Fifteen years ago this month, a small spacecraft gently touched down on a strange, new world. The European Space Agency’s Huygens lander had hitched a ride to Saturn’s largest moon, Titan, on the back of NASA’s Cassini mission, which orbited Saturn for more than a decade.
Huygens gave earthlings an unprecedented view of a world much like—and unlike—our own. Titan is the only other place in the solar system with liquid raining from clouds and pooling on its surface in lakes and rivers, but that liquid is composed primarily of methane and ethane, not water. Titan’s weather patterns refuse to match our models, and its atmosphere remains mysteriously replenished by large, complex, organic molecules.
And then there are Titan’s river deltas. Or, rather, there aren’t the river deltas.
The Missing Deltas
A river delta forms where a river flows into a slower moving or stagnant body of water, often the ocean. As the river’s flow slows, it usually breaks into smaller tributaries and spreads, sometimes creating a classic fan shape while collecting and depositing sediment. Over thousands of years, a nutrient-dense wetland forms, and the deposited sediment keeps a rich record of climate and environmental history.
“On Titan, if the deltas are there, the sediment is there, and they hold the record of Titan’s climate,” said Samuel Birch, a planetary scientist at Cornell University in Ithaca, N.Y., who presented the conundrum on 10 December at AGU’s Fall Meeting 2019 in San Francisco, Calif. Studying river deltas on Titan could help scientists figure out how the moon’s mysterious atmosphere came to be.
Studying deltas would help scientists figure out how organic molecules cycle through the moon’s atmosphere, rivers, seas, and subsurface and possibly into its internal water ocean, said Michael Malaska, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who wasn’t involved in the research. “Way down there, [organic molecules] might mix into the liquid water ocean or in brine micropockets in deep ice and create a potential habitat for life deep inside Titan.”
On Earth, structures like the Nile River delta and the Mississippi River delta are easy to spot with satellites. Scientists have even found river deltas on Mars, which helped us realize that Mars was once a much more Earth-like planet complete with flowing water—and possibly life. Next year, NASA’s Mars 2020 rover will land in a crater where one of these deltas exists to hunt for signs of past (or present) life.
On Earth, “Whenever you see a big river meet a big, open body of liquid, there’s almost always a delta sitting on the end of it,” Birch said. But on Titan, these geological formations are missing.
Through Cassini’s Eyes
Birch first wondered whether Cassini’s instruments weren’t powerful enough to image the structures. Although the spacecraft performed more than 100 close flybys of the moon, its instruments weren’t built for close surveys of landscape features. Because Titan is swathed in a thick haze of organic molecules, Cassini scientists relied on radar measurements, which could travel through the haze unperturbed, to image the moon’s surface.
To find out whether Cassini would have been able to see deltas on Titan, Birch turned Cassini’s “eyes” toward Earth. He took satellite images of large deltas around the globe and changed their resolution to mimic what Cassini might see if it had flown by Earth. It turned out that yes, Cassini could see Earth’s larger deltas, including the Nile’s, which stretches 250 kilometers across at Egypt’s northern coast. Titan’s rivers are many kilometers across, akin to some of Earth’s largest rivers. They should be creating large deltas, Birch said. So where are those deltas?
Birch and his colleagues wonder whether the different densities of the rivers and seas preclude a delta. If you compare the two, Titan’s methane and ethane seas are warmer than its rivers, which means the river’s methane will absorb more atmospheric nitrogen. More dissolved nitrogen means that the river could be up to 50% heavier than the sea. So when the colder, heavier river meets the lighter, warmer sea, “the river will keep going in the subsurface like the sea is not even there,” Birch said. So even if the river is carrying sediment, it won’t slow enough to deposit that sediment in the classic shape of a fan (also called an arcuate) delta.
Another hypothesis suggested for Titan’s missing deltas has to do with the instability of Titan’s shorelines, Birch said. Over the course of the Cassini mission, scientists spotted all the lakes and seas in Titan’s northern hemisphere, but they also observed large basins in the southern hemisphere. These observations suggest that “the south once had liquid and looked like the north and that the poles are swapping the fluids,” Birch said. Pole-swapping liquids could be occurring over 100,000-year timescales.
The movement of shorelines could smear any sediment that was laid down by rivers. “You’ll still form a deposit, but not something recognizable as a delta,” Birch said.
Birch and his colleagues are planning to test the density hypotheses in the lab, using water rather than liquid hydrocarbons (which combust easily). They’ll be able to change the density of water by adding sediment or salt or changing its temperature to see how two different densities of water interact with each other.
Birch and his colleagues’ work “is fundamental research on how geological processes can work using totally different materials,” said Malaska. “On Earth, we have silicates and water—that’s one set of parameters. On Titan, we have organic molecules and ice in methane with dissolved nitrogen. So we can compare and contrast and figure stuff out.”
Luckily, we’ll soon be getting a closer look at Titan through the eyes of a new spacecraft, Dragonfly. Just selected as NASA’s next New Frontiers–class mission, the Dragonfly spacecraft will not only land on Titan but also fly around just above its surface, exploring much of the moon’s lakes, dunes, and rivers up close. The mission is slated to launch in 2026 and would land on Titan in 2034.