Saturn’s oceanic moon Enceladus is attracting increasing attention in the search for life in our solar system. Most of what we know about Enceladus and its icy ocean comes from the Cassini mission. Cassini completed its study of the Saturn system in 2017, but scientists are still working on the data.

A new study based on Cassini data supports the idea that Enceladus contains chemicals essential for life. During its mission, Cassini discovered geyser-like columns of water erupting from Enceladus’ icy crust. In 2008, Cassini performed a close flyby and analyzed the clouds with the Space Dust Analyzer (CDA).

CDA showed that water in clouds contains a surprising mixture of volatile substances, including carbon dioxide, water vapor and carbon monoxide. He also discovered traces of molecular nitrogen, simple hydrocarbons, and complex organic chemicals.

But Cassini’s data is still being analyzed six years after it ended its mission and was sent to destroy Saturn’s atmosphere. A new paper titled “Enceladus Elemental Composition Observations Consistent with Generalized Ecosystem Models” presents some new findings. The lead author is Daniel Muratore, a postdoctoral researcher at the Santa Fe Institute. The work focuses on the discovery of ammonia and inorganic phosphorus in the ocean of Enceladus. Researchers used environmental and metabolic theory and modeling to understand how these chemicals could make Enceladus habitable.

“Beyond assumptions about threshold concentrations of biologically active compounds to sustain ecosystems, metabolic and ecological theory can provide a powerful interpretive lens for assessing the compatibility of extraterrestrial environments with living ecosystems,” the authors explain.

A critical component of ecological theory is the Redfield coefficient. It was named after American oceanographer Alfred Redfield. In 1934, Redfield published results showing that the ratio of carbon, nitrogen, and phosphorus (C:N:P) was fairly constant for ocean biomass at 106:16:1. Other researchers have found that the rate varies slightly depending on region and the type of phytoplankton present. His latest work ratio improved to 166:22:1.

The exact numbers are not necessarily the critical point. Redfield’s conclusion is an important part. The Redfield coefficient demonstrates the remarkable unity between the chemical composition of living things in the ocean depths and the ocean itself. He hypothesized that there is a balance between ocean water and plankton nutrients based on biotic feedback. He explained the chemical basis of food and simple life.

“Whatever the explanation, the similarity between the amount of bioavailable nitrogen and phosphorus in the sea and the rates at which they are used by plankton is the phenomenon of most interest,” Redfield said in the conclusion of his paper.

So how does the discovery of ammonia and phosphorus in Enceladus’ ocean relate to the Redfield coefficient and the biological potential of Enceladus?

The Redfield coefficient is common throughout the Tree of Life on Earth. “Because of its ubiquity, the Redfield ratio has been considered a target for astrobiological detection of life, especially on ocean worlds such as Europa and Enceladus,” the authors of the new paper write.

When it comes to life, Earth is all we have left. Therefore, it makes sense to use fundamental aspects of the chemistry of life on Earth as a lens to explore other potential life-supporting worlds. Analysis of Cassini data from Enceladus clouds shows high levels of inorganic phosphate in the ocean. Other geochemical simulations based on Cassini’s findings show the same thing.

“These phosphorus reports are a follow-up to previous studies that identified multiple essential components of terrestrial life (C, N, H, O) from the Enceladus cloud,” the authors explain.

Further analysis reveals that the ocean contains many chemicals common to living organisms, such as amino acid precursors, ammonium, and hydrocarbons. So Enceladus’ ocean has a rich chemical composition, and many of the chemicals reflect the chemistry of life. There is a hypothesis that Enceladus in particular may support methanogenesis.

Terrestrial archaea perform methanogenesis in a wide range of different environmental conditions on Earth and have demonstrated their viability by doing so for over three billion years. Biochemical modeling shows that methanogens on Earth are compatible with Enceladus’ ocean.

Researchers developed a new, more detailed model to see whether methanogens on Enceladus could survive there. Their model was largely based on the Redfield coefficient. They found that despite the high phosphorus level in the lunar ocean, the overall rate “may be limited to Earth-like cells.”

“The high availability of these nutrients may be due to incomplete depletion from a small or metabolically slow biosphere, a biosphere where life has just emerged,” or other causes that may cause imbalance.

We are just at the beginning of biosignature science. We can identify individual chemicals, but we cannot accurately measure the overall chemical composition of Enceladus from such a great distance. More recent biosignature research, including this paper, aims to identify how biological processes rearrange chemical elements in a detectable way. By looking at entire ecosystems, as Redfield did, scientists can discover new, less subtle biological signatures.

If we can do this, we may find that extraterrestrial life forms rearrange their chemicals in a very different way. This research is part of a new effort to identify more than individual chemical biosignatures, some of which can be false positives. For example, methane may be a biosignature, but it can also be produced abiotically. There are others, such as phosphine, which was recently discovered on Venus.

The next step is to understand ecosystems as a whole. There are a surprising number of factors to consider. Cell size, availability of nutrients, radiation, salinity, temperature. Again and again. But to understand the overall chemical environment on Enceladus, Europa, or anywhere else, we need more detailed data. Fortunately, instrument science continues to improve, and future missions to Europa will begin to paint a more complete picture. According to the authors, the next step requires more complete data and a more generalized approach.

“To better understand the implications of these findings, we recommend two priorities for further astrobiological research,” they write. “First, we echo previous calls in the astrobiology literature to explore more general concepts of metabolism and physiology.”

They also suggest that looking for direct parallels to life on Earth through biochemistry may not be the best strategy for finding life on Enceladus.

“Second, we propose that the scope of Earth-like environments be expanded to include those with extraordinary resource supply rates mirroring those proposed for Enceladus,” they explain.

As this study makes clear, our understanding of viability is increasing. There probably won’t be any epiphany where we suddenly realize this. Nature has created a wide variety of worlds, each with their own chemistry. While using tools like the Redfield ratio as lenses is a way to look at these worlds in all their unique beauty, we don’t get tunnel vision.

While much of what we can imagine about life on other worlds is fanciful and improbable, life on Enceladus could take a different path. There may be different ways life exists and chemical environments are rearranged.