Titan may harbor life, but it will be difficult to find

Titan, the largest moon of Saturn, has a surface that is covered with rivers and lakes of liquid methane. It’s dotted with icy rocks and soot-like dunes – all shrouded beneath a dense, hazy atmosphere.

This strange mix has fascinated scientists and led them to wonder whether life could exist there.

Researchers from the University of Arizona and Harvard University recently set out to explore that possibility. Their goal was to paint a more realistic picture of what life might look like on Titan, where it might survive, and how much of it could exist.

Rich in organics, but are they edible?

“In our study, we focus on what makes Titan unique when compared to other icy moons: its plentiful organic content,” said Antonin Affholder from the U of A Department of Ecology and Evolutionary Biology.

Titan has an abundance of carbon-based molecules present on its surface. They have been formed by complex atmospheric chemistry, but can these molecules support life?

To investigate, the experts used bioenergetic modeling. They examined Titan’s subsurface ocean – believed to be about 300 miles (485 kilometers) deep – and considered whether microorganisms could survive there by consuming organic compounds.

The findings suggest, even if life were possible, Titan could likely support no more than a few pounds of biomass – hardly more than the weight of a small dog.

The limits of a methane world

Titan has been described as being Earth-like on the surface with an ocean world on the inside. The moon is the focus of NASA’s upcoming Dragonfly mission.

Past estimates of its habitability have focused largely on the availability of organic material, but the researchers argue that this approach oversimplifies the situation.

“There has been this sense that because Titan has such abundant organics, there is no shortage of food sources that could sustain life,” noted Affholder.

“We point out that not all of these organic molecules may constitute food sources, the ocean is really big, and there’s limited exchange between the ocean and the surface, where all those organics are, so we argue for a more nuanced approach.”

Could fermentation fuel life on Titan?

Instead of relying on exotic or speculative ideas, the team went with one of the oldest, simplest forms of metabolism: fermentation.

This process doesn’t need oxygen and only requires organic molecules. It’s what powers breadmaking, beer brewing, and even food spoilage here on Earth.

“Fermentation probably evolved early in the history of Earth’s life, and does not require us to open any door into unknown or speculative mechanisms that may or may not have happened on Titan,” Affholder said.

He explained that early life on Earth may have survived on organic leftovers from our planet’s formation.

“We asked, could similar microbes exist on Titan? If so, what potential does Titan’s subsurface ocean have for a biosphere feeding off of the seemingly vast inventory of abiotic organic molecules synthesized in Titan’s atmosphere, accumulating at its surface and present in the core?”

A small population of microbes

The team focused on glycine, the simplest amino acid known, as a possible food source for any hypothetical microbes.

“We know that glycine was relatively abundant in any sort of primordial matter in the solar system,” said Affholder. “When you look at asteroids, comets, the clouds of particles and gas from which stars and planets like our solar system form, we find glycine or its precursors in pretty much all those places.”

But simulations showed that only a small amount of Titan’s organics could actually serve as fuel for life.

The idea is that glycine from the surface might reach the subsurface ocean through Titan’s thick ice shell. This could happen when meteorite impacts melt part of the ice, allowing surface material to sink.

“Our new study shows that this supply may only be sufficient to sustain a very small population of microbes weighing a total of only a few kilograms at most. Such a tiny biosphere would average less than one cell per liter of water over Titan’s entire vast ocean,” explained Affholder.

Life on Titan would be extremely rare

If there is life in Titan’s ocean, it may be incredibly difficult to detect. The experts noted that even a future mission might miss it entirely unless researchers look in the right place – or consider other, less obvious habitats.

“We conclude that Titan’s uniquely rich organic inventory may not in fact be available to play the role in the moon’s habitability to the extent one might intuitively think,” said Affholder.

So, while life on Titan is possible, it is highly unlikely to be abundant. Despite not teeming with life, the planet may still host a tiny, ancient microbial world – waiting, perhaps, for a spacecraft to find it.

The full study was published in the journal The Planetary Science Journal.

Image Credit: NASA/JPL-Caltech/University of Arizona/University of Idaho

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