Titan Most Likely Non-Habitable, Astrobiologists Say

Saturn’s moon Titan has an organic-rich atmosphere and surface with a subsurface ocean that may represent a habitable environment. In a new study, astrobiologists determined the amount of organic material that can be delivered from Titan’s surface to its ocean through impact cratering. Unless biologically available compounds can be sourced from Titan’s interior, or be delivered from the surface by other mechanisms, their calculations suggest that even the most organic-rich ocean world in the Solar System may not be able to support a large biosphere.

The identification of life in the outer Solar System is a significant area of interest for planetary scientists, astronomers and government space agencies like NASA, largely because many icy moons of the giant planets are thought to have large subsurface oceans of liquid water.

Titan, for example, is thought to have an ocean beneath its icy surface that is more than 12 times the volume 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 of interest when looking for extraterrestrial life,” said first author Professor Catherine Neish, an astrobiologist at the University of Western Ontario.

In the study, Professor Neish and her colleagues 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 cratering.

Comets impacting Titan throughout its history have melted the surface of the icy moon, creating pools of liquid water that have mixed with the surface organics.

The resulting melt is denser than its icy crust, so the heavier water sinks through the ice, possibly all the way to Titan’s subsurface ocean.

Using the assumed rates of impacts on Titan’s surface, the researchers determined how many comets of different sizes would strike Titan each year over its history.

This allowed them to predict the flow rate of water carrying organics that travel from Titan’s surface to its interior.

The authors found the weight of organics transferred in this way is quite small, no more than 7,500 kg/year of glycine, the simplest amino acid, which makes up proteins in life.

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

“In the past, people often assumed that water equals life, but they neglected the fact that life needs other elements, in particular carbon.”

Other icy worlds in the Solar System have almost no carbon on their surfaces, and it is unclear how much could be sourced from their interiors.

“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,” Professor Neish said.

The team’s paper was published online this month in the journal Astrobiology.

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Catherine Neish et al. Organic Input to Titan’s Subsurface Ocean Through Impact Cratering. Astrobiology, published online February 2, 2024; doi: 10.1089/ast.2023.0055