Watery plumes on Enceladus could hold signs of life
Could there be life on Saturn’s ocean moon Enceladus? And if there’s microscopic life in the ocean underneath its icy crust, could we detect it? The moon’s subsurface global ocean contains water, heat and organic material. All three are crucial to life, at least the kinds of life we’re familiar with on Earth. The ocean lies beneath an outer crust of ice. However, numerous geyser-like plumes spew ocean water into space. Researchers from the University of California, San Diego, said on December 5, 2023, that amino acids – the building blocks of proteins – could survive a tumultuous journey on the moon’s plumes.
Update: On a related note, just as this article was about to be published, NASA released some news about another intriguing discovery at Enceladus. On December 14, researchers said that an additional study of data sent back by the Cassini spacecraft confirmed the presence of hydrogen cyanide, a molecule that is key to the origin of life, in the plumes. Hydrogen cyanide is one of the most important and versatile molecules needed to form amino acids. There is also new evidence that the ocean itself contains a powerful source of chemical energy, much more than previously calculated. The energy source is in the form of several organic compounds.
Plumes on Enceladus
Enceladus’ plumes erupt like geysers through cracks, called tiger stripes, in the icy crust at the moon’s south pole. In fact, NASA’s Cassini spacecraft sampled them directly by flying through them. While it confirmed the presence of organic molecules, Cassini wasn’t designed to detect life itself. But we do know now that various kinds of organic compounds exist in the plumes. Cassini also found ice grains, salts and concentrations of sodium, potassium, chlorine and carbonate-containing compounds.
Last June, scientists also said that new analyses of Cassini data revealed phosphorus in the plumes, another key ingredient and building block for life.
The paper states:
The search for extraterrestrial life, especially within our solar system, is one of the biggest endeavors of mankind. The icy moons of Saturn and Jupiter, Enceladus and Europa, are particularly promising for hosting life, as they have shown evidence for the three important criteria: water, energy and organic chemicals. Both moons eject their subsurface ocean material as a plume of icy particles, providing the opportunity to study the ocean composition and potential habitability via plume flythrough sampling.
A matter of speed
But how many kinds of organics can survive being blasted into space? What about those that might be directly associated with life, such as amino acids? It all comes down to speed. The plumes are fast-moving, erupting at about 800 miles per hour (360 m/s). Would organic particles be destroyed at that speed, when they impact with each other? We already know that some can survive, since Cassini found them. But what about amino acids?
The study showed that amino acids in the plumes could survive at speeds up to 9,400 miles per hour (4,200 m/s). And the plumes are erupting significantly slower than that. So, the researchers concluded that amino acids – if they exist – should survive the trip into space. In fact, they could be detected with limited fragmentation up to the top velocities. The paper says, using a mass spectrometer, they could fly through Enceladus’ plumes at speeds of 9,400 miles per hour (15,000 km/h) and successfully detect intact amino acids.
Measuring impacts of single ice grains in the plumes on Enceladus
This is the first time that scientists have measured what happens when a single ice grain hits another surface. Enceladus’ plumes are made up of tiny ice grains, which form after the water vapor erupting from the cracks in the ice crust freeze.
In the experiment, the researchers created ice grains by using electrospray ionization, where water is pushed through a needle held at a high voltage. The electric charge breaks the water down into increasingly smaller droplets. Then, the researchers eject the droplets into a vacuum chamber, where they freeze.
The team was able to measure the mass and charge of the grains. Image charge detectors observed the grains as they passed through the spectrometer. Using a microchannel plate detector, the researchers accurately timed the moment of impact of the ice grains down to the nanosecond. A nanosecond is one-billionth of a second.
The fact that amino acids can withstand the impacts is crucial. It shows that similar intact amino acids could still exist in the plumes of Enceladus, or even the tentative ones of Europa. (Europa’s are not proven to exist yet, but evidence is growing).
Co-author Robert Continetti at the University of California, San Diego, said:
To get an idea of what kind of life may be possible in the solar system, you want to know there hasn’t been a lot of molecular fragmentation in the sampled ice grains, so you can get that fingerprint of whatever it is that makes it a self-contained life form. Our work shows that this is possible with the ice plumes of Enceladus.
Salt and amino acids
The new study also showed how salt can affect the detectability of amino acids in the plumes. The data from Cassini suggests that Enceladus’ ocean is salty, like oceans on Earth. Salt can change the solubility of some molecules. This means that molecules like amino acids could be detected more easily. This is because they might cluster on the surface of the ice grains in the plumes. That would make it easier for a spacecraft sampling the plumes to find those molecules.
This is exciting because it means that evidence for life – traces of molecules associated with living organisms – could be detected directly in the plumes. No need to drill through the ice crust to get to the ocean below. It will require a follow-up mission to Cassini, but it can be done. Continetti said:
The implications this has for detecting life elsewhere in the solar system without missions to the surface of these ocean-world moons is very exciting, but our work goes beyond biosignatures in ice grains. It has implications for fundamental chemistry as well. We are excited by the prospect of following in the footsteps of Harold Urey and Stanley Miller, founding faculty at UC San Diego in looking at the formation of the building blocks of life from chemical reactions activated by ice grain impact.
Return to Enceladus … and Europa
While there aren’t any scheduled missions yet back to Enceladus, scientists are eager to return. In the meantime, the amino acid research results could also useful for the Europa Clipper mission to Europa, scheduled to launch in October 2024. As the paper summarized:
Our results provide a benchmark for this orbital sampling method to successfully detect signs of life and for the interpretation of past and future data. This work has implications not only for a potential Enceladus mission but also for the forthcoming Europa Clipper mission.
So we don’t know yet if the plumes on Enceladus hold signs of life. But we are getting closer to finding out.
Bottom line: Do the plumes on Enceladus contain evidence of life from the moon’s ocean? A new study shows that amino acids could survive and be detectable by spacecraft.