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 |  | Spectacular Io
By Gentry Lee
SPACE.com Columnist
posted: 07:43 am ET
07 July 2000
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SPACE.com columnist Gentry Lee is a noted space systems engineer, TV producer and SF author. This month, he brings us a closer look at Jupiter's volcanic Jovian moon Io.
In the summer of 1979, as the Voyager spacecraft approached Jupiter for mankind’s closest look yet at the largest planet in our solar system, the Voyager navigation team was working overtime to make certain that the spacecraft’s trajectory was sufficiently precise to guarantee a subsequent encounter with Saturn. The team was very busy obtaining and processing approach photographs of Jupiter and its moons, a key data type permitting an accurate determination of the location of Voyager with respect to the Jovian system.
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In one of the enhanced approach images Linda Morabito, a member of that Voyager navigation team, discovered an illuminated plume emanating from the surface of Io, the innermost of Jupiter’s large moons. She soon verified that the origin of the plume was a strange Io surface feature that some of the scientists had speculated might be a volcano. Ms. Morabito had indeed confirmed the existence of the first active volcano beyond Earth.
An active volcanic eruption on Io was captured in this picture on February 22, 2000 by the Galileo spacecraft.
The Io that emerged from the Voyager observations was unlike any world mankind had ever seen. False-colored images of its mottled surface, with dramatic yellows, browns and reds representing regions dominated by different sulfur-based compounds, depicted an unstable world of geysers and volcanoes and brimstone. Underneath Io’s tortured exterior scientists envisioned vast pools of molten sulfur, kept in constant tidal turmoil by the attraction of nearby Jupiter. In addition, Voyager indicated that the massive Jovian magnetic field was regularly enriched by ionized gases from the eruptions on the surface of Io, creating an astonishing plasma torus between Jupiter and Io carrying an electric current of millions of amperes.
The two Voyager spacecraft each offered tantalizing glimpses of spectacular Io as they hurtled past Jupiter on their way to encounters with the other outer planets. The objective of the Galileo mission, conceived in the mid 1970s, was to extend the Voyager results by completing a comprehensive survey of the entire Jovian system. Galileo, finally launched in 1989 after numerous programmatic delays, consisted of two separate scientific spacecraft, a probe and an orbiter, which flew together in a mated ensemble from Earth to Jupiter. After separating from the mother spacecraft several months before reaching Jupiter, the Galileo probe successfully relayed its atmospheric data to Earth through the orbiter. Subsequently the orbiter fired its main engines and became the first known artificial satellite of Jupiter. The primary objective of the Galileo orbiter mission was an in-depth characterization of the major Jovian satellites, Io, Europa, Ganymede, and Callisto.
If a scientist or science fiction writer had described a world exactly like Io half a century ago, his or her colleagues would have dismissed the conceit as a figment of an overactive imagination. Today, because of Io, we are more aware of the possibilities.
The baseline design for the Galileo orbiter mission included an ingenious satellite tour featuring repeated close flybys of Europa, Ganymede and Callisto without major expenditures of propellant. The only baseline mission design close encounter with Io, admittedly one of the highest priority scientific targets, was during the initial approach to Jupiter, only a few hours before the critical probe relay and Jupiter orbiter insertion (JOI) events. Why only one scheduled close encounter with Io? The nearest of the major moons to Jupiter, Io’s location deep inside the powerful Jovian magnetic field represented a significant potential hazard for the electronic subsystems on the Galileo spacecraft. Engineering calculations indicated that repetitive passes through the intense radiation at Io’s distance from Jupiter would probably shorten the useful lifetime of the spacecraft.
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Unfortunately, due to heightened concerns about the safety of the spacecraft in December 1995, when Galileo reached Jupiter, the originally scheduled Io close encounter observations were canceled because of their proximity to the mission-critical probe relay and JOI events. Scientists eagerly anticipating a high-resolution look at the exotic, pockmarked volcanic surface of Io were forced to be patient. Throughout the remainder of the baseline mission, as Galileo made repeated, and successful, close flybys of Europa, Ganymede and Callisto, observations of Io were limited to the "windows of opportunity" that happened to occur when the nominal trajectory passed comparatively close to the innermost of the Galilean moons. As Galileo project scientist Torrence Johnson has often stressed, the images and data resulting from these "windows of opportunity" observations of Io over several years were quite significant. Because of the improved instruments on board Galileo, the quality of the Io data acquired during these observation periods was comparable to that obtained by Voyager.
Volcanic calderas, lava flows and cliffs are seen in this false color image of a region near the south pole of Jupiter's moon Io. It was created by combining two images from Galileo.
Finally, at the end of 1999 and the beginning of 2000, after the completion of an extended mission focused on the icy moon Europa, Io scientists were rewarded for their patience. Three Galileo close encounters with Io provided a veritable cornucopia of new and exciting data about the most active and exotic world yet discovered in our solar system. More than a hundred useful images with resolutions from 32 to 1,640 feet (10 to 500 meters) per picture element were returned. These images, in general an order of magnitude better than equivalent Voyager photographs, permitted detailed interpretations of the processes occurring on the surface of Io.
