Saturn’s oceanic moon, Enceladus, is attracting a lot of attention in the search for life in our solar system.
Much of what we know about Enceladus and its ice-covered ocean comes from the Cassini mission. Cassini finished exploring the Saturn system in 2017, but scientists are still working on its data.
New research based on Cassini data reinforces the idea that Enceladus has the chemicals necessary for life.
During its mission, Cassini discovered geyser-like columns of water erupting through the icy wall of Enceladus. In 2008, Cassini flew by and analyzed dust particles with its Space Dust Analyzer (CDA).
CDA showed that the water in the stems contained a surprising mix of volatiles, including carbon dioxide, water vapor, and carbon monoxide. It also found traces of molecular nitrogen, simple hydrocarbons and complex organic chemicals.
But Cassini’s data is still being analyzed, even six years after it ended its mission and was sent to destroy Saturn’s atmosphere. The new paper, titled “Observations of the elemental composition of Enceladus consistent with generalized models of theoretical ecosystems,” presents some new findings. Lead author Daniel Muratore, a postdoc at the Santa Fe Institute.
The work focuses on detecting ammonia and inorganic phosphorus in Enceladus’ ocean. The researchers used ecological and metabolic theory and modeling to understand how these chemicals might have made Enceladus suitable for life.
“Beyond speculation about threshold concentrations of bioactive compounds to support ecosystems, metabolic and ecological theory can provide powerful interpretive lenses for assessing whether extraterrestrial environments are compatible with living ecosystems,” the authors explain.
An important component of ecological theory is the Redfield ratio. It is named after the American oceanographer Alfred Redfield. In 1934, Redfield published results showing that the ratio of carbon to nitrogen to phosphorus (C:N:P) in ocean biomass was remarkably consistent at 106:16:1. Other researchers found that the ratio changed slightly depending on the area and the types of phytoplankton present. More recent work refined the ratio to 166:22:1.
The exact numbers are not necessarily the critical point. Redfield’s conclusion is the important part. The Redfield correlation shows a remarkable coherence between the life forms that live in the deep ocean and the chemistry of the ocean itself. He proposed that there is a balance between ocean water and plankton nutrients based on biotic feedback. He described the chemical framework of nutrients and simple life.
“Whatever the explanation, the correspondence between the amount of biologically available nitrogen and phosphorus in the sea and the amounts used by plankton is a phenomenon of the greatest interest,” Redfield said in the conclusion of his paper.
So how does the detection of ammonia and phosphorus in Enceladus’ ocean relate to the Redfield ratio and Enceladus’ biological potential?
The Redfield ratio is common throughout the Earth’s Tree of Life. “Because of this apparent ubiquity, the Redfield ratio is considered a target signature for the detection of astronomical life, especially on oceanic worlds such as Europa and Enceladus,” the authors of the new paper write.
When it comes to life, all we have to go is the Earth. So it makes sense to use fundamental aspects of the chemistry of life here on Earth as a lens through which to study other life-supporting worlds.
Analysis of Cassini data from Enceladus’ plumes shows high levels of inorganic phosphate in the ocean. Other geochemical simulations based on Cassini’s findings show the same.
“These reports of phosphorus follow up on previous work that identified many of the elemental components of terrestrial life (C, N, H, O) from the Enceladus plume,” the authors explain.
Even more analysis shows that the ocean contains many chemicals common to living organisms, such as amino acid precursors, ammonium and hydrocarbons.
Thus, Enceladus’ ocean has a rich chemistry, and many chemicals reflect the chemistry of life. In particular, there is a hypothesis that Enceladus may support methanogenesis.
Earth’s Archaea carry out methanogenesis in a wide range of different environmental conditions on Earth and have done so for over three billion years, proving their survival. Biochemical modeling suggests that Earth’s methanogens are compatible with Enceladus’ ocean.
The researchers developed a new, more detailed model for methanogens on Enceladus to see if they could survive and thrive there. Their model relied heavily on the Redfield correlation. They found that although phosphorus is present in high levels in the moon’s ocean, the overall ratio “may be limited to Earth-like cells.”
“Elevated supplies of these nutrients may be consistent with a small or metabolically slow biosphere, incomplete depletion of a biosphere with a recent origin of life,” or other causes that may cause an imbalance.
So where does that leave the prospects for life on Enceladus?
We are only at the beginning of biosignature science. We can detect individual chemicals, but from this great distance we cannot accurately measure Enceladus’ overall chemistry. More recent research in biology, including this paper, aims to reveal how biological processes rearrange chemical elements in signaling ways. By looking at entire ecosystems, as Redfield did, scientists can discover new biosignatures that are less ambiguous.
If we can do this, we may find that non-terrestrial life forms rearrange chemicals in completely different ways.
This research is part of a new effort to discover more than individual chemical biosignatures, some of which may be false positives. Methane, for example, can be biotic, but can also be produced abiotically. There are others, like the recently discovered phosphine on Venus.
Understanding ecosystems as a whole is the next step. There are a number of surprising factors to consider. Cell size, nutrient availability, radiation, salinity, temperature. Constantly. 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 upcoming missions to Europa will begin to provide a more complete picture. According to the authors, the next step requires more complete data and a more generalized approach.
“We suggest two priorities for further astrobiological research to better understand the implications of these conclusions,” they write. “First, we echo previous calls from the astrobiology literature to explore more generalized insights into metabolism and physiology.”
They also suggest that looking for direct parallels to life on Earth in the form of biochemistry may not be the best strategy for looking for life on Enceladus.
“Second, we recommend expanding the range of Earth-analog environments to include the extreme resource supply ratios mirrored for Enceladus,” they explain.
Our understanding of habitability is gradually increasing, as this study makes clear. There probably won’t be any revelatory moments when we suddenly realize this.
Nature has created a huge variety of worlds, each with its own chemistry. While using the Redfield ratio as a lens is one way to look at these worlds in their unique glory, we can’t get tunnel vision.
While most of our imagination’s dreams of life on other worlds are fanciful and unlikely, life could have found a different path on Enceladus. Life can exist and reorganize in chemical environments in many ways.
This article was originally published by Universe Today. Read the original article.
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