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.
One plume, from the volcano Pele, shoots upward nearly 400
kilometers (250 miles) from the surface near Io's equator. The
plume has been active for at least four years and, until now, had
been far larger than any other plume seen on Io. The images
also show a second plume about the same size, closer to Io's
north pole. This plume had never been seen before. It is
associated with a fresh eruption from the Tvashtar Catena
The observations were made during joint studies of the Jupiter
system while Cassini was passing Jupiter on its way to Saturn.
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 when the picture was taken.
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
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 before the ring
was deposited, see images 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 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.
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, 10
times farther than Galileo.
Image Credit: NASA