“Such a tiny biosphere would average less than one cell per liter of water over Titan’s entire vast ocean,” said Antonin Affholder, a postdoctoral research associate at the University of Arizona. The surprising discovery, published in the latest issue of The Planetary Science Journal, underscores the difficulty of finding life on Saturn’s largest moon, Titan. These exotic ingredients may remain too thin on Titan to allow for all but the most speculatively small mass of biomass the total meat-weight of a dog on the moon at all, despite its extensive organic chemistry and strange landscapes.
The allure of Titan lies within its extreme dichotomies. Rivers and lakes of liquid methane crisscross its surface, icy boulders dot its landscape, and soot-like dunes loom in the horizon. Below this frozen surface is a subsurface ocean, which is thought to be around 300 miles deep, that intrigues scientists. Does this hidden cache hide any life? So researchers helmed by Affholder and based at Harvard University wanted to try to answer this question, with an eye toward just how organic Titan’s permafrost environment might be relative to that of other icy moons.
The study, a back-to-basics exploration of fermentation a metabolic process that needs organic molecules, but not oxygen. This detour around the complications of respiration is well suited to the chemical environment on Titan. The team zeroed in on glycine, the most basic building block of proteins, which is prevalent in primordial matter across the solar system.
Using bioenergetic modeling, the researchers found that while Titan’s subsurface ocean could in theory support glycine-consuming microbes, the prospects would be poor. Glycine needs to move from the surface, through the thick icy shell, before reaching the ocean and exchanges between surface and subsurface are somewhat limited. Meteorite strikes can also create transient melt pools that help facilitate such transfer, but according to computer simulations, not enough glycine is available to support more than a couple of kilograms of microbial biomass.
Or, as Affholder went on, “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.” This finding challenges a previous assumption that Titan’s abundant supply of organics would naturally provide life with plenty to eat. Instead, the study makes the case for nuanced understanding, underscoring the complexity of Titan’s biochemical domain.
These results are important, with implications. Oddly, however, one of the only worlds we suspect might one day be “Earthlike on the surface, ocean world on the inside,” Titan, has always been a top target for exploration when we think astrobiology. NASA’s Dragonfly mission is set to launch in 2028 and arrive at Titan in 2034 and will study the moon’s surface and atmosphere for signs of life. But discovering life inside Titan’s subsurface ocean might be like trying to locate a needle in a haystack, according to the study. On average, the density of life forms may even be under a cell per liter of water making detection an incredible challenge.
Yet Titan, with all its challenges, continues to enthrall scientists and space enthusiasts alike. Its dense, hazy atmosphere and abundance of organic chemistry have made scientists wonder whether life could gain a foothold there. Affholder’s research, which has been assisted by the International Space Science Institute in Bern, Switzerland, gives a precarious view of Titan’s habitability. “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,” he said.
The findings of this study will stimulate additional questions about the type of life beyond the planet Earth as investigations of Titan persist. Just how could life endure in such an extreme setting? What do organic molecules do when they hit Titan’s icy crust and subsurface ocean? And what do Titan’s limited biosphere tell us about the broader search for extraterrestrial life?
The answers to these questions, however, could revolutionize how we define habitability in the universe. Titan’s beguiling landscapes and chemical idiosyncrasies have a way of reminding us that life if it exists elsewhere in the cosmos could take on shapes and habitats as far from the reach of our terrestrial minds as the stars themselves. With Titan’s subsurface ocean for now a glimmer of possible hope a little biosphere that invites us to look deeper, think harder and go farther.
