October 4, 1999: Thirty years
ago, before the Voyager probes visited Jupiter, if you had described
Io to a literary critic it would have been declared overwrought
science fiction. Jupiter's strange moon is literally bursting
with volcanoes. Dozens of active vents pepper the landscape which
also includes gigantic frosty plains, towering mountains and
volcanic rings the size of California. The volcanoes themselves
are the hottest spots in the solar system with temperatures exceeding
1800 K (1527 C). The plumes which rise 300 km into space are
so large they can be seen from Earth by the Hubble Space Telescope.
Confounding common sense, these high-rising ejecta seem to be
made up of, not blisteringly hot lava, but frozen sulfur dioxide.
And to top it all off, Io bears a striking resemblance to a pepperoni
pizza. Simply unbelievable.
Right: Digital Radiance simulation
of Pillan Patera just before the Galileo flyby. click
for animation.
Since the first volcanic plume was discovered by Voyager
in 1979, Io has remained under intense scrutiny. Astronomers
using ground-based telescopes can monitor large volcanic eruptions
from Earth by recording outbursts of infrared emission. Such
measurements combined with Voyager and Galileo data show that
some volcanoes on Io have been active for at least 20 years.
For a world dominated by fiery volcanoes, it's curious that Io
is also very, very cold. The ground just around the volcanic
vents is literally sizzling, but most of Io's surface is 150
degrees or more below 0 C. The moon's negligible atmosphere traps
little of the meager heat from the distant Sun. As soon as volcanic
gases spew into the air they immediately begin to freeze and
condense. The plumes of Io's sizzling volcanoes are very likely
made up of sulfur dioxide snow.
Above: This false color infrared
composite of Jupiter's moon Io was produced from images acquired
in July and September, 1996 by NASA's Galileo spacecraft. The
area shown is 11,420 kilometers in width. Deposits of sulfur
dioxide frost appear in white and gray hues while yellowish and
brownish hues are probably due to other sulfurous materials.
Sulfur dioxide is normally a gas at room temperatures, but it
exists on Io's surface as a frost after condensing there from
the hot gases emanating from the Io volcanoes. Bright red materials
(such as the prominent ring surrounding the currently erupting
plume Pele) and spots with low brightness or albedo ("black"
spots) mark areas of recent volcanic activity and are usually
associated with high temperatures and surface changes.
"Io has lots of thermal areas just like Yellowstone,"
says JPL's Bill Smythe. "The volcanic plumes get most of
the attention but there are probably also things like fumeroles
and geysers. On a previous flyby the Particles and Fields instruments
saw a deficit of energetic particles over Io where gas was probably
coming out of the surface -- but no plumes were seen. We call
this the 'stealth plume hypothesis.' The closest Earthly analog
to what's happening would be a water geyser like Old Faithful.
In fact, if you put Old
Faithful on Io it would be about 37 km high!"
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? Scientists hope that these
and many other questions will finally have answers after Galileo
makes two daring passes less than 620 km above Io on October
11 and November 25, 1999. In November Galileo might even pass
through the plume of Pillan Patera, making it the first spacecraft
ever to fly through an alien volcano.
"For the October flyby we'll be targeting four major volcanoes,"
says JPL's Duane Bindschadler, the manager of Galileo's Science
Planning and Operations Team, "Pillan Patera, Prometheus
(the most prominent one on the surface), Loki, and Pele. The
pictures will be great. To put this in perspective, the very
best images from Voyager had 500 m resolution. We'll be getting
over 100 images at least that good, and the best images will
show details only 7 meters across."
"But we won't just be running the camera," he continued.
"Just about every instrument on the spacecraft will be turned
on during the flyby. The most important ones for volcano science
are probably the solid state imaging camera (SSI),
the near infrared spectrometer (NIMS)
and the photopolarimeter-radiometer (PPR)."
Each of these instruments does something different.
