Io

Jupiter I

Io Facts

• Io is the fifth of Jupiter's known satellites and the third largest:
  • distance from Jupiter: 422,000 km
  • diameter: 3630 km
  • mass: 8.94e22 kg
  Io is the innermost of the Galilean moons.

• Pronounced "EYE oh" (or "EE oh")

• Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer by the jealous Hera.

• Discovered by Galileo and Marius in 1610.

• Io is slightly larger than Earth's Moon.

• Io and Europa are somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. However, Io's density (3.57 g/cm3, about the same as that of the Moon and somewhat less than that of Earth) indicates the lack of a dense iron core.

• Io's surface is radically different from any other body in the solar system. It came as a very big surprise to the Voyager scientists on the first encounter. They had expected to see impact craters like those on the other terrestrial bodies and from their number per unit area to estimate the age of Io's surface. But there are very few, if any, impact craters on Io. The surface is very young and active.

• Instead of craters, Voyager 1 found hundreds of volcanic calderas (picture 3). Some of the volcanoes are active! Striking photos of actual eruptions with plumes 300 km high were sent back by both Voyagers (picture 5). This may have been the most important single discovery of the Voyager missions; it was the first real proof that the interiors of other "terrestrial" bodies are actually hot and active. The material erupting from Io's vents appears to be some form of sulfur or sulfur dioxide. The volcanic eruptions change rapidly. In just four months between the arrivals of Voyager 1 and Voyager 2 some of them stopped and others started up. The deposits surrounding the vents also changed visibly.

  Recent images taken with NASA's Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption (picture x), confirming that the activity seen by the Voyagers continues.

• Io has an amazing variety of terrains: calderas up to several kilometers deep, lakes of molten sulfur (picture 11), mountains which are apparently NOT volcanoes (picture 13), extensive flows hundreds of km long of some low viscosity fluid (some form of sulfur?), and volcanic vents. Sulfur and its compounds take on a wide range of colors which are responsible for Io's varigated appearance.

• Analysis of the Voyager images lead scienties to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur. However, subsequent ground-based infra-red studies indicate that they are too hot for liquid sulfur. The current best theory is that Io's lavas are molten silicate rock. Recent HST observations (picture 24) indicate that the material may be rich in sodium.

• Some of the hottest spots on Io may reach temperatures of 500 K (even 900 K has been reported), though the average is much lower, about 130 K. These hot spots are the principal mechanism by which Io looses its heat.

• The energy for all this activity probably derives from tidal interactions between Io, Europa, Ganymede and Jupiter. These three moons are locked into resonant orbits such that Io orbits twice for each orbit of Europa which in turn orbits twice for each orbit of Ganymede. Though Io, like Earth's Moon always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth. (Lacking another body to perturb it, the Moon is not heated by Earth in this way.)

• Io also cuts across Jupiter's magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from Io which forms a torus of intense radiation around Jupiter (picture 23). Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere.

• Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.

• Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.

Pictures

1. (above) Io from Voyager 1 192k gif

2. Io global mosaic B&W 69k gif; 161k jpg

3. Io's South Pole 74k gif; 59k jpg

4. Explosion on Io 116k gif; 94k jpg

5. Erupting Volcano 60k gif; 21k jpg

6. Volcanic Plume B&W 45k gif; 31k jpg
7. Volcanic Eruption 46k gif; 17k jpg
8. Venting Gases 105k gif; 111k jpg

9. limb of Io with active volcanoes 38k gif

10. limb of Io with active volcanoes (Zoom) 117k gif

11. Detail of Loki Patera 348k gif

12. Loki Volcano 22k gif
13. Volcano Pele 368k gif

14. Haemus Mons 226k gif

15. another close-up view of Io 422k gif
16. Io with Narrow Angle Camera (Violet filter) 205k gif
17. Io with Narrow Angle Camera (Violet filter) 233k gif
18. Io with Narrow Angle Camera (No filter) 235k gif
19. Io with Narrow Angle Camera (Violet filter) 237k gif
20. Io with Narrow Angle Camera (Violet filter) 240k gif
21. Io with Narrow Angle Camera (No filter) 229k gif
22. Io with Narrow Angle Camera (Green filter) 216k gif
23. Io plasma torus 58k gif
24. Jupiter and Io from HST 101k gif

25. new eruption from IRTF 20k gif; caption

Movies

1. rotating Io (very nice) 158k mpeg; small html version (for Netscape 1.1 only)
2. Model of the Io Plasma Torus 552k mpg
3. Io in Rotation/Volcanoes Errupting 3800k AVI
4. Pan over Io 2600k AVI

More about Io

• from LANL
• from JPL

Open Issues

• It seems that Io is radiating heat faster than the tidal mechanism can generate it. Is the present epoch atypically active? Or is there some additional source of heat?
• What mechanism drives the volcanoes? What is the eruptive material?
• Io's surface was visibly changed in the few months between the arrivals of Voyager 1 and Voyager 2. What will Io look like when Galileo arrives?

Express to Europa