On January 7, 1610, while skygazing from his garden in Padua, Italy, astronomer Galileo Galilei was surprised to see four small "stars" near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. Collectively, these four moons are known today as the Galilean satellites.
Jupiter, the Great Red Spot and moon Io.
Galileo would be astonished at what we have learned about Jupiter and its moons in the past 30 years. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and has its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa. An icy ocean may also lie beneath the crust of Callisto. In 2003 alone, astronomers discovered 21 new moons orbiting the giant planet. Jupiter now officially has 61 moons - by far the most in the solar system. Many of the outer moons are probably asteroids captured by the giant planet's gravity.
At first glance, Jupiter appears striped. These stripes are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Within these belts and zones are storm systems that have raged for years. The southern hemisphere's Great Red Spot has existed for at least 100 years, and perhaps longer, as Galileo reported seeing a similar feature nearly 400 years ago. Three Earths could fit across the Great Red Spot. Jupiter's core is probably not solid but a dense, hot liquid with a consistency like thick soup. The pressure inside Jupiter may be 30 million times greater than the pressure at Earth's surface.
As Jupiter rotates, a giant magnetic field is generated in its electrically conducting liquid interior. Trapped within Jupiter's magnetosphere - the area in which magnetic field lines encircle the planet from pole to pole - are enough charged particles to make the inner portions of Jupiter's magnetosphere the most deadly radiation environment of any of the planets, both for humans and for electronic equipment. The "tail" of Jupiter's magnetic field - that portion stretched behind the planet as the solar wind rushes past - has been detected as far as Saturn's orbit. Jupiter's rings and moons are embedded in an intense radiation belt of electrons and ions trapped in the magnetic field. The Jovian magnetosphere, which comprises these particles and fields, balloons one to three extending more than one billion kilometers behind Jupiter - as far as Saturn's orbit.
Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise: a flattened main ring and an inner cloud-like ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe, and Adrastea. Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons. The main ring probably comes from the tiny moon Metis.
Click on the image for a diagram of Jupiter's moons.
In December 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere. Carrying six scientific instruments, the probe survived the crushing pressure and searing heat for nearly an hour, collecting the first direct measurements of Jupiter's atmosphere, the first real data about the chemistry of a gas planet. Following the release of the probe, the Galileo spacecraft began a multi-year orbit of Jupiter, observing each of the largest moons from close range several times.
778,412,010 km
Radius:
71,492 km
(11.209 of Earth's radius)
Mass:
1,898.7 x 10-27 g
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. Megaclite 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