NIMS spectra can be used to deduce the composition of plumes,
flows and other surface features. The PPR measures the polarization
and intensity of sunlight and thermal radiation. This helps scientists
understand what atmospheric and volcanic gases are made of and
how things are heated. The SSI camera takes high resolution pictures
in optical light. Each of the three can also be used like a thermometer
to measure the temperature of features on Io. The NIMS and PPR
instruments are better at reading the temperature of cool material
like plumes and ground frost. Around hots spots warmer than 700
K, where the NIMS and PPR detectors saturate, the SSI camera
can be used to estimate temperature. By using the SSI, NIMS and
PPR together scientists hope to get a more complete picture of
Io's volcanic activity.
Above: The bright spots in this
image indicate the locations of volcanic vents on Io, which are
spewing hot lava. This image and other data from NASA's Galileo
spacecraft indicate that the lava at Pillan Patera (marked Pillan)
exceeded 1,700 degrees kelvin (2,600 degrees Fahrenheit) and
may have reached 2,000 degrees kelvin (3,140 degrees Fahrenheit).
The hottest eruptions on Earth today reach temperatures of about
1,500 kelvin (2,240 degrees Fahrenheit), but hotter lava erupted
billions of years ago. [more
information]
"The biggest mystery about Io's volcanoes is why they're
so hot," says Bill Smythe, a co-investigator on JPL's NIMS
team. "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 mystery."
"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 and we'll have the
answer."
Understanding the balance between sulfur and silicate (basaltic)
volcanism is important for scientists who are trying to understand
how Io's interior is heated. Sulfur has a lower melting point
so it doesn't need as much energy to make lava. The basaltic
flows require much more internal heat.
Right: This Voyager image of Ra Patera, a
large shield volcano on Io, shows colorful flows up to 200 miles
long emanating from the dark central volcanic vent. Copyright
Calvin J. Hamilton More
information.
"Another thing we'll be going for with these close-up flybys
are high resolution pictures of the lava flows," continued
Smythe. "We really want to know what the shapes and edges
of the flows look like because that can tell us a lot about the
properties of the lava. On Earth lava flows form little side
lobes, or extrusions that look like arms, feet and toes. They
range in size from a few centimeters to meters. From experiments
on Earth, we know how to estimate the viscosity of the lava and
other material properties from the shapes and sizes of the toes.
That's what we want to do on Io, but the best resolution we have
now is 1 km. At closest approach we'll have resolutions of only
7 meters. When we start seeing how the toes form we'll know what
kind of flows these are."
Some of the most exciting results from the upcoming flybys will
result from great improvements in resolution. For example, the
best resolution of previous NIMS data is only 60 km.
"You can hide a lot in 60 km," points out Smythe. "During
the closest flybys NIMS will see things just a few hundred meters
across. That'll be a first."
"Another thing we're hoping to get in October is a plume
seen in profile as the spacecraft passes by Io and looks back
over the limb," continues Smythe. "By looking at the
polarization of sunlight passing through the plume with the PPR,
we ought to get some really valuable information about the temperature
and density of particles coming out of the vents."
Digital Io
Here on Earth scientists will be eagerly awaiting the new
data. For instance, at the University of Texas, Dr. David Goldstein
and graduate students J. Victor Austin and Ju Zhang, following
the pioneering work of Sue Keefer, have been working for years
on computer simulations of Io's volcanic plumes. Using a technique
called Monte Carlo direct simulation, they send computerized
test particles blasting out of a model volcanic vent. The University
of Texas program tracks the motion of ejected molecules taking
into account intermolecular collisions, energy input from the
Io torus, and energy lost to infrared radiation. By varying the
size of the vent, the temperature and velocity of the ejected
gas, and the temperature of the surrounding terrain they can
match the appearance of their computerized plumes with the ones
photographed by NASA space probes. Sometimes this leads to new
insights about the fluid dynamics and physics of Io's volcanoes.
"The upcoming flybys could substantially improve our models
by providing better boundary conditions," says Goldstein.
