"These images were such a delight to look at." -Emily Costello

Ganymede is one of my personal favorites. Partly because it has plate-tectonic activity that might be active, more similar to Earth than any other world. However the similarities don't go far, because Earth-like subduction and buoyancy differences between granite and basalt are off the table. Ganymede uses different rules to do different things than Earth thought to be related to ice expanding as it freezes. But there is a catch. The Sulci of Ganymede seem to take up more space than water freezing alone can explain. The simplest explanation is that the Sulci didn't rift at once, they took turns.

That's what this fantastic LPI lecture is doing, dating the relative ages of the Sulci by crater-counting.

• At 6:40 the speaker starts showing craters on trailing Sulci. Leading and trailing are important terms with moons. Jupiter is attracting impactors, and Ganymede's leading end will collect more than the trailing end. Since Ganymede is tidally locked, the leading end will always be the same.
• The slide at 7:00. Look at Tiamat Sulcus. Is it older or younger than Kishar Sulcus? The one that crosses the other is younger. Which one is younger in this image? There seems to be another Sulci inside Tiamat Sulcus that is the youngest, while Kishar is younger than the rest of Tiamat. But the youngest thing is that crater right on the intersection there, and there aren't very many craters in that image. See how fun Ganymede is?
• At 9:00 the speaker demonstrates the same law-of-superposition, (younger is on top), principal, just in a mathematically absolute manner.
• At 10:10 the speaker demonstrates a conflict between eyeballing the law-of-superposition and her mathematical model. In every case, the law-of-superposition wins. The likely explanation is that where they overlap, the newest Sulci wiped out the craters on the bottom Sulci, thus skewing the numbers. 

The bottom line is that Ganymede craters do make for an accurate method to date Ganymede's Sulci, and that the Sulci vary in age considerably. But there's another funny catch to consider going forward. Europa's double-ridges are thought to be related to where Europa's crust happens to be (since it's floating) when it pulls towards or away from Jupiter. The Europa ridges tend to come in arcs this way, and periodically appear with ongoing frequency. Ganymede has very different morphology, but the root cause may be similar. However Ganymede may not be gaining new Sulci anymore if the ice-lithosphere is thick enough to resist and insulate. Ganymede has internal heat, and the thermal expansion of ice may be in equilibrium.