(ORDO NEWS) — Saturn’s moon Titan from space is very similar to Earth: rivers, lakes and seas. While these landscapes may seem familiar, they are made up of materials that are undeniably different – streams of liquid methane flow around Titan’s icy surface, and nitrogen winds create hydrocarbon sand dunes.
The presence of these materials, whose mechanical properties differ significantly from those of the silicate materials that make up other known sedimentary bodies in our solar system, makes the formation of Titan’s landscape mysterious.
By identifying the process that allows hydrocarbon-based substances to form grains of sand or bedrock, depending on how often winds blow and streams flow, Stanford University geologist Mathieu Lapoutre and colleagues have shown how dunes, plains and labyrinths can form on Titan.
Titan, a target for space exploration because of its potential for habitability, is the only body in our solar system that today has a seasonal fluid transport cycle similar to Earth’s. A new model, published April 25 in the journal Geophysical Research Letters, shows how this seasonal cycle drives the movement of grains across the lunar surface.
“Our model adds a unifying framework that allows us to understand how all of these sedimentary environments work together,” said Lapoutre, assistant professor of geological sciences at the Stanford School of Earth, Energy and Environmental Sciences.
“If we understand how the different pieces of the puzzle fit together and what their mechanics are, then we can start using the landforms left behind by these sedimentary processes to say something about Titan’s climate or geological history – and how they might affect the outlook.” life on Titan.
The missing gear
To build a model capable of simulating the formation of Titan’s various landscapes, Lapoutre and his colleagues first had to solve one of the biggest mysteries associated with sedimentary rocks on the planet: As the main organic compounds thought to be much more fragile than the inorganic silicate grains on Earth, can turn into grains that form distinct structures, rather than just shatter and scatter into dust?
On Earth, silicate rocks and minerals on the surface eventually weather and turn into grains of sedimentary rocks, are moved by winds and currents, deposited in layers of sedimentary rocks, which eventually turn back into rocks under the influence of pressure, groundwater, and sometimes heat. These rocks then continue the process of erosion and the materials are recycled into the Earth’s layers over geological time.
On Titan, the researchers believe, similar processes have shaped the dunes, plains and labyrinths visible from space. But unlike Earth, Mars and Venus, where the predominant geological material from which the sediments are formed is silicate rocks, the sediments of Titan are believed to be composed of solid organic compounds.
Scientists have not yet been able to show how these organic compounds can turn into grains of sedimentary rock that can be transported across lunar landscapes and over geologic time.
“When the winds carry the grains, they collide with each other and with the surface. These collisions tend to reduce the size of the grains over time. We lacked a growth mechanism that could balance this and allow the sand grains to maintain a stable size over time,” Lapotre said.
The research team found the answer by studying terrestrial deposits called ooids, which are small spherical grains most commonly found in shallow tropical seas such as those found in the Bahamas. Ooids are formed when calcium carbonate is pulled out of the water column and attached in layers around grains such as quartz.
Ooids are unique in that they are formed by chemical precipitation, which allows the ooids to grow, while the simultaneous erosion process retards growth as the grains are smashed against each other by waves and storms. These two competing mechanisms balance each other out over time, forming grains of constant size, a process that the researchers suggest could also occur on Titan.
We were able to resolve the paradox of why sand dunes on Titan could exist for so long, despite the fact that the materials are very weak,” Lapoutre said. “We hypothesized that sintering, in which neighboring grains are fused into one piece, can balance abrasive wear, when the winds carry grains.”
Armed with the sedimentary hypothesis, Lapautre and co-authors of the study used existing data on Titan’s climate and the direction of wind transport of sedimentary rocks to explain individual parallel bands of geological formations: dunes near the equator, plains at mid-latitudes, and labyrinths near the poles.
Atmospheric modeling and data from the Cassini mission show that winds often blow near the equator, supporting the idea that there is less caking and therefore fine grains of sand, 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 create ever larger grains that will eventually turn into the bedrock that makes up Titan’s plains.
The grains of sand are also needed to form labyrinths of lunar landforms near the poles. The researchers think that these distinct rocks may look like limestone karsts on Earth, but on Titan they are avalanches of dissolved organic sandstones.
River flows and downpours occur much more frequently near the poles, so sediments are carried more by rivers than by winds. A similar process of sintering and abrasion during river transport may provide a local supply of large grains of sand, the source of the sandstones that labyrinths are thought to be composed of.
“We show that there is an active sedimentation cycle on Titan – like on Earth and as it used to be on Mars – that can explain the latitudinal distribution of landscapes by episodic attrition and sintering driven by the seasons on Titan,” Lapoutre said. “It’s very exciting to think that there is an alternate world so far out there where everything is so different yet so similar at the same time.”
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