By Lori Stiles - University
of Arizona News
Two NASA spacecraft
jointly observing Jupiter's moon Io this winter captured images
of a towering volcanic plume never seen before and a bright red
ring of fresh surface deposits surrounding its source.
Combined information
from images taken by the Cassini and Galileo spacecraft indicates
the new plume is about the same size -- nearly 400 kilometers
or 250 miles high -- as a long-lived plume from Io's Pele volcano.
Pele's plume and ring are also seen in the new images.
The images
and further information about them are available online from the
web sites of the Cassini Imaging Science team at the University
of Arizona, Tucson, at http://ciclops.lpl.arizona.edu,
from the University of Arizona's Planetary Image Research Laboratory
at http://pirlwww.lpl.arizona.edu/Galileo/Releases,
and from NASA's Jet Propulsion Laboratory, Pasadena, Calif., at
http://www.jpl.nasa.gov/pictures/jovianmoons.
The new plume
originates from a volcanic feature named Tvashtar Catena near
Io's north pole. Scientists were astounded to discover so large
a plume so near the pole, because all active plumes previously
detected on Io have been over equatorial regions and no others
have approached Pele's in size, said University of Arizona planetary
scientist Dr. Alfred McEwen.
Galileo might
pass right through the Tvashtar plume in August, if the plume
persists until then. The spacecraft will be flying over that part
of Io at an altitude of 360 kilometers (224 miles). Material in
the plume is tenuous enough to present little risk to the spacecraft,
and passing through it could give an opportunity to analyze the
makeup of the plume, said Dr. Torrence Johnson, Galileo project
scientist at the Jet Propulsion Laboratory in Pasadena, Calif.
Cassini is
a cooperative project of NASA, the European Space Agency and the
Italian Space Agency. JPL manages Cassini and Galileo for NASA's
Office of Space Science, Washington, D.C. JPL is a division of
the California Institute of Technology in Pasadena.
Links to Images:
http://pirlwww.lpl.arizona.edu/Galileo/Releases/
http://ciclops.lpl.arizona.edu/
http://www.jpl.nasa.gov/pictures/jovianmoons
Two tall volcanic
plumes and the rings of red material they have deposited onto
surrounding surface areas appear in images taken of Jupiter's
moon Io by NASA's Galileo and Cassini spacecraft in late December
2000 and early January 2001.
A plume near
Io's equator comes from the volcano Pele. It has been active for
at least four years, and has been far larger than any other plume
seen on Io, until now. The other, nearer to Io's north pole, is
a Pele-sized plume that had never been seen before, a fresh eruption
from the Tvashtar Catena volcanic area.
The observations
were made during joint studies of the Jupiter system while Cassini
was passing Jupiter on its way to Saturn. The two craft offered
complementary advantages for observing Io, the most volcanically
active body in the solar system. Galileo passed closer to Io for
higher-resolution images, and Cassini acquired images at ultraviolet
wavelengths, better for detecting active volcanic plumes.
The Cassini
ultraviolet images reveal two gigantic, actively erupting plumes
of gas and dust. Near the equator, just the top of Pele's plume
is visible where it projects into sunlight. None of it would be
illuminated if it were less than 240 kilometers (150 miles) high.These
images indicate a total height for Pele of 390 kilometers (242
miles). The Cassini image shows a bright spot over Pele's vent.
Although the Pele hot spot has a high temperature, silicate lava
cannot be hot enough to explain a bright spot in the ultraviolet,
so the origin of this bright spot is a mystery, but it may indicate
that Pele was unusually active.
Also visible
is a plume near Io's north pole. Although 15 active plumes over
Io's equatorial regions have been detected in hundreds of images
from NASA's Voyager and Galileo spacecraft, this is the first
image ever acquired of an active plume over a polar region of
Io.The plume projects about 150 kilometers (about 90 miles) over
the limb, the edge of the globe. If it were erupting from a point
on the limb, it would be only slightly larger than a typical Ionian
plume, but the image does not reveal whether the source is actually
at the limb or beyond it, out of view.
A distinctive
feature in Galileo images since 1997 has been a giant red ring
of Pele plume deposits about 1,400 kilometers (870 miles) in diameter.
The Pele ring is seen again in one of the new Galileo images.
When the new Galileo images were returned this month, scientists
were astonished to see a second giant red ring on Io, centered
around Tvashtar Catena at 63 degrees north latitude.
Tvashtar was
the site of an active curtain of high-temperature silicate lava
imaged by Galileo in November 1999 and February 2000. The new
ring shows that Tvashtar must be the vent for the north polar
plume imaged by Cassini from the other side of Io, This means
the plume is actually about 385 kilometers (239 miles) high, just
like Pele. The uncertainty in estimating the height is about 30
kilometers (19 miles), so the plume could be anywhere from 355
to 415 kilometers (221 to 259 miles) high.
If this new
plume deposit is just one millimeter (four one-hundredths of an
inch) thick, then the eruption produced more ash than the 1980
eruption of Mount St. Helens in Washington.
NASA recently
approved a third extension of the Galileo mission, including a
pass over Io's north pole in August 2001. The spacecraft's trajectory
will pass directly over Tvashtar at an altitude of 360 kilometers
(224 miles).
Will Galileo
fly through an active plume? That depends on whether this eruption
is long-lived, like Pele, or brief, and it also depends on how
high the plume is next August. Two Pele-sized plumes are inferred
to have erupted in 1979 during the four months between Voyager
1 and Voyager 2 flybys, as indicated by new Pele-sized rings in
Voyager 2 images. Those eruptions, both from high-latitude locations,
were shorter-lived than Pele, but their actual durations are unknown.
Before its August flyby, Galileo will get another more-distant
look at Tvashtar in May.
It has been
said that Io is the heartbeat of the jovian magnetosphere. The
two giant plumes evidenced in these images may have had significant
effects on the types, density and distribution of neutral and
charged particles in the Jupiter system during the joint observations
of the system by Galileo and Cassini from November 2000 to March
2001.
These Cassini
images were acquired on Jan. 1 and Jan. 2, 2001. The Galileo images
were acquired on Dec. 30 and 31, 2000.Cassini was about 10 million
kilometers (6 million miles) from Io, ten times farther than Galileo.
More information about the Cassini and Galileo joint observations
of theJupiter system is available online at http://www.jpl.nasa.gov/jupiterflyby.
From: Guy
Webster, Jet Propulsion Laboratory, 818-354-6278
UA Science
Contacts:
Alfred S.
McEwen, Galileo Imaging Team and Cassini Imaging Team
520-621-4573, mcewen@lpl.arizona.edu
Carolyn Porco,
Leader, Cassini Imaging Team
520-621-2390, carolyn@ciclops.lpl.arizona.edu
Laszlo Keszthelyi,
Galileo Imaging Team
520-621-8284, lpk@lpl.arizona.edu
Mitch Battros
Producer - Earth Changes TV
http://www.earthchangesTV.com
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