When it comes to microbial life on Earth, there really is no place like home
At first glance, the ocean-filled world of Enceladus (a moon of Saturn) may seem like an enticing place for a microbe (a tiny, single-celled life form) from Earth. Enceladus has an outer layer of ice that is miles-thick, but underneath is a deep ocean full of tasty organic molecules and gases that microbes could eat. But if we chose the best microbe for the job, and sent it on a micro spaceship (which don’t exist, but one can dream) to splash into Enceladus’ ocean, that microbe would starve, having found some but not all of the nutrients it needed to survive.
In fact, at least 307 of the hardiest microbes would have a tough time surviving. In a recent study, Harrison B. Smith and collaborators investigated which microbes from Earth would be able to grow in Enceladus’ ocean. The goal was not to send Earth’s microbes to invade Enceladus’ ocean but to understand if life similar to Earth’s microbes might already live there.
Because Enceladus’ ocean is hidden beneath the moon’s icy surface, we cannot observe the ocean directly. However, cracks in Enceladus’ icy shell expose sections of the ocean to the vacuum of space, causing huge eruptions of gases and frozen ocean water to spew out into space, where spacecraft can (and have) analyzed them. From measurements of these eruptions, we have a general sense of the ocean’s temperature and acidity. These are the conditions that Smith et al. used to define which earthly microbes they would consider in their study.
With this list of microbes in mind, Smith et al. used a concept called ‘network theory’ to explore how life can create a variety of molecules that it needs to grow from just a handful of starting molecules. This method is like looking at a microbe as a factory, where different chemical reactions that the microbe performs are like different factory lines. Each factory line has a different purpose, a different product it makes, but each line can only turn on when it has the right materials for its job. Some factory lines rely on the products of other lines, so they can only turn on if the others are already running. The authors of the paper used computer models of these microbial ‘factories’ and fed the model microbe a laundry list of everything that is expected to be in Enceladus’ ocean. The hope was that some factory lines would be able to use those initial molecules and turn on, producing new molecules. The new molecules would allow other factory lines to turn on, and so on and so forth.
How did the authors figure out if their chosen microbes would survive? They came up with a list of key molecules that are necessary for growth, and survival. If a microbe (or group of microbes) could scrounge together those key molecules from the laundry list of what we know to be on Enceladus, then it would survive. But the authors found that none of the microbes would be able to produce the key molecules necessary to survive.
It’s not all bad news for microbial life on Enceladus. The study also revealed what was missing from the laundry list of molecules that could have helped the microbes grow, such as phosphate, iron, and more complex molecules like amino acids. If these molecules are found in the plumes in the future, then Earth’s microbial life might have a chance of survival in Enceladus’ ocean.
But until then, earthly microbes, better to postpone your trip.
(Based on Smith et al.’s 2021 research article published in Astrobiology)
Lucas Fifer is a graduate student and researcher in the University of Washington’s Department of Earth and Space Sciences, and Astrobiology Program. His research focus is on the subsurface ocean of Enceladus (a moon of Saturn), and whether this ocean could support extraterrestrial life.