When NASA's Voyager spacecraft visited Saturn's moon Enceladus, they found a body with young, reflective, icy surface features. Some parts of the surface were older and marked with craters, but the rest had clearly reappeared. It was clear evidence that Enceladus was geologically active. The moon is also located near the Saturn E ring, and scientists believe that Enceladus could be the source of the material in this ring, further indicating the geological activity.
We have learned a lot more about the cold moon since then. It almost certainly has a warm and salty subterranean ocean beneath its icy exterior, making it a prime target in those looking for life. The Cassini spacecraft discovered molecular hydrogen – a potential food source for microbes – in feathers taken from the subterranean ocean of Enceladus, and this sparked conversations about the moon's potential to harbor life.
A new article uses modeling to better understand Enceladus chemistry. The research team behind it says the subterranean ocean could contain a variety of chemicals that could support a diverse community of microbes.
When it comes to finding life elsewhere in the solar system, Enceladus ticks a lot of boxes. Saturn's sixth largest moon is about 500 km in diameter and appears to have buried a global ocean under an ice sheet. And this ocean is probably warm and salty and has some interesting chemicals in it. Among these chemicals, there are several ways that could aid life, according to the new research.
This colored Voyager 2 picture mosaic shows the surface of Enceladus covered with water ice. Parts of the lunar surface are old and crater-like, while other parts are young and bright, indicating geological activity. Photo credit: NASA / JPL / USGS
The title of the paper is "Oxidation Processes Diversify Enceladus' Metabolic Menu". The lead author is Christine Ray, Ph.D. Student at the Department of Physics and Astronomy at the University of Texas, San Antonio. Ray is also a member of the Space Science and Technology Division at the Southwest Research Institute (SwRI). The paper was published in the journal Science Direct.
This work was driven by Cassini's discovery of molecular hydrogen in clouds of steam from Enceladus.
"The detection of molecular hydrogen (H2) in the cloud showed that free energy is available in the ocean of Enceladus," said lead author Ray in a press release. “On earth, creatures that are aerobic or oxygen-breathing use energy in organics like glucose and oxygen to create carbon dioxide and water. Anaerobic microbes can metabolize hydrogen to methane. All life can be distilled into similar chemical reactions associated with an imbalance between oxidizing and reducing agent compounds. "
The imbalance that Ray is referring to creates a vital energy gradient. The energy gradient enables the exchange of energy between an organism and its environment. Processes around these gradients are critical to many aspects of biology such as photosynthesis and respiration. When a system is in equilibrium, there is no energy gradient, which creates a barrier to life.
When molecular hydrogen was discovered in the springs from Enceladus, it drew a parallel to the hydrothermal sources of the deep sea here on earth. In these sources, hydrogen provides an energy source for an entire ecosystem. People immediately wondered if this could also apply to Enceladus.
But this study goes further. The authors wanted to know if there could be other energy pathways in the ocean of Enceladus that are conducive to life.
"We wondered if other types of metabolic pathways could also provide energy sources in Enceladus' ocean," said Ray. "Since that would require a different set of oxidizers that we haven't yet discovered in the Enceladus cloud, we ran chemical models to see if conditions in the ocean and rocky core could support these chemical processes."
Artist rendering with an inner cross-section of the Enceladus crust showing how hydrothermal activity can cause the plumes of water on the lunar surface.
Credits: NASA-GSFC / SVS, NASA / JPL-Caltech / Southwest Research Institute
When it comes to Enceladus and life, the availability of energy is key. In their work, the authors write that "compounds that could be used for metabolic reactions must be present in imbalance concentrations so that the biology can extract energy from the environment in order to bring the system towards equilibrium."
Cassini showed us that, thanks to molecular hydrogen, there is at least one life path. The necessary chemicals are in place for methanogenesis to take place. Then molecular hydrogen oxidizes with carbon dioxide to methane. This pathway is common on earth, including in the human intestine, where archaea use methanogenesis.
With no spaceship near Enceladus and no way to collect the data it needs, the team turned to models. They wondered whether there might be oxidizing agents other than carbon dioxide present at Enceladus, possibly below Cassini's detection threshold, which might represent a further path of life.
Cassini images of Saturn's moon Enceladus, backlit by the sun, show the fountain-like sources of the fine material spray that towers over the south pole region. This image was taken more or less broadly and shows the “tiger stripe” fractures observed in previous Enceladus images. It shows discrete feathers of various apparent sizes over the edge of the moon. This picture was taken on November 27, 2005. Photo credit: NASA / JPL / Space Science Institute
What this study is really about is the relationship between energy and biology. To explore this relationship with Enceladus, the authors focused on two key concepts: chemical affinity and energy flow.
