Drastic expansion of Pluto's atmosphere as revealed by stellar occultations

Moving on its eccentric orbit, Pluto is presently receding from the Sun; between 1979 and 1999 it was inside Neptune's orbit, but since then it has again been the planet most distant from the Sun. As it moves outward, the amount of solar energy that reaches its surface decreases, so its surface is expected to cool.

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.

The Arica site in Chile, where the 20 July 2002 stellar occultation by Pluto was observed, see below. Photo by Cyril Birnbaum
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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.

Figure 1: track of Pluto's shadow on 20 July 2002
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Figure 2: track of Pluto's shadow on 21 August 2002
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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.

Figure 3: Panel a: the light curve obtained near Arica during the occultation of P126 by Pluto (20 July 2002). The continuous curve superimposed to the data is a smoothed version of the light curve obtained in 1988, showing a discontinuity in slope, or "knee", absent in the 2002 data. Panel b: the light curve of 21 August 2002 observed at CFHT during the occultation of P131.1. The continuous curve represents again the 1988 data. Panel c: a vertically stretched version of Panel b, showing the "spikes", labelled S. the spikes are caused by small temperature fluctuations in Pluto's atmosphere. Note also the small but steady decrease of signal at the bottom of the light curve, probably betraying temperature contrasts between the polar and equatorial regions of Pluto.
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Figure 4: Panel a: the temperature profiles of Pluto's atmosphere derived from the P131.1 CFHT light curve. The horizontal axis represents the temperature in Kelvin, and the vertical axis represents the radius in km, that is the distance to Pluto's center. Note the sudden drop of temperature (inversion layer) at the bottom of the profile. This inversion is caused by the cold surface of Pluto which forces the atmosphere to cool down as it gets closer and closer to the surface. The radius of the latter is not known presently, but should be close to 1160 km. Panel b: the temperature gradients profiles, showing the small temperature fluctuations responsible for the spikes shown in Fig. 3. Panel c: Pluto's atmospheric pressure profiles in 2002 (right) and 1988 (left). The horizontal axis is a logarithmic pressure scale, showing that the pressure increased at all levels by a factor of more than two between 1988 and 2002.
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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.

The movie

Now, have a look to the P131.1 occultation movie below! The movie simulates what would have be observed with a very big telescope (of the class 50-m!) during the P131.1 occultation at Hawaii. As Pluto gets closer to the star, the latter gradually dims, due to the differential refraction of stellar rays. Simultaneously, the stellar image is deviated along the planet limb. Note the strong fluctuations of signal (the spikes) at the beginning and at the end of the event, caused by small temperature fluctuations in Pluto's atmosphere.

click inside to download

gzipped avi movie (340 ko).

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. Díaz, H. Calderòn, S. Carrillo, A. Carvajal, E. Recalde, L. Gaviria Cavero, C. Montalvo, D. Barría, R. Campos, R. Duffard & H. Levato : 2003
Drastic expansion of Pluto's atmosphere as revealed by stellar occultations, Nature, 10 July 2003

Bruno Sicardy (Observatoire de Paris, LESIA)
Thomas Widemann (Observatoire de Paris, LESIA)