Saturn's moon Titan was once suggested to possibly contain hidden life inside its huge subsurface ocean, but these hopes may have been scuppered by a new discovery.
Titan is likely uninhabitable from its surface to its enormous subsurface ocean, a new paper in the journal Astrobiology reveals. This comes as scientists found that there simply aren't enough amino acids in Titan's ocean to sustain life.
"In this study, we quantified the amount of organic molecules that could be transferred from Titan's organic-rich surface to its subsurface ocean through impact cratering. We found that the amount of organics that is transferred in this way is quite small – no more than 7500 kg/year of glycine in the most optimistic scenario. This is not sufficient to sustain life in Titan's subsurface ocean," paper co-author Catherine Neish, an Earth sciences and astrobiology professor at the University of Western Ontario, told Newsweek.
Titan is the largest moon of Saturn and the second-largest moon in the entire solar system after Jupiter's Ganymede. It's about 50 percent larger than Earth's moon, with an atmosphere mostly made of nitrogen and small amounts of methane and hydrogen.
Titan was hoped to be home to some form of extraterrestrial life in its huge subsurface ocean, which is composed of salty liquid water, which is coated by an outer crust of water ice. Titan also has oceans on its surface, but they are made from liquid methane and ethane rather than water.
These chemicals can exist in liquid form on Titan's surface due to the moon's extremely low temperatures and high atmospheric pressure, which is between 50 and 60 percent higher than that of Earth's surface pressure.
"Life as we know it here on Earth needs water as a solvent, so planets and moons with lots of water are of interest when looking for extraterrestrial life," said Neish.
The study attempted to quantify the amount of organic molecules, including amino acids, in the ocean, using data from the impact craters across Titan's icy surface. These impacts would have melted the surface at the crater, creating lagoons of liquid water that would have mixed with organic chemicals on the surface, sinking into the ocean through the ice.
Therefore, the rate of impacts on Titan's surface can inform the rate of water mixed with organic material flowing into the ocean.
The researchers found that only around 7,500 kilograms or 16,500 pounds of glycine—a simple amino acid needed for life—were transferred into the ocean every year. This amounts to about the weight of an African elephant.
"One elephant per year of glycine into an ocean 12 times the volume of Earth's oceans is not sufficient to sustain life," Neish said in a statement. "In the past, people often assumed that water equals life, but they neglected the fact that life needs other elements, in particular carbon."
"This work shows that it is very hard to transfer the carbon on Titan's surface to its subsurface ocean—basically, it's hard to have both the water and carbon needed for life in the same place."
This finding dooms Titan as a major candidate for finding life outside of our own planet and may also condemn other icy moons in the outer solar system, including Jupiter's Europa and Ganymede and Saturn's Enceladus.
Titan also has lakes of methane on its surface, though these are also not thought to contain life.
"I think it is unlikely that Titan hosts any life in its lakes of liquid methane. Methane is non-polar, and this makes it a poor solvent for life. It is also extremely cold (~90 K). Since reaction rates drop off exponentially with temperature, this would severely impede Titanian biochemistry," Neish said.
"In addition, any life that uses methane as a solvent would be very different to life on Earth, and it would therefore be very difficult to detect. I am, however, hopeful that we may be able to study prebiotic chemistry on Titan's surface, in locations where the icy crust has melted and mixed with organics before freezing again (like lava flows)."
Despite this disappointing news, the researchers hope to further investigate how much organic material makes it into the ocean of Titan via an exploratory mission. NASA's Dragonfly project, which will send a Dragonfly quadcopter to the surface of Titan, is planned to jet off in 2028 and will gather data on the moon's chemistry.
"It is nearly impossible to determine the composition of Titan's organic-rich surface by viewing it with a telescope through its organic-rich atmosphere," said Neish. "We need to land there and sample the surface to determine its composition."
Dragonfly will mark the second-ever lander on Titan's surface, after the Cassini–Huygens probe was landed in 2005.
"Even if the subsurface ocean isn't habitable, we can learn a lot about prebiotic chemistry on Titan, and Earth, by studying the reactions on Titan's surface. We'd really like to know if interesting reactions are occurring there, especially where the organic molecules mix with liquid water generated in impacts," said Neish.
"If all the melt produced by impacts sinks into the ice crust, we wouldn't have samples near the surface where water and organics have mixed. These are regions where Dragonfly could search for the products of those prebiotic reactions, teaching us about how life may arise on different planets.
"The results from this study are even more pessimistic than I realized with regards to the habitability of Titan's surface ocean, but it also means that more interesting prebiotic environments exist near Titan's surface, where we can sample them with the instruments on Dragonfly," she said.
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Update 2/20/2024, 12:38 p.m. ET: This article was updated with comment from Catherine Neish.
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