The three margins described on earth in the concept of plate tectonics: (1) divergent (seen above), (2) convergent (mountain building or orogeny) and (3) transform or strike-slip often associated with earthquakes, may also play a role in the geologic formation of Ganymede.
Ganymede, discovered by Galileo in January, 1610, has surface area that is greater than half of the surface area of the land mass on earth. The moon was named for Ganymede, the pretty boy who was taken by Zeus to be cup bearer to the gods (Had they been geologists it may well have been mead in those cups :-) ). Ganymede may be seen this month with binoculars:
NASA released a geologic map of Ganymede this week, the first complete geologic map of Jupiter's seventh moon. The "cup bearer" stands out quite well with a blue, dark green, and purple body and green "crater head" on the right side of the first image:
The geology of Ganymede is especially interesting to geologists because it appears there were times of tectonic movement on this icy moon. These grooves and ridges point to a similar origin to the earth's plate tectonic movement:
In addition, periods of intense crater impacts are seen in the geologic history:
One of the more interesting features on Ganymede are palimpsests, derived from the word meaning to write over older writing (similar to pentimento overpainting in art). [The word palimpsest is from the Greek for to scrape]:
Similarly, older craters are "overwritten" by younger ones as the older crater margins are eroded through time:
There were (and are) quiescent times on this frozen, icy moon (quiescently frozen :)) as well.
There is also evidence for some cryovolcanism (eruption of volatiles like ice and water, methane, and ammonia from volcanoes) on Ganymede.
The link to the NASA article contains a 37 second animation so you can also see the "Dark Side of the Moon" (The Pink Floyd song turns 40 next month!):
Looking forward to discussing Ganymede geology, nomenclature, great music, and some planetary geology jokes and puns. Do you have a favorite?
