Sunset on Io

NASA’s Galileo spacecraft captured this dramatic image of mountains on Io. In this newly released image of Jupiter’s innermost moon Io, a mountain ridge named Mongibello – three-fourths as tall as Mount Everest – gleams from the rays of an otherworldly sunset.

Other Io pictures that researchers were presenting at this week’s Geophysical scientific meeting show colorful volcanic deposits covering the ground and a recent lava flow. Io, one of four large Jovian moons, is highly volcanic with high-temperature eruptions similar to those common on Earth, indicating a similar silicon-rich composition.

Io is one of the few bodies in the solar system known to be volcanically active (along with Neptune’s moon Triton, the planet Venus, and of course, Earth). Io’s volcanic processes are driven by internal tidal friction. This tidal friction is caused by gravitational effects that pull and stretch the moon. The closer a moon is to Jupiter, the greater its tidal friction. Io is relatively close (380,000 km) to Jupiter.

IO Eruption
Observations of Io by Galileo spacecraft. Click image for higher resolution black and white. The image has a resolution of 335 meters (1,100 feet) per picture element. North is to the top of the image. Credit: Arizona/ Galileo images NASA/JPL. False color. BW Image produced by: Zibi Turtle, Planetary Image Research Lab. (PIRL), Lunar and Planetary Lab. (LPL), University of Arizona

Galileo captured the images during flybys of Io in the past three years. Scientists are studying Galileo data for a better understanding of Io’s mountains and volcanoes. They presented the images in San Francisco during this week’s fall meeting of the American Geophysical Union, in San Francisco.

Galileo has been orbiting Jupiter since December 1995. The spectacular Galileo flybys of Europa and Io are largely credited with the discovery of frozen water ice and some of the earliest examples of non-solar (tidal) heating anywhere in our solar system.

The larger moons of Jupiter, for instance, have some of the qualities that would make life possible. These moons – Europa, Io, Callisto, and Ganymede – are called the Galilean moons because they were first spotted by Galileo Galilei in 1610. (The moons were individually named by the German astronomer Simon Marius, who discovered them the same year as Galileo).

Since the Galileo spacecraft found what appears to be water-ice among the moons, scientists have speculated about some of the basic environmental ingredients for life (energy, liquid water, non-vacuum) on both the Jovian moons and to a lesser extent in Jupiter’s atmosphere. A summary (Scientific Assessment for Galileo Disposal) of how to dispose of Galileo safely offers fascinating insights into the prospects for life on Io, Europa, Callisto and Ganymede.

One indication of possible life is the presence of organic molecules, which the Galileo space probe has detected on Europa in particular. The Galileo spacecraft’s near-infrared mapping spectrometer sent back data indicating the presence of combinations of oxygen, carbon, sulfur, hydrogen and nitrogen on Europa. The data also included a suggestion of the presence of tholins, complex organic compounds. The Galileo probe also detected organic molecules on the Jovian moons Callisto and Ganymede.

For Lack of Water

But Jupiter’s moon, Io, hosts an environment very different from that of the planet’s three other giant moons. Io is liberally dotted with volcanoes, giving its sulfur-rich surface a uniquely colorful appearance. Io is a greenish-yellow moon speckled with red, orange, white, and black markings.

Galileo entered orbit about Jupiter in December 1995 on a 2-year mission to conduct intensive observations of Jupiter’s atmosphere, rings, satellites, and radiation environment. In 1997, the mission was extended for an additional 2-year period to allow for additional studies of Europa and the first close-up observations of Io. In 1999, the mission was extended again for another year to enable more studies of Io and Europa, and, in addition, to conduct concerted observations of Jupiter’s magnetosphere with the Saturn-bound Cassini spacecraft in December 2000. The spacecraft is currently on a ballistic trajectory designed to intercept Jupiter and burn up in the atmosphere in September 2003.

Sunset Shadows

The Io sunset image was taken when the Sun was low in the sky, illuminating the scene from the left, so it reveals the dramatic topographic details of Io’s surface. A low scarp, roughly 250 meters (820 feet) high, runs from the upper left toward the center of the image. Mongibello Mons, the jagged ridge at the left of the image, rises 7 kilometers (23,000 feet) above the plains of Io, higher than any mountain in North America.

Few of Io’s mountains actually resemble volcanoes found on Earth or Venus. Instead Galileo scientists believe that the mountains are formed when blocks of Io’s crust are uplifted along thrust faults. Angular mountains like the one shown are thought to be younger, while older mountains have more subdued topography, such as the rise near the top center of this image. Analysis conducted by Galileo’s instrumentation determined that the lava of Io’s volcanoes reaches temperatures of 1430 to 1730 degrees Celsius (2,600 to 3,140 degrees Farenheit), thus exceeding the temperatures of earthen lava, which only reaches about 1090 degrees Celsius (2,000 degrees Farenheit). While Io seems to have enough heat and energy to sustain life, and it does have a thin sulfur dioxide atmosphere, it lacks water. Io’s volcanoes are constantly resurfacing the planet, and the intense heat generated by this activity probably caused any water present to evaporate billions of years ago.

What’s Next

Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly like Io without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.

Jupiter itself seems to be a long shot in the search for extraterrestrial microbial life. True, the planet is warm and has plenty of organic materials. Jupiter may also have wet and dry areas like the desert and tropical regions of Earth. Telescopes on Earth and on the Galileo space probe even have detected areas on Jupiter with clouds of water that could indicate rainfall. But other qualities of Jupiter seem far too extreme to sustain life.

For instance, a great deal of Jupiter is composed of liquid metallic hydrogen, an element only possible at pressures exceeding 4 million bars. (The atmospheric pressure on Earth at sea level is a little over one bar, or 14.7 pounds per square inch.) Any life that could withstand such conditions would still have to face Jupiter’s torrential winds, which clock in at an astounding rate of 400 miles per hour, twice as fast as tornadoes on Earth.

What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.

The Galileo probe has neared the end of its investigations of the Jovian system. The latest series of mission trajectories, now referred to as the Galileo Europa Mission (GEM), conclude its highly focused follow-on to Galileo’s Jupiter system exploration and a precursor for future missions to Europa and Io. Galileo has functioned in orbit more than three times longer than its originally planned mission.

The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Galileo mission for NASA’s Office of Space Science, Washington, D.C. Additional information about Galileo and its discoveries is available on the Galileo mission home page at

Related Web Pages:

Life and Giant Planets series
Galileo Home Page
Io Image Library– JPL
Evidence of bacteria on Jupter’s moon?
The Galileo Mission to Jupiter and Its Moons (Scientific American)