[meteorite-list] The Lower Atmosphere of Pluto Revealed

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 4 Mar 2009 09:33:11 -0800 (PST)
Message-ID: <200903041733.JAA22926_at_zagami.jpl.nasa.gov>


ESO 08/09 - Science Release

2 March 2009
For Immediate Release
The lower atmosphere of Pluto revealed

Using ESO's Very Large Telescope, astronomers have gained valuable new
insights about the atmosphere of the dwarf planet Pluto. The scientists
found unexpectedly large amounts of methane in the atmosphere, and also
discovered that the atmosphere is hotter than the surface by about 40
degrees, although it still only reaches a frigid minus 180 degrees
Celsius. These properties of Pluto's atmosphere may be due to the
presence of pure methane patches or of a methane-rich layer covering the
dwarf planet's surface.

ESO PR Photo 08a/09
Pluto (Artist's Impression)

"With lots of methane in the atmosphere, it becomes clear why Pluto's
atmosphere is so warm," says Emmanuel Lellouch, lead author of the paper
reporting the results.

Pluto, which is about a fifth the size of Earth, is composed primarily
of rock and ice. As it is about 40 times further from the Sun than the
Earth on average, it is a very cold world with a surface temperature of
about minus 220 degrees Celsius!

It has been known since the 1980s that Pluto also has a tenuous
atmosphere [1], which consists of a thin envelope of mostly
nitrogen, with traces of methane and probably carbon monoxide. As Pluto
moves away from the Sun, during its 248 year-long orbit, its atmosphere
gradually freezes and falls to the ground. In periods when it is closer
to the Sun ??? as it is now ??? the temperature of Pluto's solid surface
increases, causing the ice to sublimate into gas.

Until recently, only the upper parts of the atmosphere of Pluto could be
studied. By observing stellar occultations (ESO 21/02
a phenomenon that occurs when a Solar
System body blocks the light from a background star, astronomers were
able to demonstrate that Pluto's upper atmosphere was some 50 degrees
warmer than the surface, or minus 170 degrees Celsius. These
observations couldn't shed any light on the atmospheric temperature and
pressure near Pluto's surface. But unique, new observations made with
the CRyogenic InfraRed Echelle Spectrograph (CRIRES), attached to ESO's
Very Large Telescope, have now revealed that the atmosphere as a whole,
not just the upper atmosphere, has a mean temperature of minus 180
degrees Celsius, and so it is indeed "much hotter" than the surface.

In contrast to the Earth's atmosphere [2], most, if not all, of
Pluto's atmosphere is thus undergoing a temperature inversion: the
temperature is higher, the higher in the atmosphere you look. The change
is about 3 to 15 degrees per kilometre. On Earth, under normal
circumstances, the temperature decreases through the atmosphere by about
6 degrees per kilometre.

"It is fascinating to think that with CRIRES we are able to precisely
measure traces of a gas in an atmosphere 100 000 times more tenuous than
the Earth's, on an object five times smaller than our planet and located
at the edge of the Solar System," says co-author Hans-Ulrich Kaufl. "The
combination of CRIRES and the VLT is almost like having an advanced
atmospheric research satellite orbiting Pluto."

The reason why Pluto's surface is so cold is linked to the existence of
Pluto's atmosphere, and is due to the sublimation of the surface ice;
much like sweat cools the body as it evaporates from the surface of the
skin, this sublimation has a cooling effect on the surface of Pluto. In
this respect, Pluto shares some properties with comets, whose coma and
tails arise from sublimating ice as they approach the Sun.

The CRIRES observations also indicate that methane is the second most
common gas in Pluto's atmosphere, representing half a percent of the
molecules. "We were able to show that these quantities of methane play a
crucial role in the heating processes in the atmosphere and can explain
the elevated atmospheric temperature," says Lellouch.

Two different models can explain the properties of Pluto's atmosphere.
In the first, the astronomers assume that Pluto's surface is covered
with a thin layer of methane, which will inhibit the sublimation of the
nitrogen frost. The second scenario invokes the existence of pure
methane patches on the surface.

"Discriminating between the two will require further study of Pluto as
it moves away from the Sun," says Lellouch. "And of course, NASA's New
Horizons space probe will also provide us with more clues when it
reaches the dwarf planet in 2015."


[1] The atmospheric pressure on Pluto is only about one hundred
thousandth of that on Earth, or about 0.015 millibars.

[2] Usually, air near the surface of the Earth is warmer than the air
above it, largely because the atmosphere is heated from below as solar
radiation warms the Earth's surface, which, in turn, warms the layer of
the atmosphere directly above it. Under certain conditions, this
situation is inverted so that the air is colder near the surface of the
Earth. Meteorologists call this an inversion layer, and it can cause
smog build-up.

More information

E. Lellouch et al. 2009, A&A, in press, Pluto's lower atmosphere
structure and methane abundance from high-resolution spectroscopy and
stellar occultations <http://fr.arxiv.org/abs/0901.4882>.
The team is composed of E. Lellouch, B. Sicardy, and C. de Bergh
(Observatoire de Paris, France), H.-U. K??ufl (ESO), S. Kassi and A.
Campargue (Universit?? Joseph Fourier, France).

Emmanuel Lellouch
Observatoire de Paris, France
E-mail: emmanuel.lellouch (at) obspm.fr
Phone: +33 1 450 77 672

Hans-Ulrich Kaufl
ESO, Garching, Germany
E-mail: hukaufl (at) eso.org
Phone: +49 89 3200 6414
Cell: +49 160 6365135

ESO La Silla - Paranal - ELT Press Officer: Dr. Henri Boffin - +49 89
3200 6222 - hboffin at eso.org
ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 -
vrodrigu at eso.org
Received on Wed 04 Mar 2009 12:33:11 PM PST

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