Alfred S. McEwen, 520-621-4573,
Elizabeth Turtle, 520-621-8284,
Paul Geissler, 520-621-621-2114,
Jani Radebaugh, 520-621-1632,
18 May 2000
Galileo’s sharp new pictures of Io detail amazing volcanism
By Lori Stiles
(EDITORS: For images, go to the Planetary Imaging Research Laboratory – Galileo web page: PIRL/Galileo,

When the Galileo spacecraft flew close by Jupiter’s moon, Io, late last year and early this year, it took sharpest-ever pictures of the best volcanic show in the solar system.
Galileo took more than 100 high-resolution images during Io flybys on Oct. 11, Nov. 26 and Feb. 22. Results include pictures of an active lava flow as long as the Grand Canyon, a mile-high curtain of burning lava, a unique long-lived "wandering" volcanic plume, and a 6-mile-diameter lava lake that shines steady as a beacon. The Galileo imaging team reports the results in the May 19 issue of Science.
"Io is absolutely fascinating because every single thing we see is completely new and completely unexpected," said Jani Radebaugh, a graduate student at the University of Arizona who helps analyze the photographs. "You can take what you know about volcanology and about planetary processes, but in every picture we see, there’s something new going on that we don’t understand. There’s something brand new, every time."
"Io is like a laboratory for large-scale volcanic experiments," said Alfred S. McEwen of the Galileo imaging team. McEwen directs the Planetary Imaging Research Laboratory (PIRL) at the UA Lunar and Planetary Lab (LPL). "We don’t have controlled laboratories big enough to study these processes. And large-scale volcanic fields on Earth are very modified. So here’s a place we can watch the changes and really understand how this activity might happen at this scale. With that insight, we can go back and reconsider what happened in Earth’s past."
"I like Io as a model of what might be going on under the crust of Europa," said Paul Geissler, senior research associate at LPL and member of the Galileo imaging team. "The whole (jovian moon) system is a great exercise in comparative planetology," Geissler said. "You have four separate moons, separated at birth, and they’ve grown up completely differently. So it’s very important to study them from that point of view."
The Galileo imaging scientists are trying to understand how Io’s extraordinary volcanism fits within the big geologic picture of the moon and the jovian system. They’d like to discover the composition of Io’s crust. They’d like to know if Io’s interior holds a magma ocean — a question images alone won’t answer. They use Galileo to explore how volcanoes erupt and modify the landscape, what the lavas are made of, and how Io’s strange terrain forms and evolves.
Results discussed in the Science article include —
* Pele — Pele is a very unusual volcano, even for Io, in that it consistently   shines brightly. The new images show it to be an active lava lake about 10   km (6 miles) across. Galileo imaging and spectral data show it must be at   least 1,030 degrees Celsius (1,890 Fahrenheit). Nighttime pictures of Pele   taken in October show a hot, glowing line at the margin of the Pele caldera.   The researchers interpret this line to be where molten lava pushes against   the caldera walls and is exposed, breaking through the edges of the crust   that covers the lava lake. (A caldera is a depression that forms from   collapse over a magma chamber.)
* Pillan — New high-resolution images of Pillan Patera, show a complex mix   of pits, domes, channels and possibly rafted plates near a 70-km lava flow   that erupted in June 1997. Such channels and rafted plates form when lava   erupts at flow rates violent enough to destroy stable crust — a kind of   volcanism also seen on Mars and Earth. But pits and domes, which range in   size from a few tens of meters to many hundreds of meters in diameter,   are harder to explain. Possibly, They are vents of plumes, analogous to   volcanic "rootless cones" on Earth.
  The new images also reveal color properties of the dark diffuse deposits   around Pillan that provide the first evidence that silicate particles are   spewed more than 100 kilometers high in Io’s plumes.
