this excess of gas. Hansen and Paige's model then predicts that this expansion will last till 2015 or so, before the atmosphere shrinks again.

More complications may arise as the albedo of the planet (the percentage of light reflected by the surface) probably varies during the whole process, thus changing the surface temperature, and by the same way, the amount of nitrogen sublimated into the atmosphere. Nevertheless, these kinds of models seem to adequately capture the physics of hemispheric gas exchange on Pluto.

Finally, "spikes" in the P131.1 light curve (see Fig. 3) reveal a dynamical activity in Pluto's atmosphere. The spikes are caused by small atmospheric temperature and density fluctuations, maintained either by strong winds between the lit and dark hemisphere of the planet, or by convection near the surface of Pluto.

Reference: B. Sicardy, T. Widemann, E. Lellouch, C. Veillet, J.-C. Cuillandre, F. Colas, F. Roques, W. Beisker, M. Kretlow, A.-M. Lagrange, E. Gendron, F. Lacombe, J. Lecacheux, C. Birnbaum, A. Fienga, C. Leyrat, A. Maury, E. Raynaud, S. Renner, M. Schultheis K. Brooks, A. Delsanti, O.R. Hainaut, R. Gilmozzi, C. Lidman, J. Spyromilio, M. Rapaport, P. Rosenzweig, O. Naranjo, L. Porras F. D�az, H. Calder�n, S. Carrillo, A. Carvajal, E. Recalde, L. Gaviria Cavero, C. Montalvo, D. Barr�a, R. Campos, R. Duffard & H. Levato : 2003 Drastic expansion of Pluto's atmosphere as revealed by stellar occultations, Nature, 10 July 2003

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New Horizons Pluto-Kuiper Belt mission -- Johns Hopkins University Applied Physics Laboratory
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