<P> The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value . The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible . The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal dissipation in the interior of Ganymede . This means that the last episode of the eccentricity excitation happened only several hundred million years ago . Because Ganymede's orbital eccentricity is relatively low--on average 0.0015--tidal heating is negligible now . However, in the past Ganymede may have passed through one or more Laplace - like resonances that were able to pump the orbital eccentricity to a value as high as 0.01--0.02 . This probably caused a significant tidal heating of the interior of Ganymede; the formation of the grooved terrain may be a result of one or more heating episodes . </P> <P> There are two hypotheses for the origin of the Laplace resonance among Io, Europa, and Ganymede: that it is primordial and has existed from the beginning of the Solar System; or that it developed after the formation of the Solar System . A possible sequence of events for the latter scenario is as follows: Io raised tides on Jupiter, causing Io's orbit to expand (due to conservation of momentum) until it encountered the 2: 1 resonance with Europa; after that the expansion continued, but some of the angular moment was transferred to Europa as the resonance caused its orbit to expand as well; the process continued until Europa encountered the 2: 1 resonance with Ganymede . Eventually the drift rates of conjunctions between all three moons were synchronized and locked in the Laplace resonance . </P> <P> Ganymede is the largest and most massive moon in the Solar System . Its diameter of 5,268 km is 0.41 times that of Earth, 2% larger than that of Saturn's Titan (the second - largest moon), 1.08 times Mercury's, 1.09 times Callisto's, 1.45 times Io's and 1.51 times the Moon's . Its mass is 10% greater than Titan's, 38% greater than Callisto's, 66% greater than Io's and 2.02 times that of the Moon . </P> <P> The average density of Ganymede, 1.936 g / cm, suggests a composition of about equal parts rocky material and mostly water - ices . The mass fraction of ices is between 46--50%, which is slightly lower than that in Callisto . Some additional volatile ices such as ammonia may also be present . The exact composition of Ganymede's rock is not known, but is probably close to the composition of L / LL type ordinary chondrites, which are characterized by less total iron, less metallic iron and more iron oxide than H chondrites . The weight ratio of iron to silicon ranges between 1.05 and 1.27 in Ganymede, whereas the solar ratio is around 1.8 . </P>

How big is ganymede compared to earth's moon