Saturn has 146 confirmed moons - more than any other planet in the solar system - but one called Enceladus stands out. It seems to have the ingredients for life.
From 2004 to 2017, Cassini - a joint mission of NASA, the European Space Agency and the Italian Space Agency - examined Saturn, its rings and moons. Cassini came up with spectacular findings. Enceladus, just 504 kilometers in diameter, hosts an ocean of liquid water beneath its icy crust that spans the entire moon.
Geysers at the moon's south pole shoot gas and ice grains formed from ocean water into space.
Although Cassini engineers did not expect to analyze the ice grains that Enceladus was actively emitting, they did put a dust analyzer in the spacecraft. This instrument measured the ejected ice grains individually and told researchers about the composition of the subsurface ocean.
As a planetary scientist and astrobiologist who studies ice grains from Enceladus, I am interested in whether there is life on these or other icy moons. I also want to understand how scientists like me can detect it.
Ingredients for life
Like Earth's oceans, Enceladus' ocean contains salt, most of which is sodium chloride, commonly known as table salt. The ocean also contains several carbon-based compounds and has a process called tidal heating that generates energy in the moon. Liquid water, carbon-based chemistry and energy are all important ingredients for life.
In 2023, I and other scientists found phosphate, another life-sustaining compound, in ice grains taken from Enceladus' ocean. Phosphate, a form of phosphorus, is essential for all life on Earth. It is part of DNA, cell membranes and bones. This was the first time scientists discovered this compound in an alien water ocean.
The rocky core of Enceladus likely interacts with the water ocean through hydrothermal vents. These hot, geyser-like structures protrude from the ocean floor. Scientists predict that a similar environment may have been the birthplace of life on Earth.
Detecting potential life
So far, no one has ever discovered life beyond Earth. But scientists agree that Enceladus is a promising place to look for life. How do we proceed then?
In a paper published in March 2024, my colleagues and I conducted a laboratory test that simulated whether dust analyzer instruments on spacecraft could detect and identify traces of life in the ejected ice grains.
To simulate the detection of ice grains as dust analyzers in space record them, we used a laboratory setup on Earth. Using this setup, we injected a small water jet containing bacterial cells into a vacuum, where the jet broke up into droplets. Each drop theoretically contained one bacterial cell.
We then shot a laser at the individual droplets, creating charged ions from the water and cell connections. We measured the charged ions using a technique called mass spectrometry. These measurements helped us predict what dust analyzer 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 identifying cellular material. Instruments designed to analyze individual ice grains must be able to identify bacterial cells, even if there are only 0.01% of the constituents of a single cell in an ice grain from an Enceladus-like geyser.
The analyzers could pick up a number of potential signatures from cellular material, including amino acids and fatty acids. Detected amino acids represent fragments of the cell's proteins or metabolites, which are small molecules that participate in chemical reactions in the cell. Fatty acids are fragments of lipids that make up cell membranes.
In our experiments we used a bacterium called Sphingopyxis alaskensis. The cells from this culture are extremely small - the same size as cells that could possibly fit into the ice grains ejected from Enceladus. In addition to their small size, these cells like cold environments and require only a few nutrients to survive and grow, similar to how life adapted to the conditions in Enceladus' ocean would likely be.
The specific dust analyzer on Cassini did not have the analytical capabilities to identify cell material in the ice grains. However, scientists are already designing instruments with much greater capabilities for potential future Enceladus missions. Our experimental results will inform 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 was the second highest priority when choosing their next major missions - a Uranus mission was the highest priority.
The European agency recently announced that Enceladus is the top target for its next major mission. This mission would likely include a highly capable dust analyzer for ice grain analysis.
Enceladus isn't the only moon with an ocean of liquid water. Jupiter's moon Europa also has an ocean that spans the entire moon beneath its icy crust. Ice grains on Europa float above the surface, and some scientists think Europa may even have geysers like Enceladus that shoot grains into space. Our research will also help study ice grains from Europe.
NASA's Europa Clipper mission will visit Europe in the coming years. Clipper is scheduled to launch in October 2024 and arrive at Jupiter in April 2030. One of the two mass spectrometers on the spacecraft, the SUrface Dust Analyzer, is designed for analysis of individual ice grains.
Our study shows that this instrument can find even small fractions of a bacterial cell if it is only present in a few ejected ice grains.
With the near-future plans of these space agencies and the results of our research, the prospects of upcoming space missions visiting Enceladus or Europa are incredibly exciting. We now know that scientists should be able to use current and future instruments to find out whether there is life on any of these moons.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Fabian Klenner, University of Washington Read more: Fabian Klenner is affiliated with the Europa Clipper mission (SUrface Dust Analyzer instrument). He receives funding from NASA.
