Caught in the Act
In November 1999, astronomers photographed
a large volcanic eruption on Io just as Galileo was flying by
Jupiter's fiery satellite.
Jan 17, 2000: On Thanksgiving Day in 1999, astronomers at
Mauna Kea, Hawaii were hard at work. NASA's Galileo spacecraft
was executing a dangerous
flyby of Jupiter's volcanic moon Io, and some of Mauna
Kea's most powerful telescopes were poised to observe Io
during the encounter. Volcanic activity on Io is so intense
that hot spots can sometimes be seen from Earth by the
infrared radiation they emit.
John Spencer (Lowell Observatory) and Glenn
Orton (JPL) were using NASA's Infrared Telescope Facility on
Thanksgiving when they captured an image of a towering lava
fountain arching 1.5 km above Io's surface. The eruption was
so large that it was visible nearly 400 million miles away!
Above: This false-color infrared image of
Jupiter's moon Io was taken at NASA's Infrared Telescope
Facility at Mauna Kea, Hawaii, a few hours after a November
25, 1999 close flyby of Io by the Galileo spacecraft. The
bright spot at the 1 o'clock position is the same lava
fountain seen close-up by Galileo's camera, but in this case
it is seen from Earth at a distance of 630 million kilometers
(390 million miles).
Just a few hours before Spencer and Orton
captured their dramatic picture, Galileo made a close pass
over Io. From a distance of 17,000 km the indefatigable
spacecraft captured closeup images of the lava fountain,
despite being blasted
by a heavy dose of radiation just before the encounter.
"Catching these fountains was a
one-in-500-chance observation," said Galileo scientist
Dr. Alfred McEwen from the University of Arizona in Tucson.
Astronomers making Earth-based telescopic observations see a
bright spot like this one somewhere on Io only about 20
percent of the time, so the Galileo team was fortunate to
catch one in its narrow field of view.
Below: This mosaic of images collected by
NASA's Galileo spacecraft on Thanksgiving Day, November
25,1999 shows a fountain of lava spewing above the surface of
Jupiter's moon Io. North is to the upper left of the picture
and the Sun illuminates the surface from the lower left. The
image covers an area approximately 300 by 75 kilometers
(190-by-47 miles). It was taken at a range of 17,000
kilometers (11,000 miles) by Galileo's on-board camera. [more
information]
The intense volcanism on Io results from 100 meter high tides
raised in its otherwise solid surface by nearby Jupiter and
the other Galilean satellites. Although this process is fairly
well understood, many aspects of Io's forbidding environment
remains a mystery. What makes the lava around the volcanic
vents so incredibly hot? What are the plumes made of? What
causes the many colors of Io's mottled surface? Many basic
questions are still unanswered.
The fact that the same eruption was recorded
by Galileo and by an Earth-based infrared telescope is a boon
to the research being done on Io. By combining the data,
scientists have their best chance ever to pin down
temperatures of Io's extremely hot lava.
"The biggest mystery about Io's
volcanoes is why they're so hot," said Bill Smythe, a
co-investigator on JPL's NIMS team in a 1999 interview.
"At 1800 K, the vents are about 1/3 the temperature of
the surface of the sun! Billions of years ago basaltic lava on
Earth was about that hot, but now -- thanks to mixing in
subduction zones -- terrestrial basalts have a lower melting
point. Lavas we see now on Earth are about 300 K cooler than
they used to be. It's very surprising to see lava flows on Io
as hot as these ancient flows on Earth. Why? Simply because
Io's soil has been reworked many, many times, so the melting
temperature should be lower for the same reason that Earth's
basalts melt at a lower temperature. It's a real puzzle."
Right: Digital Radiance simulation of the volcano
Pillan Patera just before an Galileo flyby. click
for animation.
"Originally we thought all the lava
flows were sulfurous, but sulfur vaporizes at ~700K. The 1800
K regions have to be basaltic. Now the questions is 'are any
of the lava flows sulfurous?' Galileo has detected areas on Io
with temperatures between 300 and 600 K. That's about right
for molten sulfur. But those could also be places where tiny
volcanic vents at ~1800 K are surrounded by cold ground. From
a distance the average temperature would appear to be 300 -
600 K. We need higher resolution data to figure out what's
going on. If we're lucky Galileo will fly right over one of
these spots in October or November [1999]"
That's exactly what happened.
In the Galileo
image most of the hot material is distributed along a wavy
line, which scientists think is hot lava shooting more than
1.5 kilometers high out of a long crack on the surface.
Initial estimates of the lava temperature indicate that it is
well above 1,000 Kelvin (1,300 Fahrenheit) and might even be
hotter than 1,600 Kelvin (2,400 Fahrenheit).
The 'Water Cycle' on Io
 Visit
IoFlyBy.com for
coverage of Galileo's close encounters with Io,
including science news and the latest images of
Jupiter's volcanic moon. |
Another feature of great interest in the image
is the flat-topped mesa on the right side of the picture. The
scalloped margins, visible in a larger
image, are typical of a process geologists call
"sapping," which occurs when erosion takes place as
fluid escapes from the base of a cliff. On Earth, such sapping
features are caused by springs of ground water. Similar
features on Mars are one of the key pieces of evidence for
past water on the Martian surface. However, on Io, the liquid
is presumed to be pressurized sulfur dioxide. The liquid
sulfur dioxide changes to a gas almost instantaneously when it
bubbles up to the near-vacuum of Io's surface, blasting away
material at the base of the cliff. The sulfur dioxide gas
eventually freezes out on the surface of Io in the form of a
frost. As the frost is buried by later deposits, it can be
heated and pressurized until it becomes a liquid. This liquid
then flows out of the ground again, completing Io's version of
the 'water cycle.'
Additional information about the Galileo
mission is available on the Galileo home page at a new web
address of http://galileo.jpl.nasa.gov.
JPL manages the Galileo mission for
NASA's Office of Space Science, Washington, D.C. JPL is
operated for NASA by the California Institute of Technology,
Pasadena, CA.
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