A new hypothesis suggests that the formation of landscapes on Saturn’s moon Titan could be explained by a global sedimentary cycle driven by seasons. According to the findings, the alien world may be more Earth-like than previously thought.
Titan, Saturn’s moon, appears to be Earth from space, with rivers, lakes, and seas filled with rain tumbling through a thick atmosphere. While these landscapes appear to be familiar, they are made of materials that are not; liquid methane streams streak Titan’s icy surface, and nitrogen winds build hydrocarbon sand dunes.
Titan’s landscape formation is enigmatic due to the presence of these materials, whose mechanical properties differ greatly from those of silicate-based substances 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, which is a target for space exploration because of its potential habitability, is the only other body in our solar system known to have an Earth-like, seasonal liquid transport cycle today. The new model, published in Geophysical Research Letters April 25, shows how that seasonal cycle drives the movement of grains over the moon’s surface.
“Our model provides a unifying framework that enables 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 (Stanford Earth). “Once we understand how the various pieces of the puzzle fit together and their mechanics, we can start using the landforms left behind by those sedimentary processes to say something about Titan’s climate or geological history – and how they might impact the prospect for life on Titan.”
We were able to resolve the paradox of why there could have been sand dunes on Titan for so long even though the materials are very weak. We hypothesized that sintering — which involves neighboring grains fusing together into one piece — could counterbalance abrasion when winds transport the grains.
Mathieu Lapôtre
A missing mechanism
In order to build a model that could simulate the formation of Titan’s distinct landscapes, Lapôtre and his colleagues first had to solve one of the biggest mysteries about sediment on the planetary body: 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 just wearing down and blowing away as dust?
On Earth, silicate rocks and minerals on the surface erode into sediment grains over time, moving through winds and streams to be deposited in layers of sediments that eventually turn 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 similar processes formed the dunes, plains, and labyrinth terrains seen from space on Titan. Titan’s sediments, however, are thought to be composed of solid organic compounds, as opposed to Earth, Mars, and Venus, where silicate-derived rocks are the dominant geological material from which sediments are derived. Scientists have yet to demonstrate how these organic compounds can form sediment grains that can be transported across the moon’s landscapes and through geologic time.
“As winds transport grains, the grains collide with each other and with the surface. These collisions tend to decrease grain size through time. What we were missing was the growth mechanism that could counterbalance that and enable sand grains to maintain a stable size through time,” Lapôtre said.
An alien analog
The researchers discovered the answer 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 formation through chemical precipitation, which allows ooids to grow while simultaneously slowing growth as the grains are smashed into each other by waves and storms. These two competing mechanisms balance each other out over time to form a constant grain size, a process that the researchers believe is also occurring on Titan.
“We were able to resolve the paradox of why there could have been sand dunes on Titan for so long even though the materials are very weak, Lapôtre said. “We hypothesized that sintering — which involves neighboring grains fusing together into one piece — could counterbalance abrasion when winds transport the grains.
Global landscapes
Armed with a hypothesis for sediment formation, Lapôtre and the study’s co-authors used existing data about Titan’s climate and the direction of wind-driven sediment transport to explain its distinct parallel bands of geological formations: dunes near the equator, plains in the mid-latitudes, and labyrinth terrains near the poles.
Winds are common near the equator, according to atmospheric modeling and data from the Cassini mission, supporting the idea that less sintering and thus fine sand grains could be created there – a critical component of dunes. The authors of the study predict a lull in sediment transport at mid-latitudes on either side of the equator, where sintering could dominate and produce coarser and coarser grains, eventually forming the bedrock that makes up Titan’s plains.
Sand grains are also required for the formation of the labyrinth terrains near the poles on the moon. These distinct crags are thought to be collapsed features made of dissolved organic sandstones, similar to karsts in limestone on Earth. River flow and rainstorms are much more common near the poles, so sediments are more likely to be transported by rivers rather than winds. A similar process of sintering and abrasion during river transport could provide a local supply of coarse sand grains, which is thought to be the source of the sandstones that make up labyrinth terrains.
“We’re showing that on Titan, just like 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 episodic abrasion and sintering driven by Titan’s seasons,” Lapôtre said. “It’s pretty fascinating to think about how there’s this other world out there, where things are so different, yet so similar.”
This study was funded by a NASA Solar System Workings grant.