* Zamama — The new images show the primary vent at the westernmost end   of the 100-km-long dark flow (roughly the length of Arizona’s Grand Canyon)   with lava flows forming a radial pattern around the vent. The 75-km Zamama   plume comes not from the primary vent, but from the center of a lava field   extending to the east.
* Prometheus — Prometheus is Io’s most faithful, persistent plume. It has   been seen in every image of the area taken by Voyager, Galileo and Hubble   Space Telescope. Remarkably constant in size, shape and brightness   throughout its observed history, Prometheus had "wandered" between 75 to   95 km west in the 20 years between the 1979 Voyager flyby and the 1996   Galileo flyby. In 1996, Galileo also saw a new dark flow extending between   the Voyager-era and the Galileo-era plume sources.
  McEwen and others on the Galileo imaging team had suggested that   Prometheus comes not from the primary vent, but from the end of a lava   flow — an argument that some of their peers questioned. The new Galileo   evidence strengthens the UA researchers’ case.
  The latest Galileo images and spectra show two main hot spots. One is 15   km south of the caldera and marks where lava spews from a fissure to the   surface. The other hot spot, 80 km to the west, is where the 100-km-high   Prometheus plume rises above active lava beds.
  "The new Io images pretty strongly confirm the view that the plume comes   from the lava flow, not the volcano," McEwen said. "But that doesn’t mean we   understand it." Although Prometheus erupts at 10 times the rate of Kilauea,   Earth’s most active volcano, it in many ways resembles Kilauea, only on a   larger scale, McEwen said. However, he added, "Prometheus’ plume and its   behavior is totally alien from anything seen on Earth."
* Emakong Patera — Emakong is one of the largest calderas on Io without an   observed "hot spot." It features a brightly colored surface that may be sulfur   lava flows. The images show that at some point lava filled the entire 40-km   diameter caldera and overflowed. This has happened at a much smaller scale   at Kilauea in Hawaii.
  Emakong and many other calderas on Io are irregular in shape, rather than   circular, as are Earth’s calderas, Radebaugh said."That tells us there are   interesting things going on underneath the surface of the crust. There’s   probably fractures and some stresses that make those irregular shapes   when the surface collapses.
* Tvashtar Catena — A lava "curtain", or line of lava fountains, rises to 1.5   km above a linear fissure within one of the calderas. On Io, as on Earth, lava   erupting from a fissure can create a curtain of fire along the fissure. But   because Io’s atmospheric pressure is a billion times less than the Earth’s,   because Io’s gravity is lower and its lava is hotter, Io’s lava fountains can   reach 100 times higher than those on Earth.
* Mountains and related landforms — Researchers know of about 100   mountains on Io that do not appear to be volcanoes. Io’s mountains resemble   tilted blocks bounded by steep scarps, said LPL research associate Elizabeth   Turtle of the Galileo imaging team. The highest mountain known on Io is 16   km high, Turtle said. The new photos suggest an intriguing association   between mountains and calderas, which may help explain how both form,   she added.
  The sharp new Galileo photos show that several mountains have calderas   cut into their sides. The photos also suggest that mountains and calderas   are geologically related in othe
r ways. For example, Turtle said, rising   plumes of new material in Io’s mantle may concentrate stresses acting in   Io’s crust, causing it to fail, forming mountains by thrust faulting. As the   mountains are tilted up, layers of weak, sulfur-rich material deposited by   Io’s plumes may fail, resulting in tremendous landslides.
  From a mosaic of new pictures of the lava-filled depression called Hi’iaka   Patera and its two bordering mountains, UA graduate student Windy Jaeger   speculates that Io’s crust there might have been laterally pulled apart. It   may be only coincidence that the north and south parts, if pushed together,   would fit together like pieces of a puzzle, Jaeger said. At this point,   evidence is only circumstantial.
  But if Hi’iaka Patera was once pulled apart, McEwen said, "that would be   very surprising, because on Earth a movement on that scale is associated   with plate tectonics. But we see no evidence for plate tectonics on Io.   There again might be some unique Ionian process involved."