"Jupiter was condensed while in
the solar-system nebula, and began its individual existence and
its evolutionary career long before Mercury was formed. The
matter now in Ganymede, however, doubtless remained part of the
Jupiter-system nebula till after Mercury's creation, and, being
part of so great a mass, did not cool very rapidly. I should say
that this satellite has about the same relation to Jupiter that
Jupiter has to the sun, and is therefore younger in point of time
as well as of development than the most distant Callisto, and
older, at all events in years, than Europa and Io, both of which
are nearer. This supposition is corroborated by the fact that
Europa, the smallest of these four, is also the densest, having a
specific gravity of 2.14, its smallness having enabled it to
overtake Ganymede in development, notwithstanding the latter's
start. In the face of the evidence before us we must believe
this, or else that, perhaps, as in the case of the asteroid
Hilda, something like a collision has rejuvenated it. This might
account for its size, and for the Nautical Almanac's statement
that there is a 'small and variable' inclination to its orbit,
while Io and Europa revolve exactly in the plane of Jupiter's
equator.
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