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Fri. Aug 18, 2000

Pizza Pie in the Sky: Understanding Io's Riot of Color

By Robert Roy Britt
Senior Science Writer
posted: 10:54 am ET
16 March 2000
Galileo Team Celebrates 21 Years, Stunning Images

Galileo Girds Itself for Third Swing past Io

Galileo Gives Closest Look Yet at Io

Closest-Ever Io Flyby Yields Dramatic Details


If some insane NASA project were to build a pizza the size of Earth’s moon, the results might look a lot like Io.

The innermost moon of Jupiter, Io is the most volcanic body in the solar system. It’s pockmarked with some three dozen active volcanoes, about a hundred towering mountains or plateaus, and 300 basin-shaped calderas -- the collapsed remains of spent volcanoes.

But it's the colors that make Io so interesting. A riot of green, white, black, red and more have led scientists to liken Io to a pizza.


The volcano Prometheus is surrounded by Io's riot of colors.

While any couch potato might recognize the green olives on a pizza pie, and maybe even have a vague idea of what pepperoni is made of, scientist are just beginning to grasp how Io's mountains and volcanoes are made and what generates the engaging culinary colors.

Since 1996, the Galileo spacecraft has been returning increasingly more detailed images of the pizza moon, moving in for a closer look on each of more than two dozen passes.

The green

"When we finally got to see [Io's features] close-up...we learned that the greenish tinge was confined to lava flows and calderas in volcanic centers that appear to be coated by sulfurous deposits," said University of Arizona researcher Paul Geissler. Geissler presented some of the latest research on Io this week at Lunar and Planetary Science Conference held at Johnson Space Center.

"Our best guess is that the green materials consist of sulfur compounds that are contaminated by, or have reacted with the lava, but we can't rule out other possibilities like green silicates (rock)," Geissler told

The white

White areas on Io have been confirmed to be sulfur dioxide frost, said Geissler, who works out of the university's Lunar and Planetary Laboratory.

Infrared imaging had found traces of the compound in areas that were not visibly white, and researchers had begun to question their interpretations. As it turned out, the infrared imagers were spotting thin, virtually transparent layers of sulfur dioxide that did not show up in visible light.

The black

Scientists have also been curious about a handful of curious black spots that are associated with extensive deposits of volcanic ash, and appear to be different from the other outflows on Io. A recent infrared image showed that the black spots are made of rock.

"Instead of fountains of black sulfur, we now know that they are Io's versions of Pinatubo or Mt. St. Helens, spewing rocky ash to heights of up to 100 kilometers (62 miles) above the surface," Geissler said.

He points out that while the eruptions may resemble terrestrial volcanoes, the stuff flung out is not the same as what emanates from a volcano on Earth.

The elongated, dark caldera below and to the right of center is Monan Patera. Red areas are thought to be a compound of sulfur associated with erupting lava. The white circle is probably sulfur dioxide. The mountain on the far right is about 8 kilometers (26,000 feet) high.

The red

"Io is a unique body in the solar system," says Jani Radebaugh, another Lunar and Planetary Laboratory scientist who also spoke at this week's conference. "Its high volcanic activity produces a great number of calderas of large sizes."

Calderas, broad depressions in the surface, are created by shifting underground magma. They are similar to the Yellowstone basin on Earth. But at only some 50 miles (80 kilometers) in diameter, the Yellowstone caldera pales in comparison to those on Io, which can stretch for 125 miles (200 kilometers).

The floors of the calderas are black, indicating recent flows of lava.

"Lava erupts from Io at such a high rate that sometimes lava fills up a caldera and flows great distances away from it," Radebaugh said in an e-mail interview. "Diffuse red material, possibly due to the eruption of [sulfur] gas, is often found along the margins of Io's calderas."

The force

The incredible pace of volcanic action on Io is driven by external forces. Giant Jupiter tugs on Io with colossal gravity, which wrenches the tiny moon's insides, stirring up and heating the core.

While researchers have long focused on Io's volcanoes, they are now also beginning to understand its huge mountains, the tallest of which towers some 50,000 feet (16 kilometers) above the surface. The highest point on Earth is Mt. Everest, in the Himalayas at 29,022 feet (8.8 kilometers).

Io's mountains are not created in chains, as they often are on Earth, but instead are evenly distributed around the moon. And, in a recent surprise to scientists, they do not appear to be created by volcanoes. But lava has been spotted spurting from the bases of the mountains.

Other researchers suggested this week that Io's mountains might be built when large blocks of surface crust crunch together and are pushed up by the heat from below. The mountainous blocks were described as resting on "a mushy magma ocean," and over time they are thought to collapse.

Stay tuned

With the help of the Galileo spacecraft, scientists are painting an increasingly coherent picture of the solar system's most colorful and active volcanic object. But there is much to learn, as recent passes by the spacecraft have produced even more detailed views of the pizza moon.

Geissler said "spectacular" new color images will be revealed in an upcoming issue of the journal Science.

This Weekend: Vacation with the Astronauts and Cosmonauts.

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