There are three things about Ganymede requiring explanation, i.e....Ganymede is unique among moons in that it has a magnetic field surrounding it, something even Mars does not possess. In December 1995, the Galileo spacecraft entered orbit around Jupiter. During a flyby of Ganymede at an altitude of only 838 kilometers, Galileo discovered a dipole magnetic field similar to the one surrounding Earth. The signature of Ganymede’s electric flux tube (the electric current that connects it with Jupiter) can be seen in Jupiter’s polar aurora.
With a mean diameter of 5262 kilometers, Ganymede is the largest moon orbiting any planet, and is the fourth largest rocky object after the planet Mars. Its magnetic field is supposedly created by the moon’s core in a “dynamo” of sorts—again like the Earth’s core is supposed to generate its magnetic field. There is an ambiguity, however: Ganymede’s core is thought to be too hot to hold on to permanent magnetism. On the other hand, Ganymede is so small that, according to conventional astrogeology, it should have cooled off billions of years ago and not have a liquid core in the first place.
The ad hoc explanation that NASA scientists announced also creates its own conundrum....
1. The ultra-low moment of inertia.
2. The intrinsic magnetosphere.
3. The large rock areas (some as large as Switzerland) which appear embedded in the ice.
The standard claims don't work. They involve claiming that the low moment of inertia is produced by a 600km deep outer mantel of salt water; that the salt water provides the conductivity to generate the magnetosphere via a dynamo effect of some sort; and hoping that people will sort of forget about the rocky areas...
In real life, Ganymede is too small to generate any sort of a dynamo effect, and rocks don't float on water or end up embedded in ice on top of a salt water outer mantel. The salt-water outer mantel is pure hypothesis, there is very little detectable sodium on or around Ganymede.
Cosmos in Collision posits an outer mantel of pumice rather than salt water. The combination of pumice (with its huge surface to mass ratio and P-holes (the things which make rock surfaces partially conductive) would make the pumice highly conductive, and the magnetosphere was fused in early on by Birkeland currents. In fact our own intrinsic magnetosphere also appears to be a remnant and not an artifact of dynamo effects. The fact of our magnetosphere weakening with time strongly suggests that.
With out relatively tiny eyes, lack of a decent sense of smell, lack of fur, and basic adaptation to an aquatic environment, humans would have been hideously maladapted to the conditions of this planet 60,000 years ago. We could not plausibly have come from this planet.
Jupiter on the other hand, 60,000 years ago, would have been within the sun's habitable zone as is normal for gas giants in our galaxy. Ganymede with its oxygen atmosphere, fresh water oceans, anchored islands and floating bergs of pumice, and its intrinsic magnetosphere to block out radiation, would have been an ideal original home world for humans.
http://www.cosmosincollision.com