Saturn’s largest moon likely uninhabitable

A study led by Western astrobiologist Catherine Neish shows that the underground ocean of Titan, Saturn’s largest moon, is most likely a non-habitable environment, meaning any hope of finding life in the icy world is dead in the water.

The discovery means it is much less likely that space scientists and astronauts will ever discover life in the outer solar system, home to the four “giant” planets: Jupiter, Saturn, Uranus and Neptune.

“Unfortunately, we will now have to be a little less optimistic when searching for extraterrestrial life forms in our own solar system,” said Neish, professor of earth sciences. “The scientific community is very excited about discovering life on the icy worlds of the outer solar system, and this discovery suggests that this may be less likely than we previously thought.”

Identifying life in the outer solar system is an important area of ​​interest for planetary scientists, astronomers and government space agencies like NASA, largely because many of the giant planets’ icy moons are thought to have large underground oceans of liquid water. Titan, for example, would have an ocean beneath its icy surface whose volume is more than 12 times greater than that of Earth’s oceans.

“Life as we know it here on Earth needs water as a solvent, so planets and moons with lots of water are interesting for the search for extraterrestrial life,” said Neish, a member of the Institute of Western’s exploration of Earth and space.

In the study, published in the journal AstrobiologyNeish and his collaborators attempted to quantify the amount of organic molecules that could be transferred from Titan’s organic-rich surface to its subsurface ocean, using data from impact craters.

Comets impacting Titan throughout its history melted the moon’s icy surface, creating pools of liquid water that mixed with organic matter on the surface. The resulting melt is denser than its icy crust, so heavier water flows through the ice, perhaps to Titan’s underground ocean.

Using assumed impact rates on Titan’s surface, Neish and his collaborators determined how many comets of different sizes would hit Titan each year over its history. This allowed researchers to predict the flow of water carrying organic matter moving from Titan’s surface to its interior.

Neish and the team found that the weight of organic matter transferred in this way is quite small, no more than 7,500 kg/year of glycine, the simplest amino acid that makes up the proteins of life. That’s about the same mass as a male African elephant. (All biomolecules, like glycine, use the element carbon as the backbone of their molecular structure.)

“One elephant per year injecting wisteria into an ocean 12 times the volume of Earth’s oceans is not enough to sustain life,” Neish said. “In the past, people often thought that water was synonymous with life, but they overlooked the fact that life requires other elements, especially carbon.”

Other icy worlds (like Jupiter’s moons Europa and Ganymede and Saturn’s moon Enceladus) have almost no carbon on their surfaces, and it’s unclear how much might come from their interiors. Titan is the most organically rich icy moon in the solar system, so if its subterranean ocean isn’t habitable, that doesn’t bode well for the habitability of other known icy worlds.

“This work shows that it is very difficult to transfer carbon from Titan’s surface to its subsurface ocean. Basically, it is difficult to have both the water and the carbon needed for life in the same place.” , Neish said.

The flight of the dragonfly

Despite this discovery, there is still much to learn about Titan, and for Neish, the big question is: what is it made of?

Neish is a co-investigator on NASA’s Project Dragonfly, a space mission planned for 2028 aimed at sending a robotic rotorcraft (drone) to the surface of Titan to study its prebiotic chemistry, or how organic compounds formed and self-organized for the origin of life. on Earth and beyond.

“It is almost impossible to determine the composition of Titan’s organic-rich surface by looking at it with a telescope through its organic-rich atmosphere,” Neish said. “We need to land there and sample the surface to determine its composition.”

To date, only the Cassini-Huygens international space mission, in 2005, has succeeded in landing a robotic probe on Titan to analyze samples. It is the first spacecraft to land on Titan and the farthest landing from Earth that a spacecraft has ever made.

“Even though the subsurface ocean is not habitable, we can learn a lot about the prebiotic chemistry of Titan and Earth by studying reactions on Titan’s surface,” Neish said. “We would really like to know if any interesting reactions occur there, particularly where organic molecules mix with liquid water generated during impacts.”

When Neish began her latest study, she worried it would negatively impact the Dragonfly mission, but it actually led to even more questions.

“If all the melt produced by the impacts buried itself in the ice crust, we would not have samples near the surface where water and organic matter mixed. These are regions where Dragonfly could search the products of these prebiotic reactions, teaching us how life can arise on different planets,” Neish said.

“The results of this study are even more pessimistic than I thought regarding the habitability of Titan’s surface ocean, but it also means that there are more interesting prebiotic environments near Titan’s surface , where we can sample them with Dragonfly’s instruments.”