<P> Galileo steadfastly refused to use Marius' names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names . The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede, and Callisto respectively . Galileo used this system in his notebooks but never actually published it . The numbered names (Jupiter x) were used until the mid-20th century when other inner moons were discovered, and Marius' names became widely used . </P> <P> Galileo was able to develop a method of determining longitude based on the timing of the orbits of the Galilean moons . The times of the eclipses of the moons could be precisely calculated in advance, and compared with local observations on land or on ship to determine the local time and hence longitude . The main problem with the technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship; a problem that Galileo tried to solve with the invention of the celatone . The method was used by Cassini and Picard to re-map France . </P> <P> Some models predict that there may have been several generations of Galilean satellites in Jupiter's early history . Each generation of moons to have formed would have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's proto - satellite disk, with new moons forming from the remaining debris . By the time the present generation formed, the gas in the proto - satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits . Other models suggest that Galilean satellites formed in a proto - satellite disk, in which formation timescales were comparable to or shorter than orbital migration timescales . Io is anhydrous and likely has an interior of rock and metal . Europa is thought to contain 8% ice and water by mass with the remainder rock . These moons are, in increasing order of distance from Jupiter: </P> <Table> <Tr> <Th> Name </Th> <Th> Image </Th> <Th> Model of interior </Th> <Th> Diameter (km) </Th> <Th> Mass (kg) </Th> <Th> Density (g / cm3) </Th> <Th> Semi-major axis (km) </Th> <Th> Orbital period (d) (relative) </Th> <Th> Inclination (°) </Th> <Th> Eccentricity </Th> </Tr> <Tr> <Td> Io Jupiter I </Td> <Td> </Td> <Td> </Td> <Td> 7003366000000000000 ♠ 3660. 0 7003363740000000000 ♠ × 3637. 4 7003363060000000000 ♠ × 3630. 6 </Td> <Td> 0893! 7022893000000000000 ♠ 8.93 × 10 </Td> <Td> 7000352800000000000 ♠ 3.528 </Td> <Td> 7005421800000000000 ♠ 421 800 </Td> <Td> 1.769 (1) </Td> <Td> 0.050 </Td> <Td> 0.0041 </Td> </Tr> <Tr> <Td> Europa Jupiter II </Td> <Td> </Td> <Td> </Td> <Td> 7003312160000000000 ♠ 3121. 6 </Td> <Td> 0480! 7022480000000000000 ♠ 4.8 × 10 </Td> <Td> 3.014 </Td> <Td> 7005671100000000000 ♠ 671 100 </Td> <Td> 3.551 (2) </Td> <Td> 0.471 </Td> <Td> 0.0094 </Td> </Tr> <Tr> <Td> Ganymede Jupiter III </Td> <Td> </Td> <Td> </Td> <Td> 7003526820000000000 ♠ 5268. 2 </Td> <Td> 1480! 7023148000000000000 ♠ 1.48 × 10 </Td> <Td> 1.942 </Td> <Td> 7006107040000000000 ♠ 1070400 </Td> <Td> 7.155 (4) </Td> <Td> 0.204 </Td> <Td> 0.0011 </Td> </Tr> <Tr> <Td> Callisto Jupiter IV </Td> <Td> </Td> <Td> </Td> <Td> 7003482060000000000 ♠ 4820. 6 </Td> <Td> 1080! 7023108000000000000 ♠ 1.08 × 10 </Td> <Td> 1.834 </Td> <Td> 7006188270000000000 ♠ 1882700 </Td> <Td> 16.69 (9.4) </Td> <Td> 0.205 </Td> <Td> 0.0074 </Td> </Tr> </Table>

Which of jupiter’s moons is believed to be covered with water ice