Planetary scientists will be reaping the benefits of the Saturn Cassini mission for many decades to come. It orbited the ringed jewel of the solar system for thirteen years, taking incredible data of the planet, the rings, and the fleet of bizarre moons.
The biggest moon around Saturn — and the second biggest in the whole solar system — is the aptly named Titan. We’d learned quite a bit about it from Earth-bound telescopes, but there’s nothing like being there. Cassini passed the planet-sized moon many times, taking images of it at lots of different wavelengths. We’ve known for a long time that it has an atmosphere, which is very cool, but also a pain: carbon-based molecules floating around there are really good at absorbing and reflecting visible light, so all see we when we look at those wavelengths is an orange fuzzball.
But… those molecules are completely ambivalent to certain wavelengths of infrared light, letting them through. That means that equipped with the right filters, Cassini’s Visual and Infrared Mapping Spectrometer could see light reflected from the surface, piercing the haze! After a dozen flybys a lot of images were taken, allowing scientists to put together a pretty decent map of the entire surface of Titan. Earlier maps showed lots of seams in the mosaics, due in part to different lighting (and weather!) conditions between Cassini passes, but now they have released new images that are just stunning.
Check. This. Out.
Whoa. These images show a lot of interesting things on the surface of Titan. Bear in mind they’re in the infrared, and they’re not constructed like normal images. Instead of using a single filter to represent each color, they used ratios of filters. So what you see as blue is actually an image taken at 1.27 microns (very roughly twice the reddest wavelength you can see with your eye) divided by one at 1.08 microns. Green is 2.03/1.27, and red 1.59/1.27. This is pretty clever; by using ratios the contrast of surface features is enhanced significantly. That makes it easier to see them, and also reduces lighting differences between Cassini visits. Still, it must have taken a lot of work to put these all together; the images were taken at different distances, different geometric angles, different perspectives. What a task!
But so worth it. Titan looks gorgeous. And there’s so much to see.
The southern and northern high latitudes appear pretty featureless — though they aren’t at all; that’s where the lakes of liquid methane are, but they’re hard to see here. They show up far better in radar images. But the equator is a different story.
It’s mostly flat, without much in the way of mountains. But there are very interesting things to be seen… or not seen. For example, there are very few impact craters on Titan’s surface. It gets hit as much as any other Saturnian moon (more, since it’s bigger and has gravity to pull them in), so the lack of craters means the surface gets repaved, so to speak, on a short timescale. Erosion from weather may be the culprit here.
Yes, weather. Titan is cold, so water is frozen harder than granite there, but methane can exist as a liquid, solid, and gas, just as water does here on Earth*. So it can evaporate from lakes, form clouds, and rain down elsewhere: in other words, erosion.
The equatorial region also has vast dune fields! But these aren’t silicate sand as on Earth, but hydrocarbons that have condensed out of the atmosphere. Winds blow these grains around, forming dunes all around the equator. Two huge fields of them can be found in areas called Shangri-La (the reddish area on the right in the top middle image) and Xanadu (the vast bright purple/white field on the left in the lower left image).
The dune fields were discovered using radar and are an important clue to Titan’s environment. How the grains form, how they grow, how they blow, and the size and spacing of the dunes tells planetary scientists a lot about conditions on the moon’s surface. Weirdly, longitudinal dunes on Titan are similar to those on Earth, despite the very different conditions.
On the other hand, as bizarre a thing as it is to say, Titan really is very similar to Earth in many ways. Sure, it’s a lot colder, but it has an atmosphere that’s mostly nitrogen, like ours, has a molecule (methane) that can exist in three phases in the same area (as water does on Earth), and has lakes of liquid with eroded channels where it flows — some radar maps look just like maps of Earth were water flows.
And we think life needs a medium in which to form, a liquid where nutrients and such can flow, accumulate, and concentrate. Titan has that! I’m not saying there’s life there, but I am saying we need to broaden our attitude about where life can arise. And even if it’s lifeless, Titan is an amazing place with processes both familiar and alien, and very much worth our study.
And if we can also make gorgeous maps of it from space? Well, bonus.
* Well, this turns out to be more complicated than I first thought. Methane can technically be solid (like snow) high up in Titan's atmosphere, but lower down it will likely mix with ethane and nitrogen, which lowers its freezing point by enough that it's unlikely to be found on the surface in a solid state. In my defense, most papers I found mention solid methane being possible, but when I posted this article I got a tweet coorrecting me by Dr. Mike Malaska, who studies Titan, which led to a fun discussion about all this (which included Dr. Jennifer Hanley, who studies these molecules under Titan-like conditions). Who knew that studying alien worlds might be complex and subtle?