"We need to know lots of things. What are the particle velocities
and temperatures coming out of the volcanic vents? How do the
molecules interact with the surface? Do they stick immediately
or do they bounce? etc..."
"Right now when we look at a photograph of a plume, we really
are not certain what we're seeing. It might be gas, it might
be dust entrained in the gas, or gas that has condensed out to
form ice crystals. We assume that whatever it is traces volcanic
gas but we can't be sure. Hopefully the flybys will resolve some
of these issues."
In one of the University of Texas sample models,
pictured right, gas erupts from a vent at 2.7 times the velocity
of sound into Io's tenuous atmosphere. The ejecta soar to a height
of about 120 km. Much of the material lands about 150 km away
where it hits the ground and bounces. Just before it hits the
surface, the gas passes through a shock wave and heats up to
200 - 300 K.
Right: This image is one frame from
a computer simulation of gas flowing in a volcanic plume. The
vent is at the intersection of the vertical and horizontal axes.
To view the flowing gas click on the image for a 0.4 MB MPEG
animation. Colors in this picture represent gas temperature.
Red is warm and blue is cooler. The gas in this model starts
out at 200+ K near the vent. It cools as it rises and expands,
then heats up again as it passes through the canopy shock. An
important feature can be seen about 150 km from the vent where
falling gas strikes the ground. The gaseous ejecta heats up and
possibly scours away the sulfurous surface frost, exposing dirt
and rock underneath. [more
information]
"One of the most important features of this model is the
canopy shock," points out Victor Austin. "It's where
the gas rises to its apex and then falls back on itself. One
way to think of the shock is to imagine a running water hose
held straight up in the air. The water decelerates due to gravity
and then comes back down. The same thing happens to a volcanic
plume. It rises into the atmosphere and then gravity pulls it
back. On Io the rising gaseous 'fluid' is supersonic so it forms
a shock near the turnaround point."
"This is where the Io flybys might confirm some of our results.
Gas cools very quickly after it passes through the shock, so
it's possible that a layer of SO2
snow will form in the postshock region. Vertically the shock
is very thin, so the layer of ice crystals is probably going
to be thin, too. That's something we might see in the high resolution
Galileo images."
"Another thing," said
Goldstein, "many of our simulations show that warm material
from the plume crashes down about 150 km from the vent. We think
this might explain the dark rings we see in some of the spacecraft
images (see left). These could be 'scouring rings' -- places
where the SO2 frost is worn down to dirt and rock.
Victor's calculations show that these rings themselves are wide
but the edges are sharp. If we can see those edges in the close-up
photos, it would help confirm our results. Some of the models
also show a secondary scouring ring much further from the vent.
We'll be looking for those in the new images, too. Scouring is
one possibility, but the dark coloration may also simply be due
to deposition of a different colored material coming out of the
vent."
Above: Volcanoes on Jupiter's moon
Io are compared in these images from NASA's Galileo spacecraft
(right) taken in early September 1996, and from the Voyager spacecraft
(left) taken in 1979. Prometheus (bright ring in upper right)
was first seen as an erupting volcano by the Voyager spacecraft
and still features an active plume. A smaller active plume was
discovered at the volcano Culann Patera (dark feature at lower
left) by the Galileo spacecraft. [more
information]
Whatever the upcoming flybys reveal, the data are bound to improve
our understanding of Io's volcanoes. "The programs we're
running are unique," says Goldstein, "and we're looking
forward to running our code with the very latest data."
Galileo has been orbiting Jupiter and its moons since December
1995. Its primary mission ended in December 1997. The spacecraft
is currently near the end of a two-year extended mission that
will culminate in two daring flybys of volcanoes on Io later
this year. More information about the Galileo mission is available
at: http://www.jpl.nasa.gov/galileo/
JPL manages Galileo for NASA' s Office of Space Science, Washington,
D.C. JPL is a division of the California Institute of Technology,
Pasadena, CA. |