A small, icy moon of Saturn called Enceladus is one of the prime targets in the search for life elsewhere in the solar system. A new study strengthens the case for Enceladus being a habitable world.
Author
- Flynn Ames
PhD candidate, Meteorology, University of Reading
The data for those new research findings comes from the Cassini spacecraft, which orbited Saturn from 2004-2017. In 2005, Cassini discovered geyser-like plumes of water vapour and ice grains erupting continuously out of cracks in Enceladus' icy shell.
In the latest study, Nozair Khawaja, from the Free University of Berlin, and his team set out to re-analyse a Cassini sample of material from Enceladus' south pole.
Most analyses of solid particles from Enceladus' plumes had been done on Saturn's E-ring. The E-ring is an outer diffuse ring in the majestic ring system that surrounds the planet. It is continuously replenished with material from Enceladus' plumes. But this material is not fresh - and exposure to radiation in space can alter its characteristics.
The younger material analysed by Khawaja and colleagues was sampled by Cassini during a particularly fast flyby over Enceladus' south pole. The use of freshly ejected plume material guaranteed the removal of any possible interference from radiation.
So what do these and other analyses of plume samples tell us about Enceladus? Early Cassini samples were found to contain sodium salts , suggesting the plumes are fed by an underground liquid water ocean in contact with a rocky bottom. Later observations of Enceladus' "wobble" (slight shifts in its rotation) relative to Saturn demonstrated that its icy exterior shell is probably completely detached from the rocky core below.
This means that Enceladus' underground ocean (sandwiched between the ice and rock) is global , extending across the entire moon. The ocean is likely sustained by tidal flexing, where the varying gravitational tug of Saturn on Enceladus stretches and squeezes it, causing Enceladus to heat up and preventing the ocean from freezing and preventing the ocean from freezing.
The ability to (albeit, indirectly) sample the ocean has permitted a more comprehensive investigation of Enceladus' habitability - that is, whether Enceladus contains the necessary ingredients for life as we know it (namely a suitable energy source and mix of chemical elements).
Sampling the plumes
Analysis of Cassini's plume samples was made possible by a technique called "mass spectrometry". The process began with the high velocity impact between Cassini (flying at speeds of kilometres per second) and the solid plume material it collected.
This broke up the material smaller, charged fragments. After impact, an instrument exposed the fragments to an electric field which moved them towards a detector.
The timing of impact of the chemical fragments with the detector was then used to determine their mass and charge. Scientists could then "piece the jigsaw" back together to figure out the identity of the molecules that the fragments once formed.
When attempting to determine habitability, there are certain molecules to look out for in the data. Organics are simply molecules that contain carbon. Because life on Earth is fundamentally carbon-based, detecting carbon-bearing molecules of any form is a good start.
Organics have been detected with confidence in plume material, including "amines" , which can be precursors to amino acids (which in turn, can be precursors to proteins). Much larger "macromolecules" have also been seen. But their exact identity is currently uncertain owing to the limitations of the Cassini instrumentation.
Carbon is one of the "CHNOPS" elements (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur) which form the majority of atoms within living organisms on Earth. Apart from sulphur, these have all been detected with confidence in plume material.
Mass spectrometry can also indicate the types of energy sources available within an ocean. Photosynthesis, the primary energy source for life on Earth, is unlikely to be viable within Enceladus because its ocean is buried under kilometres of ice. Photosynthesis requires light and the ocean is almost certainly dark.
Luckily, there are other ways that life can extract energy from its environment. In the late 1970s, vast ecosystems were discovered at Earth's ocean depths, around hydrothermal vents - fissures on the ocean floor from which hot water rich in minerals emerges.
The microbes surviving around hydrothermal vents are forms of "chemosynthetic" life. They use the various substances found in hydrothermal waters to perform chemical reactions to get the energy they need.
It appears that ingredients for some chemosynthetic pathways such as carbon dioxide and hydrogen are available in sufficient quantities within Enceladus' ocean to theoretically make it a viable energy source.
In fact, the amount of hydrogen in plume material is so large that it would require a present-day source in Enceladus' ocean to explain it, most likely hydrothermal vents .
Recent study
Of course, we need to be careful in using plume material to infer what is inside the ocean. Processes during the formation of the plume (as it travels through the ice into space), may either dilute or concentrate certain substances. The harsh radiation could also cause chemicals within the plumes to react, leaving the material unrepresentative of the ocean it came from.
By analysing the freshly ejected plume material, the latest study removes that problem. Owing to the higher speed, samples obtained during this flyby should have fragmented in a way that would allow more types of molecules to become visible in the data.
And the samples collected did include new substances, as well as some that were already known, confirming that they came from within Enceladus, not from radiative alteration. Some of the newly detected substances further hint at a hydrothermal origin.
With knowledge of Enceladus' potential habitability, the European Space Agency is planning a mission, launching in the 2040s , that will perform flybys of Enceladus, and possibly even orbit and land on its surface.
With an upgraded suite of instruments, the mission will aim to look for evidence of life within plume material. If life resides around hydrothermal systems at Enceladus' depths, its journey to the ocean top and out into space may be long and arduous .
Yet recent work by Fabian Klenner from the University of Washington and colleagues, showed that even a single bacterial cell within an ice grain could be detectable via mass spectrometry. Thus the hope remains that if life resides within Enceladus, the evidence of it may be floating in space waiting for us to come and see it.
Flynn Ames has previously received funding from STFC
