The Moon of Saturn From space, Titan seems quite similar to Earth, with rivers, lakes, and seas filled with rain tumbling through a thick atmosphere. While these landscapes appear to be familiar, they are made of things that are not—liquid methane streams stripe Titan’s frozen surface, while nitrogen winds produce hydrocarbon sand dunes.
Titan’s landscape creation is perplexing due to the presence of these materials, whose mechanical characteristics are radically different from those of silicate-based compounds found in other known sedimentary bodies in our solar system. Stanford University geologist Mathieu Lapôtre and his colleagues demonstrated how Titan’s distinct dunes, plains, and labyrinth terrains could be formed by identifying a process that would allow hydrocarbon-based substances to form sand grains or bedrock depending on how frequently winds blow and streams flow.
Titan, a potential habitable body in our solar system, is the only other body in our solar system known to have an Earth-like, seasonal liquid transport cycle today. The new model, which was published in Geophysical Research Letters on April 25, explains how the seasonal cycle causes grain mobility across the moon’s surface.
“Our model provides a unifying framework that helps us to understand how all of these sedimentary environments interact,” said Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy, and Environmental Sciences. “Once we understand how the various puzzle pieces fit together and their mechanics, we can start using the landforms left behind by those sedimentary processes to indicate something about the climate.”
“If we understand how the different pieces of the puzzle fit together and their mechanics, then we can start using the landforms left behind by those sedimentary processes to say something about the climate or the geological history of Titan—and how they could impact the prospect for life on Titan.”
Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences
“Our model provides a unifying framework that helps us to understand how all of these sedimentary environments interact,” said Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy, and Environmental Sciences. “Once we understand how the various parts of the puzzle fit together and their mechanics, we can begin using the landforms left behind by those sedimentary processes to indicate something about Titan’s temperature or geological history—and how they might impact the chance for life on Titan.”
A mechanism is missing.
To create a model that could simulate the formation of Titan’s distinct landscapes, Lapôtre and his colleagues had to first solve one of the planet’s biggest mysteries: how can its basic organic compounds, which are thought to be much more fragile than inorganic silicate grains on Earth, transform into grains that form distinct structures rather than simply wearing down and blowing away as dust?
On Earth, silicate rocks and minerals on the surface disintegrate into sediment grains over time, travelling through winds and streams to be deposited in layers of sediment that eventually convert back into rocks thanks to pressure, groundwater, and, in some cases, heat. These rocks are then eroded, and the materials are recycled through the Earth’s layers over geologic time.
Researchers believe that comparable processes generated the dunes, plains, and labyrinth terrains observed from space on Titan. Titan’s sediments, however, are assumed to be formed of solid organic molecules, as opposed to Earth, Mars, and Venus, where silicate-derived rocks constitute the major geological material from which sediments are derived. Scientists have yet to explain how these organic molecules might form sediment grains that can be carried across the moon’s landscapes and over geologic time.
As the grains are carried by the wind, they collide with one another and with the surface. These encounters tend to reduce grain size over time. What we lacked was a growth mechanism that could counteract this and allow sand grains to keep a consistent size throughout time, according to Lapôtre.
An analogy to an alien
The researchers discovered the solution by studying ooids, which are small, spherical grains commonly found in shallow tropical seas such as those around the Bahamas. When calcium carbonate is drawn from the water column and attaches in layers around a grain, such as quartz, ooids form.
What distinguishes ooids is their development by chemical precipitation, which permits ooids to expand while simultaneously slowing growth as the grains are smashed into one another by waves and storms. These two conflicting mechanisms balance each other out over time to form a consistent grain size, a process that the researchers believe is also occurring on Titan.
“We were able to address the dilemma of how sand dunes could have existed on Titan for so long despite the fact that the materials are quite weak,” Lapôtre remarked. “We believed that sintering—which involves neighboring grains fusing together into one piece—could counteract abrasion when the grains are transported by wind.”
Landscapes across the world
Lapôtre and the study’s co-authors used known data on Titan’s temperature and the direction of wind-driven sediment movement to explain its different parallel bands of geological formations: dunes around the equator, plains in the mid-latitudes, and labyrinth terrains towards the poles.
According to atmospheric models and data from the Cassini mission, winds are abundant around the equator, supporting the concept that less sintering and hence finer sand grains could be generated there—a fundamental component of dunes. The authors of the study foresee a break in sediment transit at mid-latitudes on either side of the equator, where sintering could dominate and produce coarser and coarser grains, eventually forming the bedrock that forms Titan’s plains.
Sand grains are also required for the creation of the labyrinth terrains near the poles of the moon. These different crags are thought to represent collapsed landforms made of dissolved organic sandstones, similar to karsts in limestone on Earth. River flow and rainstorms are significantly more common towards the poles, so sediments are more likely to be transported by rivers than winds. A similar process of sintering and abrasion during river transit could offer a local supply of coarse sand grains, which is assumed to be the source of the sandstones that make up labyrinth terrains.
Through episodic abrasion and sintering driven by Titan’s seasons, we’re demonstrating that on Titan, just as on Earth and what used to be the case on Mars, we have an active sedimentary cycle that can explain the latitudinal distribution of landscapes through an active sedimentary cycle. It’s intriguing to think about how there’s this other universe out there, where things are so different, yet so similar.
