Bruno Sicardy

Paris, France

Two stellar occultations observed on 20 July 2002 and 21 August 2002
from South America and
Hawaii, respectively, revealed a drastic increase of pressure in Pluto's
nitrogen atmosphere since
the previous  occultation observations in 1988.

A large campaign was organized to observe the July event with fixed and
portable telescopes
(ranging from 20-cm to 8.2-m in diameter) in Venezuela, Ecuador, Peru,
Chile, Brazil and Argentina.
The August event  was observed with the 3.6-m Canada-France-Hawaii
telesope (CFHT) on
Mauna Kea, Hawaii.

The analysis of the light curves obtained from Chile and Hawaii shows
that the thermal structure of
Pluto's did not change since 1988, with an isothermal upper atmosphere
with T~100 K, and a sharp
inversion layer cooling down to T~60 K in the last tens km above the
surface.

However, it appears that the pressure at all levels has been increasing
by a factor of two or more
since 1988, a probable effect of seasonal changes on Pluto. In effect,
the South Pole of the planet went
into sunlight in 1988, after more than 120 years in darkness, probably
causing an outgassing of the
nitrogen surface, thus explaiing  the increase of pressure.

Furthermore, spikes obsreved in the CFHT light curves betray a dynamical
activity in the atmosphere,
either dure to strong winds on Pluto, or a convection layer near the
surface.
 
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