Someone has done lab work that indicates extremely dry rock is too viscous for plate tectonics to work. The explanation for why Venus doesn't have plate tectonics has been around a while. The problem is we don't have a reason why Earth does that I know of. We know how it works currently, but we don't know why it started. That left the door open that it may have required a massive geological disturbance like the Moon forming impact. This new data suggests plate tectonics may be the norm on Earth like worlds with oceans even if we don't understand how they start. Further detailed studies of Io, Europa, Ganymede, Enceladus, Titan, Mars, and Venus and even Miranda may help answer this question. Callisto might also be a usefull null set since it is so similar to Ganymede in many ways. The similarly sized Mercury and Titan are also proving geologically surprising. I think we are experiencing the best period of planetary studies so far even if the wonder level is not as high as the first flybyes.

Twins we can learn from

Earth/Venus

Ganymede/Callisto (Titan is somewhat of a fraternal twin to these two)

Triton/Pluto (Eris might be a triplet, but we are unlikely to learn much about it for quite some time)

Europa and Enceladus aren't twins, but they also have a lot of interesting similarities and differences I'm sure comparative planetologists will find enteresting (do we need a new word there too?).

Regards,
Mark