[OT} Bode's Law

I was looking at ratios of the planets' orbital periods and noticed that the
periods of Io:Europa:Ganymede are very close to 1:2:4 (for those who are
interested, Callisto's period is about 7/3 of Ganymede's).  Can any of
you educated blokes point me to a source (preferably on the Web) which may
explore possible reasons for Bode's Law?