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, upper right, 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 at far right 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,
lower left. 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. (To see a
comparison from befor the ring was deposited, see PIA-01604 or
PIA-02309.) Tvashtar was the site of an active curtain of
high-temperature silicate lava imaged by Galileo in November 1999 and
February 2000 (image PIA-02584). 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 405 kilometers (221 to 252 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. 2, 2001, except for the
frame at the far right, which was acquired a day earlier. 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

More information about the Cassini and Galileo joint observations of
the Jupiter system is available online at .

Cassini 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 Galileo
and Cassini missions for NASA’s Office of Space Science, Washington,