Last week, researchers released the first-ever geological map of Ganymede, Jupiter's largest moon. Here's the map again, flattened into a 2D, rectangular map.

I sometimes prefer these planar versions of maps to their three-dimensional counterparts. Their geologic nature (and planetary provenance) are less obvious this way. And sometimes that's the point – to enjoy the beauty of a thing for the sake of its beauty alone. Like a caption-less photo of the Australian Outback, a map in this format can more closely resemble a piece of art* than a reference tool. That the variegated view of Earth's Moon pictured below is, at first glance, unrecognizable as a geologic map is one of the reasons I'm so fond of it:

And so it is, also, with this map of Ganymede. Removed from its lunar sphere and flattened out like wrapping paper, its visual impact is more keenly felt, its raw aesthetic more readily appreciated.


Of course, more practically speaking (i.e., in a manner more suited to its original, geological purpose), 2D maps also enable the observation of large-scale features and patterns. Furrows, grooves and craters can be considered in a more global context – are there more signs of impact events on one side of the object than the other? Is its surface divided into global highlands and lowlands, perhaps in a hemispheric dichotomy similar to the one astronomers observe on Mars?

Above: The Red Planet's hemispheric dichotomy. Simply put: the planet's Southern hemisphere has a rugged, crusty surface, while much of the planet's Northern hemisphere remains relatively smooth.


These are questions similar to the one I had while examining the map of Ganymede at the top of this post, namely: what is the deal with that vertical gap of terrain, about two thirds of the way across (highlighted below in red)?

Puzzled, I emailed planetary scientist Geoffrey Collins, who led the team responsible for the map's creation. It turns out the gap is an artifact of sorts, a patch of map devoid of data. Not the sexiest or most satisfying answer, but Collins was good enough to dispense with a fair bit of insight into his team's process, and a little map-making trivia, to boot:

Yes, that stripe… I wish that I could have convinced the USGS publications folks to change the center longitude on the map to put it at the edge, because that's a complete artifact… One subtlety that only extreme map aficionados might notice is that the color changes on the map at that boundary don't have any black lines on them, which means "no mapped contact" between the units. What this means in plain english is that it represents a transition where we couldn't see how the surface changed from one type of terrain to the other. The reason we couldn't see it is because right at that longitude there is a sharp break between very detailed images to the west, and much less detailed images to the east. If you look at the two little "rainbow" maps in the lower left corner of the full map sheet, they show the changes in the quality of the imaging data that we had to work with when constructing the map. (Map makers a few centuries ago didn't have data quality keys, they just drew sea monsters and mermaids on the parts where they had low data quality.)

Any line of inquiry that begins with a map of Ganymede and ends with mermaids and sea monsters is a win in my book. Aren't maps just the best?

*I in no way mean to suggest that maps can not also be pieces of art. In fact, I'd venture that most maps can be regarded as such. Map-making is an art form in itself, after all; surely maps are, too, themselves.