Cassini's last shot of Titan, taken by the probe's narrow-angle camera on September 13, 2017. Credit: NASA

A new map of Titan

It’s been a long time since I’ve obsessed over Titan, primarily because after the Cassini mission ended, the pace of updates about Titan died down, and because other moons of the Solar System (Europa, Io, Enceladus, Ganymede and our own) became more important. There have been three or four notable updates since my last post about Titan but this post that you’re reading has been warranted by the fact that scientists recently released the first global map of the Saturnian moon.

(This Nature article offers a better view but it’s copyrighted. The image above is a preview offered by Nature Astronomythe paper itself is behind a paywall and I couldn’t find a corresponding copy on Sci-Hub or arXiv nor have I written to the corresponding author – yet.)

It’s fitting that Titan be accorded this privilege – of a map of all locations on the planetary body – because it is by far the most interesting of the Solar System’s natural satellites (although Europa and Triton come very close) and were it not orbiting the ringed giant, it could well be a planet of its own accord. I can think of a lot of people who’d agree with this assessment but most of them tend to focus on Titan’s potential for harbouring life, especially since NASA’s going to launch the Dragonfly mission to the moon in 2026. I think they’ve got it backwards: there are a lot of factors that need to come together just right for any astronomical body to host life, and fixating on habitability combines these factors and flattens them to a single consideration. But Titan is amazing because it’s got all these things going on, together with many other features that habitability may not be directly concerned with.

While this is the first such map of Titan, and has received substantial coverage in the popular press, it isn’t the first global assessment of its kind. Most recently, in December 2017, scientists (including many authors of the new paper) published two papers of the moon’s topographical outlay (this and this), based on which they were able to note – among other things – that Titan’s three seas have a common sea level; many lakes have surfaces hundreds of meters above this level (suggesting they’re elevated and land-locked); many lakes are connected under the surface and drain into each other; polar lakes (the majority) are bordered by “sharp-edged depressions”; and Titan’s crust has uneven thickness as evidenced by its oblateness.

According to the paper’s abstract, the new map brings two new kinds of information to the table. First, the December 2017 papers were based on hi- and low-res images of about 40% of Titan’s surface whereas, for the new map, the authors write: “Correlations between datasets enabled us to produce a global map even where datasets were incomplete.” More specifically, areas for which authors didn’t have data from Cassini’s Synthetic Aperture Radar instrument for were mapped at 1:2,000,000 scale whereas areas with data enabled a map at 1:8,000,000 scale. Second is the following inferences of the moon’s geomorphology (from the abstract the authors presented to a meeting of the American Astronomical Society in October 2018):

We have used all available datasets to extend the mapping initially done by Lopes et al. We now have a global map of Titan at 1:800,000 scale in all areas covered by Synthetic Aperture Radar (SAR). We have defined six broad classes of terrains following Malaska et al., largely based on prior mapping. These broad classes are: craters, hummocky/mountainous, labyrinth, plains, lakes, and dunes [see image below]. We have found that the hummocky/mountainous terrains are the oldest units on the surface and appear radiometrically cold, indicating icy materials. Dunes are the youngest units and appear radiometrically warm, indicating organic sediments.

SAR images of the six morphological classes (in the order specified in the abstract)

More notes once I’ve gone through the paper more thoroughly. And if you’d like to read more about Titan, here’s a good place to begin.