Saturn’s moon Titan looks a lot like Earth from space, with rivers, lakes and seas filled with rain falling through a thick atmosphere. While these landscapes may look familiar, they are made of materials that are certainly different: currents of liquid methane cut across Titan’s icy surface, and nitrogen winds build hydrocarbon sand dunes.
The presence of these materials, whose mechanical properties are very different from those of the silicate-based substances that form other known sedimentary bodies in our solar system, makes the formation of Titan’s landscape enigmatic. By identifying a process that would allow hydrocarbon-based substances to form grains of sand or bedrock depending on how often winds blow and streams flow, Stanford University geologist Mathieu Lapôtre and colleagues have shown how the various dunes, plains and labyrinthine terrains of Titan could be formed.
Titan, which is a target for space exploration due to its potential habitability, is the only other body in our solar system known to have a similar seasonal liquid transport cycle to Earth today. The new model, published in Geophysical Investigation Letters April 25 shows how that seasonal cycle drives the movement of grains on 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 in the Stanford School of Earth, Energy and Environmental Sciences. “If we understand how the different pieces of the puzzle and their mechanics fit together, then we can start to use the landforms left behind by those sedimentary processes to say something about Titan’s climate or geological history, and how they might affect perspective.” for life on Titan.
a missing mechanism
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 sediments in the planetary body: How can its basic organic compounds, which are thought to be much more brittle than inorganic silicate grains on Earth, morph into grains that form distinct structures rather than just wear off and fly away like dust?
On Earth, silicate rocks and minerals at the surface erode into sediment grains over time, moving through winds and currents to be deposited in layers of sediment that eventually, with the help of pressure, the groundwater, and sometimes heat, turn back into rock. Those rocks then continue through the erosion process and the materials are recycled through the Earth’s layers over geologic time.
On Titan, researchers believe similar processes formed the dunes, plains, and labyrinthine terrain 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 have not been able to show how these organic compounds can be converted into sediment grains that can be transported across the moon’s landscapes and over geologic time.
“As grains are carried by winds, the grains collide with each other and with the surface. These collisions tend to decrease the size of the grains over time. What we were missing was the growth mechanism that could counteract that and allow that the grains of sand maintain a stable size over time”. Lapotre said.
an alien analog
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 extracted from the water column and adheres in layers around a grain, such as quartz.
What makes ooids unique is their formation through chemical precipitation, which allows them to grow, while the simultaneous process of erosion slows growth as waves and storms crush the grains together. These two competing mechanisms balance each other out over time to form a constant grain size, a process the researchers suggest could also be occurring on Titan.
“We were able to solve the paradox of why there could be sand dunes on Titan for so long even though the materials are very weak,” Lapôtre said. abrasion when the winds transport the grains”.
Armed with a hypothesis about sediment formation, Lapôtre and study co-authors used existing data on 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 mid-latitudes and labyrinthine terrain near the poles.
Atmospheric models and data from the Cassini mission reveal that winds are common near the equator, supporting the idea that less sintering and thus fine sand grains, a critical component of dunes, could be created there. . The study authors predict a pause in sediment transport in mid-latitudes on both sides of the equator, where sintering could dominate and create increasingly coarse grains, eventually becoming the bedrock that makes up Titan’s plains.
Sand grains are also necessary for the formation of the moon’s labyrinthine terrain near the poles. The researchers think these distinct ridges might be like karsts in limestone on Earth, but on Titan, they would be collapsed features made of dissolved organic sandstones. River flow and rain storms occur much more frequently near the poles, making sediment more likely to be transported by rivers than by winds. A similar process of sintering and abrasion during river transport could provide a local supply of coarse sand grains, the source of the sandstones thought to form labyrinthine terrains.
“We are 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 seasonally driven sintering.” of Titan,” Lapôtre said. . “It’s quite fascinating to think about how this alternate world exists so far away, where things are so different and yet so similar.”
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Mathieu GA Lapôtre et al, The Role of Seasonal Sediment Transport and Sintering in Shaping Titan’s Landscapes: A Hypothesis, Geophysical Investigation Letters (2022). DOI: 10.1029/2021GL097605
Provided by Stanford University
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