What one Earth (or elsewhere) is a “palimpsest”?
The official definition is:
- A manuscript, typically of papyrus or parchment, that has been written on more than once, with the earlier writing incompletely erased and often legible.
- An object, place, or area that reflects its history: “Spaniards in the sixteenth century... saw an ocean moving south... through a palimpsest of bayous and distributary streams in forested paludal basins” (John McPhee).
What I suspect the astronomers of having hinted at happening here is “lots of stuff” — but what it actually looks like is a laboratory-made electrical discharge area. Note the sort-of “un-crater” at the centre?
The next question, of course, is “where did all of the electrickery come from?” — but of course Ganymede is a large moon right next to this huge, electrically-busy planet known as Jupiter.
Looking back at the Shoemaker-Levy-9 impact on said planet, it was massively dominated by electrical discharges, hinting that in this particular location bulk power is not so very hard to run across.
This would have made — to say the least — an interesting spectacle (to watch from a safe distance, presuming that there was one at the time).