The eruption took place in February 2001,
though image analysis was only recently completed by a team
of University of California, Berkeley, astronomers. The group
was co-led by postdoctoral research associate Franck Marchis
and Imke de Pater, professor of astronomy and of earth and
Their results are published in the November
issue of the planetary sciences journal Icarus.
"It is clear that this eruption is the most
energetic ever seen, both on Io and on Earth," Marchis said.
"With the end of the Galileo mission, ground-based telescopes
equipped with adaptive optics systems are the best tools for
monitoring volcanic activity of Io. It is clear that future
monitoring of Io's volcanism lies in the hand of terrestrial
Adaptive optics employs a technique to remove
the twinkle from stars, by flexing segmented mirrors fast
enough to stabilize and focus the bouncing image created by
turbulent air in the atmosphere.
Io, one of four large Jovian moons, is highly
volcanic with high-temperature eruptions similar to those
common on Earth, indicating a similar silicon-rich composition.
The 2001 Io eruption was very close to Surt, the site of a
large eruption in 1979 that took place between the Voyager
1 and Voyager 2 flybys.
"The Surt eruption appears to cover an area
of 1,900 square kilometers, which is larger than the city
of Los Angeles and even larger than the entire city of London,"
Marchis said. "The total amount of energy being released by
the eruption is amazingly high, with the thermal output from
this one eruption almost matching the total amount of energy
emitted by all of the rest of Io, other volcanoes included."
The area covered by the Io lava is considerably
larger than the entire cone of one of Earth's most active
volcanoes, Etna in Italy, and far larger than Etna's most
recent eruption in 1992.
"This eruption is truly massive," said Ashley
Davies, PhD, a scientist at NASA's Jet Propulsion Laboratory
who aided in modeling the eruption. "The observed energy indicates
the presence of a vigorous, high-temperature volcanic eruption.
The kind of eruption to produce this thermal signature has
incandescent fire fountains of molten lava which are kilometers
high, propelled at great speed out of the ground by expanding
gases, accompanied by extensive lava flows on the surface."
Io's volcanism has been monitored for the
last eight years by the Galileo spacecraft and now, with the
advent of adaptive optics systems, by Earth-bound astronomers.
Ground-based observations with an adaptive optics system,
which produces very high-resolution images, provide a competitive
alternative to the limited temporal and spatial coverage of
Io by space missions, Marchis said. The spatial resolution
is 105 kilometers (66 miles) per pixel, comparable to many
infrared observations obtained by the Galileo Near Infrared
Mapping Spectrometer from orbit around Jupiter.
"We were lucky to detect the beginning of
an outburst eruption," de Pater said. "Thanks to the high-resolution
capabilities of the adaptive optics system, it was possible
to pin-point the location of the eruption, and the wavelength
coverage allowed us to apply constraints to the nature of
With a specialized infrared camera, the Keck
telescope captured images of Io on two days, Feb. 20 and 22,
at three different wavelengths. On the first day, Io was mostly
quiet, with visible surface features such as dark calderas
and relatively bright areas rich in sulfur dioxide frost.
Two days later, however, what seemed a small hot spot on the
surface had become a large bright eruption.
"We observed the same side of the satellite
and were amazed to see a very bright eruption that had suddenly
appeared," Marchis said. The UC Berkeley team quickly obtained
data before Io entered the shadow of Jupiter.
The data were analyzed using advanced image
processing techniques and a package called MISTRAL, developed
by France's Office National d'Etudes et de Recherche Aerospatiales
(ONERA) MISTRAL yields clear images of a quality comparable
to observations taken above the Earth's atmosphere. The data
showed that the temperature of the erupting lava was about
1,500 Kelvin, similar to that commonly seen on Earth at locations
such as the Hawaiian volcanoes.
The investigative team consists of Marchis
and de Pater of UC Berkeley, Davies of the Jet Propulsion
Laboratory, UC Berkeley graduate student Henry G. Roe, Thierry
Fusco of ONERA, David Le Mignant of the W. M. Keck Observatory,
Pascal Descamps of the Institut de Mécanique Céleste, Bruce
A. Macintosh of Lawrence Livermore National Laboratory and
Renée Prangé of the Institut d'Astrophysique Spatiale.
This observational study of Io was supported
by the France-Berkeley Fund, the National Science Foundation
and the Technology Center for Adaptive Optics managed by UC
The W. M. Keck Observatory, located at the
summit of Mauna Kea, provides astronomers access to two 10-meter
optical telescopes, the world's largest. Each telescope features
a revolutionary primary mirror composed of 36 hexagonal segments
that work in concert as a single piece of reflective glass
to provide unprecedented power and precision. Both Keck telescopes
are equipped with adaptive optics.
Funding for the telescopes and the Keck II
adaptive optics system was provided by the W.M. Keck Foundation.
The observatory is operated by the California Association
for Research in Astronomy, a partnership of the California
Institute of Technology, the University of California and
the National Aeronautics and Space Administration (NASA).