Io: Fire World

Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.

Discovery1610, Galileo Galilei1610, Galileo Galilei1610, Galileo Galilei1610, Galileo Galilei
Diameter2,273 miles
3,660 km
1,944 miles
3,130 km
3,270 miles
5,262 km
2,985 miles
4,806 km
Distance from Jupiter262,000 miles
422,000 km
416,700 miles
671,000 km
665,000 miles
1.1 million km
1.2 million miles
1.9 million km
Orbital Period1.8 days3.6 days7.2 days16.7 days

Other than the Sun, portions of Io offer the hottest surface in the solar system. Several hundred volcanoes dot the surface, and they belch sulfur-rich lava that is hundreds of degrees hotter than the hottest lava on Earth.

Io’s interior is heated by a tug-of-war between Jupiter and the planet’s other big moons. Io is “locked” so that the same hemisphere always faces the planet, just as the same hemisphere of our own moon always faces Earth. But as the other moons move past Io, their gravity tugs at it, too. That heats Io’s interior enough to melt some of its rock, which “bubbles” to the surface.

Europa: Water World?

For decades, Mars was considered the most likely home for life in the solar system. As observations continued to show a sterile, desolate world, though, scientists began turning their attention to Europa. Europa’s icy crust appears to cover a large ocean of liquid water, where life may have gained a foothold.

Tidal gravity may have created the ocean by heating Europa enough to melt some of its ice.

There is abundant evidence of an ocean. The Galileo spacecraft found two types of terrain that may be related to water. “Chaotic” terrain looks like icebergs breaking off glaciers on Earth. The other terrain consists of smooth plains marked by ridges that are hundreds of miles long. The ridges may form as Jupiter’s gravity rips apart the thin ice sheet. In addition, the motions of salty water below the surface may generate Europa’s weak magnetic field.

Ganymede and Callisto: Ice Worlds

Ganymede is the solar system’s largest moon — larger than the planet Mercury. It consists of about half ice and half rock and metal. Grooves and ridges that crisscross its surface indicate that it has undergone great changes over the eons.

During several passes, the Galileo spacecraft saw mountains of ice, plus sheets of ice that erupted from volcanoes. It also saw deep canyons and broad, smooth plains created by the motions of Ganymede’s crust.

Like Europa, Callisto’s icy surface may conceal an ocean. The case for an ocean is more tentative, but it is bolstered by a huge basin on one side of the moon. It was created by a powerful impact billions of years ago.

But there is no jumble of rocks and mountains on the opposite side of Callisto, as there is with big impact basins on our own moon. A deep ocean could have cushioned the impact, preventing shock waves from piling up rocks half a world away.

1. Metis
2. Adrastea
3. Amalthea
4. Thebe
5. Io
6. Europa
7. Ganymede
8. Callisto
9. Themisto
10. Leda
11. Himalia
12. Lysithea
13. Elara
14. S/2000 J11
15. Iocaste
16. Praxidike
17. Harpalyke
18. Ananke
19. Isonoe
20. Erinome
21. Taygete
22. Chaldene
23. Carme
24. Pasiphae
25. S/2002 J1
26. Kalyke
27. Magaclite
28. Sinope
29. Callirrhoe
30. Euporie
31. Kale
32. Orthosie
33. Thyone
34. Euanthe
35. Hermippe
36. Pasithee
37. Eurydome
38. Aitne
39. Sponde
40. Autonoe
41. S/2003 J1
42. S/2003 J2
43. S/2003 J3
44. S/2003 J4
45. S/2003 J5
46. S/2003 J6
47. S/2003 J7
48. S/2003 J8
49. S/2003 J9
50. S/2003 J10
51. S/2003 J11
52. S/2003 J12
53. S/2003 J13
54. S/2003 J14
55. S/2003 J15
56. S/2003 J16
57. S/2003 J17
58. S/2003 J18
59. S/2003 J19
60. S/2003 J20
61. S/2003 J21
62. S/2003 J22


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