1. The descent probe measured atmospheric elements and found that their relative abundances were different than on the Sun, indicating Jupiter's evolution since the planet formed out of the solar nebula.

2. Galileo made a first observation of ammonia clouds in another planet's atmosphere. The atmosphere seems to create ammonia ice particles of material from lower depths, but only in "fresh" clouds.

3. Io's extensive volcanic activity may be 100 times greater than that found on Earth's. The heat and frequency of eruption are reminiscent of early Earth.

4. Io's complex plasma interactions in Io's atmosphere include support for currents and coupling to Jupiter's atmosphere.

5. Evidence supports a theory that liquid oceans exist under Europa's icy surface.

6. Ganymede is the first satellite known to possess a magnetic field.

7. Galileo magnetic data provide evidence that Europa, Ganymede and Callisto have a liquid-saltwater layer.

8. Europa, Ganymede, and Callisto all provide evidence of a thin atmospheric layer known as a 'surface-bound exosphere'.

9. Jupiter's ring system is formed by dust kicked up as interplanetary meteoroids smash into the planet's four small inner moons. The outermost ring is actually two rings, one embedded with the other.

10. Galileo was the first spacecraft to dwell in a giant planet magnetosphere long enough to identify its global structure and to investigate its dynamics.

Technical firsts

• First flyby and imaging of an asteroid (Gaspra and later, Ida).
  
  
• Discovered first moon around an asteroid (the moon Dactyl orbits asteroid Ida).
  
  
• Only direct observation of Comet Shoemaker-Levy 9's impact into Jupiter's atmosphere.
  
  
• First spacecraft to deploy an entry probe into an outer planet's atmosphere. The descent probe measured the atmospheric composition and structure of Jupiter's atmosphere and provided clues to the origin of Jupiter and giant planets in other star systems.
  
  
• First and so far the only spacecraft to orbit an outer planet. (Cassini at Saturn in 2004 will become the second spacecraft to enter orbit around one of the outer planets.)