by Saturn has 146 confirmed moons – more than any other planet in the solar system – but one known as Enceladus stands out. It seems that the ingredients are for life.
From 2004 to 2017, Cassini – a joint mission between NASA, the European Space Agency and the Italian Space Agency – investigated Saturn, its rings and its moons. Cassini delivered amazing results. Enceladus, only 313 miles (504 kilometers) in diameter, harbors an ocean of liquid water beneath its icy crust that spans the entire moon.
Geysers at the moon’s south pole expel gas and ice grains formed from ocean water into space.
Although the Cassini engineers did not expect to analyze the ice grains that Enceladus was actively emitting, they did pack a dust analyzer on the spacecraft. This instrument measured the ice grains that were emitted individually and told the researchers about the composition of the ocean below the surface.
As a planetary scientist and astronomer who studies ice grains from Enceladus, I am interested in whether or not life exists on this icy moon. I also want to understand how scientists like me could detect it.
Ingredients for life
Just like Earth’s oceans, Enceladus’ ocean contains salt, most of which is sodium chloride, commonly known as table salt. The ocean also contains various carbon-based compounds, and has a process called tidal heating that generates energy within the moon. Liquid water, carbon-based chemistry and energy are key ingredients for life.
In 2023, I and other scientists found phosphate, another life-supporting compound, in ice grains that came from the ocean of Enceladus. Phosphorus, a form of phosphorus, is vital to all life on earth. It is part of DNA, cell membranes and bones. This was the first time scientists detected this compound in an extraterrestrial water ocean.
Enceladus’ rocky core probably interacts with the water ocean through hydrothermal vents. These hot, geyserlike structures protrude from the ocean floor. Scientists predict that a similar location could be the birthplace of life on Earth.
Sensing a possible life
Until now, no one has detected life outside of Earth. But scientists agree that Enceladus is a very promising place to look for life. So how do we go about watching?
In a paper published in March 2024, my colleagues and I conducted a laboratory test in which we simulated whether dust analyzer instruments on spacecraft could detect and identify traces of life in the emitted ice grains.
To simulate the detection of ice grains as recorded by dust analyzers in space, we used a laboratory setup on Earth. Using this setup, we injected a tiny beam of water containing bacterial cells into a vacuum, where the beam disintegrated into droplets. In theory, each droplet contained one bacterial cell.
Next, we fired a laser at the individual droplets, creating charged ions from the water and cell compounds. We measured the charged ions using a technique called mass spectrometry. These measurements helped us predict what dust analysis instruments on a spacecraft should find if they encountered a bacterial cell in an ice grain.
We found that these instruments would do a good job of identifying cellular material. Instruments designed to analyze individual ice grains should be able to identify bacterial cells, even if an ice grain from a geyser like Enceladus contains only 0.01% single-cell constituents.
The analyzers could pick up a number of potential signatures from cellular material, including amino acids and fatty acids. Detected amino acids are fragments of the cell’s proteins or metabolites, which are small molecules that participate in chemical reactions within the cell. Fatty acids are lipid fragments that make up cell membranes.
In our experiments, we used named bacteria Sphingopyxis alaskensis. The cells in this culture are extremely tiny – the same size as cells that could fit into grains of ice emitted from Enceladus. In addition to their small size, these cells like cold environments, and need only a few nutrients to survive and grow, similar to how life would have adapted to the conditions in the Enceladus ocean.
The specific dust analyzer on Cassini did not have the analytical capabilities to identify cellular material in the ice grains. However, scientists are already designing instruments with much greater capabilities for potential future Enceladus missions. Our experimental results will guide the planning and design of these instruments.
Future missions
Enceladus is one of the main targets for future missions by NASA and the European Space Agency. In 2022, NASA announced that a mission to Enceladus had the second highest priority when choosing their next major missions – a mission to Uranus had the highest priority.
The European agency recently announced that Enceladus is the main target of its next big mission. This mission would likely include a very capable dust analyzer for ice grain analysis.
Enceladus is not the only moon with an ocean of liquid water. Jupiter’s moon Europa also has an ocean that spans the entire moon under its icy crust. Ice grains on Europa float high above the surface, and some scientists think Europa may have geysers like Enceladus that shoot grains into space. Our research will also help study ice grains from Europa.
NASA’s Europa Clipper mission will visit Europa in the coming years. Clipper is scheduled for launch in October 2024 and will arrive at Jupiter in April 2030. One of the two mass spectrometers on the spacecraft, the Surface Dust Analyzer, is designed for individual ice grain analysis.
Our study shows that this instrument will be able to find even small fractions of a bacterial cell, if they are only present in a few emitted ice grains.
With the plans of these space agencies close to the future and the results of our studies, the prospects for future space missions visiting Enceladus or Europa are extremely exciting. We now know that scientists should be able to find out if there is life on any of these moons with current and future instrumentation.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. It was written by Fabian Klenner University of Washington
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Fabian Klenner is an affiliate of the Europa Clipper mission (SUrface Dust Analyzer instrument). It receives funding from NASA.
