Saturn's Bull's-Eye Marks Its Hot Spot
NASA astronomers using the Keck I telescope in Hawaii are
learning much more about a strange, thermal "hot spot" on the tip
of Saturn's south pole.
Image above: This is the sharpest image of Saturn's temperature emissions taken from the ground; it is a mosaic of 35 individual exposures made at the W.M. Keck I Observatory, Mauna Kea, Hawaii on Feb. 4, 2004. The images to create this mosaic were taken with infrared radiation. Image credit: NASA/JPL.
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In the most precise reading of Saturn's temperatures ever taken
from Earth, a new set of infrared images suggests a warm "polar
vortex" at Saturn's south pole - the first warm polar cap ever to
be discovered in the solar system. The vortex is punctuated by a
compact spot that is the warmest place on the planet. The
researchers report their findings in the Feb. 4 issue of the
The images can be viewed at:
A polar vortex is a persistent, large-scale weather pattern,
likened to a jet stream on Earth in the upper atmosphere. On
Earth, the Arctic Polar Vortex is typically located over eastern
Canada and plunges arctic air to the northern plains in the
United States. Earth's cold Antarctic Polar Vortex, centered
over Antarctica, traps air and creates unusual chemistry, such as
the effects that create the "ozone hole".
Polar vortices on Earth, Jupiter, Mars and Venus are colder than
their surroundings. But new images from the W. M. Keck
Observatory show the first evidence of such a polar vortex at
much warmer temperatures than their surroundings. And the even
warmer, compact region at the pole itself is quite unusual.
"There is nothing like this compact warm 'cap' in the Earth's
atmosphere," said Dr. Glenn S. Orton, senior research scientist
at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and lead
author of the paper. "Meteorologists have detected sudden warming
of the pole, but on Earth this effect is very short-term. This
phenomenon on Saturn is longer-lived because we've been seeing
hints of it in our data for at least two years."
Data for these observations were taken in the imaging mode of the
Keck facility instrument, the Long Wavelength Spectrometer, on
Feb. 4, 2004, by Orton and Dr. Padma Yanamandra-Fisher, the
paper's co-author, also a research scientist at JPL.
The puzzle isn't that Saturn's south pole is warm; after all, it
has been exposed to 15 years of continuous sunlight, having just
reached its summer Solstice late in 2002. But both the distinct
boundary of a warm polar vortex some 30 degrees latitude from the
southern pole and a very hot "tip" right at the pole were
completely unexpected. If the increased southern temperatures are
the result of the seasonal variations of sunlight, then
temperatures should increase gradually with increasing latitude.
But they don't – the tropospheric temperature increases toward
the pole abruptly near 70 degrees latitude from 88 to 89 Kelvin (-
301 to -299 degrees Fahrenheit) and then to 91 Kelvin (-296
degrees Fahrenheit) right at the pole. Near 70 degrees latitude,
the stratospheric temperature increases even more abruptly from
146 to 150 Kelvin (-197 to -189 degrees Fahrenheit) and then
again to 151 Kelvin (-188 degrees Fahrenheit) right at the pole.
The abrupt temperature changes may be caused by a concentration
of sunlight-absorbing particulates trapping heat in Saturn's
upper atmosphere. This theory would explain why the hot spot
appears dark in visible light and contains the highest measured
temperatures on Saturn. However, this alone would not explain
why the particles themselves are constrained to a compact area at
Saturn's south pole. One possible explanation would be
downwelling of dry air, which is also consistent with deeper
clouds observed at the southern pole. Researchers plan more
observations to check that possibility.
More detail about the temperatures and possible chemical changes
in these regions may be available from an infrared spectrometer
on the Cassini spacecraft, in orbit around Saturn. The discovery
of the hot spot at Saturn's south pole has prompted Cassini's
composite infrared spectrometer science team, including Orton, to
redirect some future observations to this area.
"One of the obvious questions is whether Saturn's north pole is
abnormally cold and whether a cold polar vortex has been
established there. That's something we can't see from Earth, and
Cassini's instruments will be in a unique position to observe
it," said Orton.
Funding for this research was provided by NASA's Office of Space
Applications, Planetary Astronomy Discipline, and the NASA
Cassini project. The Cassini-Huygens mission is a cooperative
project of NASA, the European Space Agency and the Italian Space
Agency. The Jet Propulsion Laboratory, a division of the
California Institute of Technology in Pasadena, manages the
Cassini-Huygens mission for NASA's Science Mission Directorate,
The W.M. Keck Observatory is operated by the California
Association for Research in Astronomy, a non-profit scientific
partnership of the California Institute of Technology, the
University of California, and NASA. On the Web at
Carolina Martinez (818) 354-9382
NASA's Jet Propulsion Laboratory
Laura K. Kraft (808) 885-7887
W. M. Keck Observatory, Mauna Kea, Hawaii