Alongside Earth and Mars, Titan is the third planetary body in the Solar System to show evidence for widespread and diverse sedimentary environments, including lakes, rivers, alluvial fans or deltas, eroded canyonlands, dissected plateaux, and sand dunes. 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. In a new study, planetary scientists from Stanford University and NASA’s Jet Propulsion Laboratory explored the hypothesis that a combination of abrasion, when grains are transported by winds or methane rivers, and sintering, when they are at rest, could produce sand grains that maintain an equilibrium size. The team’s model demonstrates that seasonal sediment transport could produce sand on Titan and could explain the distribution of Titan’s landscapes.
This view of Titan is among the last images NASA’s Cassini spacecraft sent to Earth before it plunged into the giant planet’s atmosphere. Image credit: NASA / JPL-Caltech / Space Science Institute.
“Our model adds a unifying framework that allows us to understand how all of these sedimentary environments work together,” said Dr. Mathieu Lapôtre, a researcher in the Department of Gelogical Sciences at Stanford University.
“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.”
In order to build a model that could simulate the formation of Titan’s distinct landscapes, Dr. Lapôtre and colleagues first had to solve one of the biggest mysteries about sediment on the planetary body: how can its basic organic compounds 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.
Those rocks then continue through the erosion process and the materials are recycled through Earth’s layers over geologic time.
On Titan, planetary 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,” Dr. Lapôtre said.
“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.”
The authors 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, while 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,” Dr. Lapôtre said.
“We hypothesized that sintering — which involves neighboring grains fusing together into one piece — could counterbalance abrasion when winds transport the grains.”
The moons of our Solar System are brimming with unusual landscapes. However, sometimes they look a little more familiar, as in this new radar image from the Cassini orbiter. The image shows dark streaks carved into dunes reminiscent of those we might find on a beach on Earth, or raked with flowing lines in a Japanese Zen garden — but this scene is actually taking place on Saturn’s moon Titan. While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye. While previous images have spotted these eerily familiar patterns on Titan’s dunes, this new image (colorized) shows them in greater detail. The image was obtained by Cassini’s radar mapper on July 10, 2013. The vertical seam near the center is an artifact of radar image data processing. Image credit: NASA / JPL-Caltech / Sci-News.com.
Armed with a hypothesis for sediment formation, the scientists 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 at the mid-latitudes, and labyrinth terrains near the poles.
Atmospheric modeling and data from NASA’s Cassini mission reveal that winds are common near the equator, supporting the idea that less sintering and therefore fine sand grains could be created there — a critical component of dunes.
The team predicts 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.
Sand grains are also necessary for the formation of the moon’s labyrinth terrains near the poles.
The researchers think these distinct crags could be like karsts in limestone on Earth — but on Titan, they would be collapsed features made of dissolved organic sandstones.
River flow and rainstorms occur much more frequently near the poles, making sediments more likely to be transported by rivers than winds.
A similar process of sintering and abrasion during river transport could provide a local supply of coarse sand grains — the source for the sandstones thought to make up labyrinth terrains.
“We’re showing that on Titan, we have an active sedimentary cycle that can explain the latitudinal distribution of landscapes through episodic abrasion and sintering driven by Titan’s seasons,” Dr. 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.”
The results were published in the journal Geophysical Research Letters.
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Mathieu G. A. Lapôtre et al. The Role of Seasonal Sediment Transport and Sintering in Shaping Titan’s Landscapes: A Hypothesis. Geophysical Research Letters, published online April 1, 2022; doi: 10.1029/2021GL097605
