There could be about a dog’s worth of microbes on Titan. That’s one conclusion from a recent study that refused to rule out the possibility that Saturn’s moon Titan could have microbial life.

The key process, researchers say, is a basic process called glycine fermentation.

“In our study, we focus on what makes Titan unique when compared to other icy moons: its plentiful organic content,” said Antonin Affholder, who is a postdoctoral research associate at the University of Arizona.

Fermentation on Titan

You’ve probably heard of Europa and Enceladus as moons that could hold life. These icy moons have subsurface oceans with hydrothermal vents similar to those teeming with life on Earth. But what if a very different kind of life could be hiding on Titan? That’s the question scientists led by Antonin Affholder of the University of Arizona set out to explore in a new study published in The Planetary Science Journal.

At a glance, Titan is a frozen world, cloaked in a golden haze. It’s cold, unforgiving, and with a weird chemistry. But in some ways, Titan is one of the most Earth-like bodies in the solar system. It has a thick nitrogen-rich atmosphere and surface lakes and rivers (albeit of methane and ethane). This liquid surface hosts a diverse organic chemistry driven by sunlight interacting with atmospheric gases.

Organic molecules alone, however, don’t mean life also exists. You have the building blocks, but whether the processes that arrange these bricks into living creatures took place is unclear. Then there’s another problem. Titan’s atmosphere doesn’t really have oxygen. So, even if life forms emerged, could they have survive?

Life needs energy to grow, reproduce, and maintain itself over time. Without a steady energy supply, even the simplest life would fizzle out. On Earth, that energy usually comes from sunlight (photosynthesis) or from redox reactions using compounds like oxygen, nitrate, or sulfate (respiration). None of those are guaranteed on Titan. So, instead, Affholder focused on one of the most primitive processes that could support life: fermentation.

Fermentation is what microbes do when oxygen isn’t around. It’s simple, ancient, and doesn’t require external electron acceptors. Glycine fermentation in particular is like a minimalist survival trick: it lets microbes squeeze out just enough energy from glycine to stay alive and replicate — albeit slowly.

“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, adding that life on Earth could have first emerged as feeding on organic molecules left over from Earth’s formation. 

“We asked, could similar microbes exist on Titan?” Affholder said. “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?”

Modelling fermentation

It’s intriguing to assume that microbes on Titan could survive with fermentation, but does it pass any scrutiny?

To answer that question, the researchers turned to bioenergetic modeling. This means crunching the numbers on whether the fermentation of organic molecules in Titan-like conditions could, in principle, support microbial life.

Affholder and colleagues focused on glycine, the simplest amino acid and one of the most easily produced in prebiotic chemistry. It shows up in meteorites, comets, and even in lab experiments simulating Titan’s atmosphere. If any molecule could serve as a starter fuel for simple life on Titan, glycine is a good candidate.

The researchers found that across a range of plausible ocean chemistries and temperatures, microbes could get enough energy from the reaction. But there’s a catch: it only works well if glycine concentrations are high, which is a relatively optimistic assumption. Lower concentrations drastically reduce the energy available.

Even in the best-case scenario, where glycine is delivered in fairly generous amounts, the size of the hypothetical biosphere would be… tiny.

How tiny? Affholder’s team estimates that Titan’s entire ocean — enough water to fill multiple Earth oceans — could support only 1014 to 1017 cells. That’s just a few kilograms of biomass.

“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 — equivalent to the mass of a small dog,” Affholder said. “Such a tiny biosphere would average less than one cell per liter of water over Titan’s entire vast ocean.”

So, if life exists on Titan, it might not resemble the thriving, teeming ecosystems we know. Instead, think of isolated oases — tiny, slow-growing communities, perhaps tucked away in warmer pockets, clustered around deposits of concentrated organics.

It’s not life as you know it. But it’s life as it might be.

The study was published in The Planetary Science Journal.