Chemical affinity is the ability of different chemicals to form compounds, and according to the authors it is also "… the amount of free energy available from a metabolic reaction". The energy flow "… determines how much biomass can be supported in the stationary state", according to the authors. Much of this is due to oxidizing agents. In essence, oxidizer production determines how much energy is available for life.
The authors wanted to model oxidant production on Enceladus for two reasons: “1) constrain the oxidant budget or the concentrations and flows of metabolically significant oxidants in the ocean, and 2) determine whether these additional metabolic pathways could provide sufficient energy for life. ”
To examine all of this, they modeled three different cases, as shown in the following figure.
This table from the study shows the total number of cells that could be supported from our list of metabolic redox reactions based on the energy requirements for maintaining the chemostat cell cultures. Cells can be assisted by all of our aerobic and anaerobic responses under consideration, except in a few cases of Case II where the energy flows were insufficient to sustain life. Thus, the corresponding cell numbers are zero. Photo credit: Ray et al 2020.
What did you find?
"This new paper is another step in understanding how a little moon can sustain life in ways that completely exceed our expectations!"
Hunter Waite, co-author, SwRI program director.
They found that according to their models, enough oxidant could be produced to create other ways of life. Surface ice could be broken down by energy and release oxidants. This can be done in a number of ways.
“We have shown that the production of radiolytic oxidants on Enceladus can lead to redox imbalances in the ocean, which can provide energy to support putative life. The radiolysis of surface ice in connection with the transport of ice to the ocean in the geologically active tiger strip region can deliver up to 9.4 × 1015 mol O2 and 3.3 × 1016 mol H2O2, ”they write in their study.
In 2005, data from Cassini showed that the so-called "Tiger Stripe" features in the south pole region of Enceladus are warm spots. Image: NASA / JPL / GSFC / SwRI / SSI
They also found that the decay of a radioactive isotope of potassium (K) could produce oxidants.
"Electrons and gamma rays released by the decay of 40K atoms in the ocean can generate another 4.2 × 1016 moles of O2 and 1.4 × 1015 moles of H2O2 directly in the ocean."
"We compared our free energy estimates with the ecosystems on earth and found that our overall aerobic and anaerobic metabolism levels met or exceeded the minimum required," Ray said in the press release. "These results suggest that oxidant production and chemistry could help support a potential life and metabolically diverse microbial community on Enceladus."
This figure from the study shows the amount of molecular hydrogen, molecular oxygen, and hydrogen peroxide that is produced due to the breakdown of potassium 40 over time. Photo credit: Ray et al., 2020.
That in itself is exciting. But there is more.
"Now that we have identified potential food sources for microbes, the next question is, what is the nature of the complex organics that come out of the ocean?" Said SwRI Program Director Dr. Hunter Waite, co-author of the new paper. "This new paper is another step in understanding how a little moon can sustain life in ways that completely exceed our expectations!"
"We have to be careful, but I find it exciting to wonder if there are any strange life forms that use these sources of energy that seem to be fundamental to the work of Enceladus."
Dr. Christopher Glein, co-author, SwRI Senior Research Scientist
Like much modeling and research on other worlds, these results are both tempting and a little frustrating. We'll have to wait for another mission to Enceladus to confirm or dismiss it. We need another spaceship, hopefully with finer instruments, to travel through Enceladus' feathers and take more measurements.
The "fountains" of Enceladus. Photo credit: NASA / JPL / SSI
"A future spaceship could fly through the Enceladus cloud to test this paper's predictions about the abundance of oxidized compounds in the ocean," said Dr. Christopher Glein, Senior Research Scientist at SwRI, is another co-author. "We have to be careful, but I find it exciting to wonder if there are any strange life forms that use these sources of energy that seem to be fundamental to the work of Enceladus."
A mission for Enceladus is currently in the concept phase. NASA's Enceladus Life Finder is a proposal to send a solar powered orbiter to Enceladus. The Life Finder would orbit Saturn but would repeatedly fly through Enceladus' clouds of steam and take measurements. It was proposed in 2017 but not selected, but the idea is still alive.
An artistic illustration of the feathers of Enceladus. Photo credit: NASA / JPL-Caltech
The last word goes to the authors for now. In their conclusion, they write: “We have shown that in addition to methanogenesis, aerobic and / or anaerobic reactions can in each of our three cases meet the minimum free energy requirement for life on earth, Gmin, and provide maintenance energy to support cell life in Enceladus. "
"The production of radiolytic oxidants and redox chemistry in the ocean and on the seabed of Enceladus can therefore support metabolic processes beyond methanogenesis and create the possibility for a metabolically diverse microbial community in the ocean of Enceladus."