In 2023, we reported on the detection of bio-essential compounds in Enceladus’s plumes – a discovery that strengthened the moon’s reputation as one of the most promising places to search for extraterrestrial life. “We’re satisfied that Enceladus has favorable conditions to develop and sustain life,” said Frank Postberg, Professor of Planetary Sciences at the Institute of Geological Sciences, Freie Universität in Berlin.
But new laboratory simulations suggest that at least some of the simple organic molecules identified by NASA’s Cassini spacecraft might not have come from the subsurface ocean at all.
To find out more, we spoke with Grace Richards, postdoctoral researcher at INAF-IAPS in Rome working as part of the SERENA instrument team for BepiColombo, who led the research.
What inspired this work – and what was your main finding?
I am very interested in surface processes, for example the question of if we were able to land a spacecraft on Enceladus, what would we see there and why? We know that the surface is predominantly composed of water ice, with some carbon dioxide, but apart from that it is relatively unconstrained and any minor component detection remains tentative. I wanted to experiment with simple ice compositions to see how Saturn’s radiation environment may affect them. But there are also other surface processes to consider, such as plume deposition. This leads to the question of whether we could actually detect this more “complex” material formed by the radiation environment – because it is constantly being covered up by plume particles that get ejected from the subsurface ocean beneath the ice and deposited all over the surface. And due to the fact that the plumes are composed of material that is similar to the surface, we wonder whether it would be possible to differentiate between the two, especially as these irradiative processes affect plume material too.
As our experiments progressed, we realized that some of the products we formed had also been detected in the plumes. Because these simple ice compositions are found in the plumes, and the plumes are also subject to radiolysis, it is plausible that these molecules could be formed within the plumes from simple ice grains, rather than originating from the subsurface.
Why did you choose FTIR spectroscopy for this study, and how does it complement the mass spectrometry approach used on Cassini?
FTIR is a powerful analytical technique that is widely used in the study of planetary surfaces. Many ices, for example H2O, CO2, NH4 and CH4 (the composition used in our study), have distinct spectral signatures in the IR range. FTIR also gives us information about the structure of the ice, for example whether it is crystalline or amorphous. We did use a mass spectrometer to help us identify radiolytic products, however this data was not presented in our paper as it was only used to verify the FTIR measurements.
Cassini’s Cosmic Dust Analyzer (CDA) provided huge insight into the chemical/elemental inventory of the plumes, and that data is still being reanalyzed to better constrain our knowledge today. The CDA worked by impact ionization, meaning that it detected plume material that flew directly into the detectors and fragmented upon impact. The mass spectrometry and IR spectroscopy techniques complement each other because mass spectrometry can provide in situ information on the composition of icy fragments, and FTIR provides context to the ice structure, chemical bonds occurring, and can help interpret which ice mixtures could provide the fragments observed by the CDA.
What were the biggest analytical or experimental challenges in recreating Enceladus’s environment in the lab?
Many of the components we used are very volatile, meaning that they like to be in the gas phase, even at very low temperatures. We did quite a lot of preliminary tests with the experimental team at the HUNREN institute for Nuclear Physics in Hungary, working to get an ice mix that was stable at Enceladus-like temperatures (70 K), and didn’t have various components just sublimating immediately. This is why our compositions are a little different for each component, and the ratios of minor components (CO2, CH4, NH3) are higher than we expect to see on Enceladus’ surface. However in the lab, we wanted to amplify the products to be able to detect them, and investigate the chemistry occurring within the ice. This chemistry is very much expected to be occurring on the icy surface, but it is likely that lower abundances of the radiolytic products are being formed on Enceladus’ surface than what we formed in the lab.
Did any findings or reaction pathways surprise you?
A lot of the radiolytic products, such as OCN-, CO, and NH4+ have been detected across radiation experiments, including with UV irradiation, electron irradiation and single ion irradiation. It was reassuring to detect these because it validated our approach and we were able to use existing work to identify material. It really depends on the starting composition: many studies use ices that contain a few of the components we used, and we used these studies to inform what we expected to detect and help us identify the radiolytic products. There have been other studies that used similar ice compositions to ours, even in the context of Enceladus, but utilized different irradiation types, so our experiments build on the body of existing studies that have informed our results.
While we were working on the heating part of our experiments – simulating some of the different temperatures across the Enceladus surface and revealing some of the products within the ices – it was exciting to see the spectra change shape as the temperature changed. Sometimes when looking at the spectra, the peaks that you form are quite small and difficult to see immediately, and it requires a bit of data processing to ensure that what you are observing is valid. During the heating, at around 160 K, there were three very distinct peaks in the spectra that emerged, so it was reassuring to know we had definitely formed some new molecules!
Overall, how does your work shift our understanding of Enceladus’s potential habitability?
I don’t see this work as necessarily shifting our understanding of the habitability, but more as adding validation to the expectation that material undergoes various transformations as it is ejected from the ocean into the plumes. We have just highlighted one of the processes that may modify material as it is exposed to the space environment.
We always need to be cautious when looking for signs of life, as if we detect biosignatures, we want to be confident that these chemical indicators are there because of biological processes. It is difficult because these indicators can sometimes be formed through abiotic processes as well. The Enceladus system is incredibly interesting – that’s why I love to study it! It is the smallest geologically active body in our solar system and anomalously hot, indicating subsurface hydrothermal regions that could be similar to places on Earth where we think life may have originated. We still have so much to understand about the Saturnian system, and it's an exciting time to be working in this field.
What do you see as the next analytical step in exploring icy moons like Enceladus?
Laboratory studies investigating the fundamental properties of ices and space weathering processes occurring on these ocean worlds will help interpret the data we get from future missions. We are about to get a wealth of information from missions like JUICE and Europa Clipper, that will study the icy moons of Jupiter and investigate the habitability of Europa, which in many ways is similar to Enceladus. We will learn a huge amount about the structure and evolution of these moons from these missions, which will in turn inform our understanding of the Saturnian system.
There is also – potentially – an upcoming ESA mission to Enceladus which is in the beginning stages of planning, so it is possible that in my lifetime we will have a spacecraft that lands on the surface of Enceladus and is able to verify whether this tiny moon is habitable.
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