Even at high resolution, no impact craters were identified on Io. The absence of impact craters confirmed that the surface is constantly being reworked by active geologic processes and sets an upper bound for the surface age at an astonishingly low value of 1 million years. Extrapolation of the high-resolution data identifying hot spots on Io suggested that several hundred volcanoes may be simultaneously active on this amazing world.
After the Voyager encounters, a vigorous scientific debate ensued about the nature of the volcanoes on Io. Some scientists argued, on the basis of what were perceived to be flows of molten sulfur and a potentially wide range of sulfur compounds on the surface, that Io volcanism was sulfur-based and distinctly different from the volcanism on Earth. Others said that the existence of 6.2-mile- (10-kilometer-) high mountains on Io required the presence of silicate rocks and that Io volcanism must, therefore, be similar to ours. The Galileo observations have resolved this debate. The ability to measure surface temperatures over smaller regions has confirmed that the very hot lava on Io is predominantly silicate. The lava consists, in fact, of magnesium-rich silicate, hotter and denser than typical basalt lavas on Earth.
Galileo captured this view of a 60-mile (100-kilometer) high volcanic plume in the Masubi region of Io on July 3, 1999.
The lava flows on Io are absolutely massive, more than a hundred times longer than any recent flows on Earth. Scientists are seeing on Io processes that hearken back to the early days of our own planet, when mammoth lava flows like those in the Columbia River valley in the state of Washington altered the surface of huge areas. Observing the dynamic evolution of lava flows on Io thus provides a fascinating insight into the processes that shaped early Earth.
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There are many astounding geologic entities on Io. The volcanic region called Pele features a bright red circular deposit with a diameter of 750 miles (1,200 kilometers). The material is deposited by a plume of gases, rich in both sulfur and sulfur dioxide, that spews more than 250 miles (400 kilometers) high in the near vacuum of Io’s atmosphere. An extremely large and powerful volcano named Loki has been continuously active for the 20 years since it was first discovered by Voyager. Although there is some variation in the intensity of Loki’s eruptions and lava flows, ground-based and Hubble observations have confirmed major volcanic activity in the region throughout the entire period. Temperatures on the floor of Loki’s huge central caldera suggest the existence of a lava lake of enormous size.
Of all the features on Io, however, perhaps the most fascinating is Prometheus. First observed by Voyager, and more recently the subject of intensive study by the Galileo science teams, the plume from the Prometheus volcano has somehow wandered roughly 50 miles (80 kilometers ) to the west over the last 20 years. In spite of this bizarre movement, the fundamental geometric and optical properties of the plume have remained the same. Nothing even remotely like this has ever been observed on Earth.
These four Galileo images from 1997 show plumes of gas and dust rising from Io's volcanoes Zamana and Prometheus.
What is Prometheus all about? In a paper in the May 19 edition of Science Magazine, which contains several superb articles about the recent Io flybys, Susan W. Kieffer, et al. offer an intriguing explanation for the origin and dynamics of Prometheus. She and her colleagues postulate that the existence and motion of the plume are due to the "vaporization of a sulfur dioxide and/or sulfur snowfield over which a lava flow is moving." Using phase diagrams to buttress their argument, Ms. Kieffer et al. show how solid, liquid and vapor forms of sulfur dioxide all exist and interact in the Prometheus region to produce the unusual observed behavior. Not even in the pages of science fiction has such an exotic and altogether different world been imagined.
The International Astronomical Union has a nomenclature subcommittee that specifies the names for planetary features. Names on Io are associated, fittingly, with myths of fire and the underworld. Pele, for example, is not named after the great Brazilian soccer player. In Polynesian mythology, Pele is the god of fire and volcanoes. The landscapes of Io are rife with exotic, image-conjuring names. The volcano Zamama is named after the ancient Babylonian sun, war and corn god. Tvashtar Catena draws its name from an Indian sun god and smith who was famous for forging thunderbolts.
Prometheus, of course, is the legendary Titan who incurred the wrath of the Olympian gods by stealing fire and making it available to mankind. As punishment for his insurrection, the gods bound Prometheus forever and constrained his enormous energy. But by bringing fire to humanity, the mythical Prometheus catalyzed the imagination and development of the human species.
Similarly, the existence of spectacular, volcanic Io kindles a sense of wonder in even the most staid observer. How can any human being not marvel at a universe that contains a world where active volcanoes and lava flows dot the landscape in all directions and sulfurous gases are spewed hundreds of miles (kilometers) into the atmosphere? Io is a fascinating, intriguing world whose very existence opens our minds to new and different thoughts. If a scientist or science fiction writer had described a world exactly like Io half a century ago, his or her colleagues would have dismissed the conceit as a figment of an overactive imagination. Today, because of Io, we are more aware of the possibilities.
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