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.

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

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.

* avi movie (1MB)
* gif movie (800KB)
* mpeg movie (2MB)


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


[Image: (67KB)]
The Arica site in Chile, where the 20 July 2002 stellar occultation by Pluto was
observed. Photo by Cyril Birnbaum.

[Figure 1: (211KB)]
Track of Pluto’s shadow on 20 July 2002.

[Figure 2: (114KB)]
Track of Pluto’s shadow on 21 August 2002.

[Figure 3: (59KB)]
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.

[Figure 4: (71KB)]
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.