For astrobiologists, the scientists dedicated to the search for life beyond Earth, the moons of Saturn are a virtual treasure trove of possibilities. Enceladus is especially compelling because of the active plumes of water emanating from its southern polar region. Not only are these vents thought to be connected directly to an ocean beneath the moon’s icy surface, but the Cassini mission detected traces of organic molecules and other chemicals associated with biological processes. Like Europa, Ganymede, and other “Ocean Worlds,” astrobiologists think this could indicate hydrothermal activity at the core-mantle boundary.
Both NASA and the ESA are hoping to send missions to Enceladus that could study its plumes in more detail. These include the Enceladus Orbitlander recommended in the Planetary Science and Astrobiology Decadal Survey 2023-2032 and the ESA’s Enceladus Moonraker, which could depart Earth in the next decade, taking advantage of a favorable alignment between the planets. In anticipation of what these missions could find, an international team of researchers used data from the Cassini mission to establish how samples of plume material could constrain how much biomass Enceladus has within it.
The team was led by Antonin Affholder, a post-doctoral researcher at the University of Arizona and the Institute of Biology at the Ecole Normale Superieure, University of Paris. He was joined by researchers from the Paris Observatory’s Institute of Celestial Mechanics and Ephemeral Calculations (IMCCE), the Institut Universitaire de France (IUF), the Observatoire de la Côte d’Azur, and the International Research Laboratory for Interdisciplinary Global Environmental Studies (iGLOBES). The paper that describes their findings was recently published in the Planetary Science Journal.
While the Cassini orbiter revealed a great deal about Enceladus and its surface features, scientists have been speculating about the existence of plume activity. The first hints came from the Voyager 1 and 2 probes, which flew through the Saturn system in November 1980 and August 1981 (respectively). Based on the reflective surface and its location near the core of Saturn’s E ring (largely composed of water ice and silica), scientists speculated that the moon was geologically active and the source of material in the ring (possibly through the venting of water vapor).
The Cassini probe first encountered indications of plume activity during its passage in 2005, which were interpreted as signs of possible cryovolcanism. During its many subsequent encounters, Cassini flew directly through Enceladus’ plumes and obtained data from its spectrometer and magnetometer. Combined with imaging and ionic data, the readings obtained suggested that the southern polar plumes emanated from pressurized subsurface chambers. Since then, multiple studies have been conducted that have attempted to constrain the type of subsurface environment.
This includes numerical simulations showing how water could circulate in the core at high temperatures and detecting hydrogen in the plume. According to Affholder, who spoke to Universe Today via email, these findings point toward possible hydrothermal activity and methanogenesis similar to what has existed here on Earth for billions of years:
“On Earth, hydrogen is generated by hydrothermal processes in the ocean. Additionally, this hydrogen generation in Earth’s ocean supports methanogenesis, a metabolism known to be used by life on Earth very early on. This metabolism is also ubiquity found in ‘anoxic’ (oxygen-poor) environments. The antiquity and ubiquity of methanogenesis, and the elements supporting the existence of hydrothermalism on Enceladus together make the case that methanogenesis could occur in Enceladus’s ocean. This hypothesis might even predate findings of the Cassini mission, as it was suggested in 1999 by Thomas McCollom that methanogenesis could support primary production on Jupiter’s moon Europa which is in many ways similar to Enceladus.”
Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS/NASA/JPL-Caltech/SwRI
In the coming years, a dedicated mission may be sent to Enceladus to investigate its plume activity more closely – like the Enceladus Orbitlander or Moonraker. Upon arrival, this mission will either fly directly through the plumes and/or dispatch a lander to the surface to obtain samples of the plumes’ gas and grains, then analyze them for evidence of potential biosignatures. To get a better idea of what this might look like, Affholder and his colleagues calculated the likely abundance of certain biosignatures in the plume material based on the assumption that a methanogenic ecosystem exists.
“Using these calculations, we deduce the sample size required to capture at least one cell with a 95% chance or more,” said Affholder. “Other than cells, we have also looked at the production of small organic molecules such as amino acids. We used our ecosystem model and the assumption of Earth-like life to predict the concentration of amino acids in the plume under the hypothesis that an ecosystem exists and under the competing hypothesis that these amino acids are produced ‘naturally’ in hydrothermal settings.”
They found that (accounting for uncertainties) capturing at least one cell would require up to 1 milliliter (0.034 fluid ounces) of plume material to be sampled. This may sound like a tiny amount but based on the mission architecture for NASA’s Orbitlander, it would take hundreds of passes through the plume material to obtain this amount. Affholder and his colleagues acknowledge that these results are subject to large uncertainties concerning the chemical and physical processes that might affect the concentration of amino acids in the sample. For starters, Affholder conveyed that their calculations are a bit optimistic:
“Indeed, the calculations assume that most cells retain their structure upon traveling across the ocean in rising warm water and being outgassed into space. A variety of things can happen during this journey to the plume: cells could be destroyed during outgassing, but they could also be consumed by ‘heterotrophs’ (organic matter-eating organisms). Heterotrophs are almost always present in ecosystems on Earth. Hence, it would seem that assuming the presence of methanogens without that of heterotrophs does not make complete sense from an ecosystem perspective.”
“Concerning the detection of amino acids, we place the threshold concentration below which biotic production is unlikely at an achievable limit of detection for onboard instruments eg those envisioned for the Orbilander concept. It is important that we define the required limits of detection to allow the interpretation of ‘negative results.’ Maybe one could interpret our result as an argument not to prioritize cell detection instruments over broader spectrum biosignature detection instruments such as gas chromatography or mass spectroscopy, which are able to identify molecules such as amino acids.”
The “Ocean Worlds” of the Solar System. Credit: NASA/JPL
Therefore, further research is needed to address the role of heterotrophs and establish tighter constraints on the amount of organic matter that could be detected. This will ensure that future missions don’t run the risk of mistaking negative results for the absence of biotic production (instead of being merely too small to detect). Nevertheless, this research constitutes an important first step in establishing what a future mission to Enceladus should look for and how the results could be interpreted. This will likely have significant implications for future mission architectures – and not just for Enceladus.
Throughout the outer Solar System, there are “Ocean Worlds” that have icy surfaces and subsurface oceans. Most of these bodies are moons that orbit gas giants, like Jupiter’s Europa, Ganymede, and Callisto, Saturn’s Titan and Dione, and Neptune’s Triton. However, astronomers and astrobiologists have begun eying other bodies like Ceres, Pluto, Charon, and other “dwarf planets” more closely in recent years. Several missions are planned to explore some of these bodies, all of which will benefit from research that helps establish what they could find. Said Affholder:
“What we do for Enceladus may translate at least in part for similar bodies – icy moons. Titan also has a global ocean overlain by ice and is the target of the upcoming NASA Dragonfly mission. Europa in the Jovian system is an icy moon that should be reached by the NASA Europa Clipper and by the ESA’s JUpiter ICy Explorer (JUICE) in the 2030s. I believe we are at the beginning of a very exciting era of exploration of icy moons, for which I am excited.”
Further Reading: Planetary Science Journal
