Is Titan’s subsurface ocean habitable?

Titan's subsurface ocean: Planet-like body half in shadow with lakes of liquid near its North Pole.
View larger. | Titan is well-known for its lakes and seas of liquid methane and ethane. This image from Cassini in 2017 shows light glinting off the lakes. But Titan is also thought to have an ocean of liquid water deep underground. Is Titan’s subsurface ocean habitable? Image via NASA/ JPL-Caltech/ University of Arizona/ University of Idaho.

Saturn’s large moon Titan teems with liquid. We’ve long known about its liquid methane and ethane lakes and seas. More recent evidence suggests a subsurface ocean of liquid water. Could Titan’s underground ocean be habitable? Earlier this year, a new study suggested it likely isn’t. The study said there probably isn’t enough organic material transferring from Titan’s surface to the ocean below to sustain life.

Astrobiologist Catherine Neish and her colleagues at Western University in Ontario, Canada published their peer-reviewed findings in the journal Astrobiology on February 2, 2024.

What does habitability mean to astronomers?

Is Titan’s subsurface ocean habitable?

NASA’s Cassini spacecraft found evidence that Titan has a deep ocean beneath its outer icy crust. This is similar to other moons such as Europa, Enceladus, Ganymede and others. But is it habitable, by earthly standards? Even with water, life still requires a source of heat, organic material and chemical nutrients. And all life on Earth uses water as a solvent to develop in. Neish said:

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.

Cometary impacts

We don’t yet know the exact conditions in Titan’s subsurface ocean. In the new study, however, Neish and her colleagues wanted to test how much organic material can make it from Titan’s surface down into the ocean. Organics, of course, including amino acids, are essential building blocks of life on Earth. The researchers used impact cratering data to determine how much organic material might be in Titan’s ocean. Those organics originated from the impacts on the surface.

Titan is blanketed in organics, with its hydrocarbon dunes, lakes and seas. Even its atmosphere is filled with a thick hydrocarbon. But does any of that organic material makes it down into the ocean? Impacts from comets – which can also have their own organics – can temporarily melt the icy surface. The meltwater could then sink through the ice.

The researchers estimated how many comets have impacted Titan throughout its history. Knowing this, the team could then estimate how much water has flowed from the surface down through the ice, possibly all the way to the ocean.

Not enough organics for life in Titan’s subsurface ocean

As it turned out, the results suggest that there wouldn’t be enough organics getting into the ocean to make life feasible. There would only be about 16,000 pounds (7,500 kg) per year of glycine, the simplest amino acid. That’s about the same mass as a single African male elephant. Amino acids are essential as they are the building blocks of proteins. Neish said:

One elephant per year of glycine into an ocean 12 times the volume of Earth’s oceans is not sufficient to sustain life. 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.

The paper stated:

Unless biologically available compounds can be sourced from Titan’s interior, or be delivered from the surface by other mechanisms, our calculations suggest that even the most organic-rich ocean world in the solar system may not be able to support a large biosphere.

Still much to learn about Titan’s subsurface ocean

It would be disappointing if the ocean isn’t well-suited for life. But Titan is still a fascinating world, with plenty of prebiotic chemistry occurring on its surface and in its atmosphere. As Neish noted:

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.

NASA’s upcoming Dragonfly mission to Titan will be able to sample in spots where meltwater from impacts has mixed with the ice. Dragonfly is currently scheduled to launch in 2026 and arrive in 2034. Neish continued:

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.

Moon-like sphere with cutaway view showing various layers inside it, and planet with edge-on rings in background.
View larger. | Titan has a global subsurface ocean of water, represented here by the dark blue layer. Image via NASA.

Pessimism about other ocean moons

The study puts forth a pessimistic view of life on the other ocean moons in the solar system as well. As Neish explained:

Unfortunately, we will now need to be a little less optimistic when searching for extraterrestrial lifeforms within our own solar system. The scientific community has been very excited about finding life in the icy worlds of the outer solar system, and this finding suggests that it may be less likely than we previously assumed.

The study argues that other ocean moons such as Europa and Enceladus have less organics and carbon on their surfaces to begin with. Therefore, they might have even less organics in their oceans.

Other moon’s oceans may still be habitable

However, other studies have pointed to those two oceans, Enceladus in particular, as being promisingly habitable. In the case of Enceladus, the Cassini spacecraft detected a variety of organic molecules in the water vapor plumes, which originate from the ocean below. There is also evidence for hydrothermal vents on the ocean floor, which would provide heat and nutrients. Enceladus’ ocean even contains phosphorus, another key building block of life.

Last September, researchers said that carbon dioxide ice deposits on Europa’s surface likely originated from its internal ocean. So at least in one way, the scenario is opposite that of Titan. The organic carbon rises to the surface through cracks, instead of sinking down through the ice. This shows that organics can indeed be present in such underground oceans, without having to get there from the surface. The organics in Enceladus’ plumes also suggest this.

Bottom line: Saturn’s largest moon Titan has an underground ocean of water. But is Titan’s subsurface ocean habitable? A new study casts doubt.

Source: Organic Input to Titan’s Subsurface Ocean Through Impact Cratering

Via Western University

Read more: Titan’s magic islands appear and disappear in liquid seas

Read more: Did Europa’s carbon dioxide come from its ocean?

April 5, 2024

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