Alongside Earth and Mars, Saturn’s moon Titan is the third planetary body in the solar system to show evidence of widespread and diverse sedimentary environments, including lakes, rivers, alluvial fans, or deltas, eroded canyonlands, dissected plateaux, and sand dunes.
The latitudinal distribution of Titan’s terrains, with sand dunes largely concentrated around the moon’s equatorial belt, undifferentiated plains at mid-latitudes, and labyrinth terrains and lakes near the poles, suggests a strong control of climate on Titan’s surface processes and landscape formation.
On Titan, loose solid particles (or sediments) are likely made of soft hydrocarbon grains, prone to rapid breakdown into dust. Yet, Titan’s equatorial dunes have been active for up to several hundreds of thousands of years, suggesting that some mechanism must produce sand-sized particles at these latitudes.
Stanford University geologist Mathieu Lapôtre and his colleagues have shown how Titan’s distinct dunes, plains, and labyrinth terrains could be formed. They did so by identifying a process that would allow for hydrocarbon-based substances to form sand grains or bedrock depending on how often winds blow and streams flow.
Titan, which is a preferred 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 on April 25, shows how that seasonal cycle drives the movement of grains over the moon’s surface.
“Our model adds a unifying framework that allows us to understand how all of these sedimentary environments work together,” said Lapôtre, an assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “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.”
Researchers explored 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 – with the help of pressure, groundwater, and sometimes heat – turn back into rocks. Those rocks then continue through the erosion process, and the materials are recycled through Earth’s layers over geologic time.
On Titan, researchers think similar processes formed the dunes, plains, and labyrinth terrains seen from space. But unlike on Earth, Mars, and Venus, where silicate-derived rocks are the dominant geological material from which sediments are derived, Titan’s sediments are thought to be composed of solid organic compounds. Scientists haven’t been able to demonstrate how these organic compounds may grow into sediment grains that can be transported across the moon’s landscapes and over 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.
The research team found an answer by looking at sediments on Earth called ooids, which are small, spherical grains most often found in shallow tropical seas, such as around the Bahamas. Ooids form when calcium carbonate is pulled from the water column and attaches in layers around a grain, such as quartz.
What makes ooids unique is their formation through chemical precipitation, which allows ooids to grow. At the same time, the simultaneous process of erosion slows the growth as the grains are smashed into each other by waves and storms. These two competing mechanisms balance each other out through time to form a constant grain size – a process the researchers suggest could also be happening 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.”
“Climate models predict that Titan’s single-celled, pole-to-pole Hadley circulation splits into two cells as atmospheric circulation reverses near equinox, possibly leading to intense mid-latitude and equatorial storms that drive significant sediment transport by winds and rivers. Global circulation model (GCM) predictions, combined with models for eolian transport, reveal that winds capable of transporting dune sands typically occur ∼0.1%–10% of the time at all latitudes. Except for two equinox storms, equatorial winds are relatively steady throughout the year, whereas sand transport may occur through infrequent but strong wind gusts at higher latitudes.” Study quotes.
Titan flybys by Cassini revealed that the moon’s methane cycle includes methane rainstorms and even inundation, with more storms near the poles than near the equator. Despite the relative infrequency of equatorial storms, surface changes and dust plumes were detected near the equinox.
The study authors predict a lull in sediment transport at mid-latitudes on either side of the equator, where sintering could dominate and create coarser and coarser grains, eventually turning into bedrock that makes up Titan’s plains.
“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 alternative world so far out there, where things are so different, yet so similar.”
“If proven correct, our model will, in turn, provide a key to decipher Titan’s recent climate history through the lens of its global source-to-sink sedimentary pathways.” Study quotes.
- Mathieu G. A. Lapôtre, Michael J. Malaska,Morgan L. Cable. The Role of Seasonal Sediment Transport and Sintering in Shaping Titan’s Landscapes: A Hypothesis. Geophysical Research Letters 49, e2021GL097605, DOI: 10.1029/2021GL097605