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Pluto's atmosphere was observed by an international
team of astronomers including French researchers from the Laboratoire
d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA
- CNRS, Observatoire de Paris, Universités de Paris VI and Paris
VII) and the Institut de Mécanique Céleste et de Calcul
des Ephémérides (IMCCE - CNRS, Observatoire de Paris). Two
occultation phenomena, that is, when Pluto passes between us and a star,
made it possible to study the atmosphere of the planet. These occultations
were observed in northern Chile with a 30-cm telescope on July 20, 2002,
and with the Canada-France-Hawaii 3.6-m telescope on August 21, 2002.
Contrary to expectations, the atmospheric pressure was found to be twice
as much as when it was measured in 1988 when Pluto was closer to the sun.
This could be the result of the sublimation(1) of the nitrogen ice located
on the south polar ice cap. This research is published in the journal
Nature dated July 10, 2003.
Pluto is the only planet in the solar system
still unvisited by spacecraft. We still know very little about this very
distant planet. Observations have shown that the surface is covered with
methane, carbon monoxide and nitrogen ice and that its atmosphere is very
thin. It is thought that this atmosphere is the result of the sublimation
of nitrogen gases and is therefore a function of the quantity of energy
coming from the sun and to which the planet is exposed. Pluto, whose very
eccentric orbit takes it as close to the sun as Neptune, is subject to
major variations in sunlight, causing its atmosphere to vary according
to its distance from the sun.
Pluto's atmosphere can be observed when the planet passes in front of
a star, creating the phenomenon known as occultation. Before the light
from the star disappears, hidden by the planet, its intensity gradually
decreases in the presence of an atmosphere. The light rays from the star
are increasingly refracted by the atmosphere depending on the altitude
– the higher the level of radiation, the greater the refraction of
the lower levels of the atmosphere.
It was precisely this phenomenon that was observed in 1988 for Pluto.
The occultation had revealed a nitrogen atmosphere with a pressure of
several microbars (10-6 bars, or a millionth of the Earth's atmospheric
pressure which is 1 bar at the surface). Moreover, the light curves from
1988 revealed a sudden variation of the brightness of the star that was
perhaps due to either the presence of a gaseous nitrogen "fog"
or the presence of an inversion layer in Pluto's atmosphere.
Pluto's occultation of a star was observed on July 20, 2002, in northern
Chile. A team of astronomers from the Observatoire de Paris, assisted
by amateur astronomers, used a 30-cm telescope fitted with a CCD camera
for this observation. One month later, on August 21, 2002, a second occultation
of a star by Pluto was observed with the 3.6-m Canada-France-Hawaii telescope.
The analysis of the results shows that the atmospheric pressure of Pluto
was twice as much as in 1988 when Pluto was much farther from the sun
and, therefore, colder. Moreover, small star scintillation phenomena in
Pluto's atmosphere were observed; these phenomena were less evident in
1988.
The fact that Pluto has a denser atmosphere when it is exposed to less
light from the sun could be explained by a seasonal phenomenon. After
its closest passage to the sun in 1989, the south pole, covered with a
nitrogen ice cap, was gradually exposed to sunlight following a 120-year
winter. The nitrogen would then have been sublimated and atmospheric gas
would have been transported by a circulation effect towards the north
pole. This pole, cloaked in darkness and, therefore cold, would have made
it possible for the nitrogen to condense once again, thus decreasing the
density of the atmosphere. But this movement and the resulting decrease
in atmosphere would have taken several years before it became effective,
beginning in about 2015.
Small variations in the light curves would probably be due to fluctuations
in temperature and the atmospheric pressure of Pluto that could be explained
by strong winds resulting from the difference between the dark hemisphere
and the light hemisphere of the planet or by convection movements very
close to Pluto's surface.
Therefore, even if the atmosphere of Pluto is very thin, it is subject
to major variations over the course of time - variations that are not
just related to the amount of sunlight that the planet receives but also
to factors directly related to Pluto's seasons.
For more information, visit the
Observatoire de Paris WEB site:
http://www.obspm.fr/actual/nouvelle/jul03/pluto.fr.html
(French version)
http://www.obspm.fr/actual/nouvelle/jul03/pluto.en.html
(English version)
1 - Direct passage from the solid
state to the gaseous state.
Researcher
contacts:
Bruno Sicardy, Observatoire de Paris.
Tel: +33 1 45 07 71 15
E-mail: sicardy@despace.obspm.fr
INSU/CNRS contact:
Philippe Chauvin
Tel: +33 1 44 96 43 36
E-mail: philippe.chauvin@cnrs-dir.fr
Press Contact
CNRS:
Martine Hasler
Tel: +33 1 44 96 46 35
E-mail: martine.hasler@cnrs-dir.fr
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