First full map of Titan assembled
Now it’s time to find out more about ‘the Earth of the outer Solar System’.
By Richard A. Lovett
Scientists working with data from NASA’s Cassini spacecraft have assembled the first planet-wide geological map of Titan – an important step in helping to understand the overall history and evolution of Saturn’s giant moon.
“Geologic mapping like this helps grapple with the bewildering complexity of Titan’s landscape,” says Ralph Lorenz, a planetary scientist at Johns Hopkins University's Applied Physics Laboratory in the US, who was not part of the study team.
The data, assembled from 126 Titan flybys between 2004 and 2017, combines a mix of infrared and radar images. (Visible images weren’t used because Titan’s thick, hazy atmosphere hinders our view of its surface.)
Individually, many of the images took the form of long ribbons, referred to by the Cassini scientists as “noodles”.
Each covered only a single swath, but there were enough of them to cover about half of Titan’s surface at high resolution, with the remaining area filled in by lower-resolution images.
Based on this, says Rosaly Lopes, a planetary geologist at NASA’s Jet Propulsion Laboratory in California, it was possible to identify six basic types of landforms.
These ranged from plains and sand dunes to lakes (of liquid methane, not water), ancient highlands, impact craters, and “labyrinth terrains” that appear to be plateaus, deeply cut by river channels.
One of the finds, Lopes says, is that there is a “really distinct” correlation of terrain types with latitude: sand dunes are largely near the equator, plains are largely in the mid-latitudes, and lakes and labyrinth terrain are largely at the poles.
One possible explanation, she says, is that this is related to variations in (methane) rainfall: the poles receive enough rain to form lakes, the equator is dry enough to have loose sand, and the mid-latitudes are too dry for lakes but moist enough for surface materials to consolidate into plains.
“It’s been suggested before that the mid-latitudes are wet,” Lopes says, “but there are still many questions we have to answer. “What we have to do now is work with people who do atmospheric circulation models to figure out why.”
At the moment, she says, one theory is that material from the dunes is being carried north and south by prevailing winds until it reaches a moist-enough latitude that it gets sedimented into plains.
But it’s also possible that the reverse is happening, with materials wind-sculpted off the plains being carried to the equatorial belt to form the dunes.
“Now that we have this complete geological map, we have to figure out details like how the wind is blowing,” Lopes says.
Meanwhile, her research also shows geologists which terrains are the oldest and which are the youngest, based on looking at the contacts between them and determining which lies on top of which.
From this, she says, it’s possible to show that the dunes and the lakes are the youngest – and are probably still active.
“We know that there may be changes in the lakes due to the amount of rainfall and the seasons, she says. “[And] although we haven’t detected any changes in the dunes, it’s very likely that they are still being formed and moving.”
If all of this sounds very Earthlike, Lopes agrees. “We call Titan the Earth of the outer Solar System because it has so many of the geological process that the Earth has,” she says.
Her team’s findings are published in the journal Nature Astronomy.