Working with data from NASA’s Cassini mission, scientists have produced the first global map of Titan’s geological features.
Scientists have created a new map of Saturn’s moon Titan that shows its unique geological features and how they’ve evolved over time. The map is all the more fascinating because it shows that the same geological processes that work on Earth also work on Titan. Only on Titan, the temperature is so achingly cold (98 K, or -290°F) that any liquid carving its surface is not water but hydrocarbons, mainly methane and ethane.
To create the global map, Rosaly Lopes (Caltech) and her colleagues combined data collected by NASA’s Cassini mission, which flew by the moon more than 120 times. In addition to high-resolution radar mapping that covered almost half of the moon’s surface, the spacecraft also characterized the surface using lower-resolution radar, infrared images, and infrared spectra.
High-resolution radar captured details as small as 1 kilometer (0.6 mile) across. The global map, on the other hand, has a scale of 1:20,000,000, which means that every centimeter on the map corresponds to 200 km on the moon’s surface. By filling in the full globe, even at lower resolution, the map not only charts the diverse geological formations on Titan, but it also enables planetary scientists to see how those features evolved over time.
In November 18th’s Nature Astronomy, Lopes and colleagues describe six main types of geological features — all of them eerily familiar to ones on Earth. Based on how the features overlap each other, the researchers are also able to tell their relative ages.
Dunes dominate Titan’s equatorial latitudes and take up about a fifth of the surface overall. Previous measurements have shown that these dunes are much like the ones that snake across deserts on Earth, except that the “sand” is made of solid hydrocarbons. They’re about 1–2 km wide, 1–4 km apart, and as tall as 80–130 meters, reaching or even surpassing the height of the Statue of Liberty. Dunes appear to be among the youngest features on Titan.
NASA’s Dragonfly mission, targeting a 2034 landing on the faraway moon, will land on and investigate the Shangri-La Dune.
Lakes, like dunes, are newer features on Titan that tend to cluster around the poles. Lakes may be dry or wet, but most of the wet ones are found around the north pole — global climate changes might be responsible for the dichotomy. Large lakes appear to have flooded in, while smaller lakes have steep sides that scientists still can’t explain — though one idea researchers have floated is that the smaller lakes are explosion craters, formed long ago when liquid molecular nitrogen deposits blew holes in Titan's crust.
Unfortunately, there’s no overlap between the polar lakes and the equatorial dunes, so there’s no way to see which of these is younger than the other. The separation of the two features reveals a global variation in climate, from the arid equator to the more humid poles.
Plains are featureless, flat regions that make up almost two-thirds (65%) of Titan’s surface. They appear dark in radar images, but spectra show that they tend to hold material swept in by the wind from nearby regions. For example, plains near the dunes tend to have some dune material on them, and plains near the labyrinths likewise show signatures of labyrinth material. The plains are intermediate in age.
Labyrinths are fascinating structures that appear to be like karstic terrain on Earth. They cover about 2% of Titan’s surface. Like Earth’s karsts, Titan’s labyrinths probably formed when rain (albeit, of the methane variety) dissolves rock, creating steep channels over time. The labyrinths are older than the dunes and lakes, but younger than Titan’s hummocks.
Craters make up less than 1% of Titan’s surface. Spectra show that the impacts have excavated some water ice. The ones that are there could be several hundred million years old, but various processes that erode Titan’s surface have washed away any older ones. The lake-filled poles, in particular, are almost craterless.
Regions designated as hummocky consist of rolling hills and mountains typically a few to tens of kilometers long and a couple kilometers high. These are among the oldest features on Titan. As with the craters, spectra of the hummocky regions reveal water-ice, supporting the idea that the irregular terrain, along with the excavated craters, could represent exposed remnants of an icy crust that’s dusted over with hydrocarbons.
The new map of Titan is not the first to portray these features, but it provides — for the first time — a global view, revealing patterns in their appearance on Saturn's largest moon.