Saturn’s largest moon Titan may be among the most Earth-like worlds in the solar system, covered in lapping rivers and lakes, but new research suggests it likely couldn't support much life — if at all.
Scientists are interested in Titan because it appears to have the organic ingredients for life — the kinds that humans know about, at least. Whether the moon harbors any microbial aliens has become a top exploration priority, helping to spur NASA's $3.35 billion Dragonfly mission. The helicopter-like robotic spacecraft is expected to visit the moon in the 2030s.
A new study led by the University of Arizona and Harvard University tried to answer that question without the 880 million-mile space trip. Their goal was to figure out how much energy life might be able to get from Titan’s environment.
What they found was unexpected: Titan’s underground ocean could probably only support a few pounds' worth of life, equal to about one small dog.
"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 Antonin Affholder, who co-led the study, in a statement.
The study, published in The Planetary Science Journal, focused on Titan’s hidden ocean. While those at its surface are made of liquid methane and ethane, the underground ocean, which lies beneath 60 to 120 miles of ice and could be 300 miles deep, is believed to be water.
The research relied on a computer simulation method known as bioenergetic modeling, a way of calculating how much energy living things would need and how much food is available. But rather than imagining some foreign form of biochemistry, the scientists based their model on fermentation, a simple and well-known metabolic process on Earth.
Fermentation is how yeast helps bread rise and bacteria spoils food. Unlike respiration, which requires oxygen, fermentation only needs organic material. As Titan lacks atmospheric oxygen, the researchers reasoned fermentation makes a good candidate for alien life.
Then they looked closely at glycine, a building block for proteins that is also found in comets, asteroids, and other cosmic objects. Since Titan has similar ingredients, the researchers wondered if tiny microbes could survive on glycine as a food source.
But there's a pretty big obstacle. Most of Titan's organic material is at the surface, not in the underground ocean. Though this same team of researchers has previously suggested that meteorites slamming into the moon could create small pools of water that then sink through the ice carrying nutrients, the amount would be limited.
All this is to say that, although Titan has lots of organic material, it doesn't necessarily mean it's full of life. And if it does have many living things, the chances of finding them could be like searching for a needle in a haystack.
Granted, there were limitations to the study. The researchers only focused on glycine, and even primitive life may be able to metabolize a variety of other molecules. Plus, they only considered one potential habitable environment at Titan. Furthermore, there may be other forms of alien metabolisms that Earthlings don't even know about.
Still, the purpose of the study was to argue for a more nuanced consideration of Titan's potential for life, Affholder said.
"There has been this sense that because Titan has such abundant organics, there is no shortage of food sources that could sustain life," Affholder said. "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."