Proposed Mission to find life on Saturn’s moon Enceladus

Saturn’s moon Enceladus has a subsurface ocean covered by a layer of ice. Some liquid escapes into space through cracks in the ice, which is the source of one of Saturn’s rings. In October 2015, the Cassini spacecraft flew directly through the plume of escaping material and sampled its chemical composition. They found that the plume contains molecular hydrogen, H2, a sign that the water in Enceladus’ ocean is reacting with rocks through hydrothermal processes. This drives the ocean out of chemical equilibrium, in a similar way to water around Earth’s hydrothermal vents, potentially providing a source of chemical energy.

Saturn’s moon Enceladus has an ice-covered ocean; a plume of material erupts from cracks in the ice. The plume contains chemical signatures of water-rock interaction between the ocean and a rocky core. We used the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft to detect molecular hydrogen in the plume. By using the instrument’s open-source mode, background processes of hydrogen production in the instrument were minimized and quantified, enabling the identification of a statistically significant signal of hydrogen native to Enceladus. We find that the most plausible source of this hydrogen is ongoing hydrothermal reactions of rock containing reduced minerals and organic materials. The relatively high hydrogen abundance in the plume signals thermodynamic disequilibrium that favors the formation of methane from CO2 in Enceladus’ ocean.

Planetary bodies with global oceans are prime targets in the search for life beyond Earth owing to the essential role of liquid water in biochemical reactions that sustain living organisms. In addition to water, life requires energy and a source of essential chemical elements (C, H, N, O, P, and S). Although there is compelling evidence for liquid water and many of the essential elements on several ice-covered planetary bodies in our solar system and beyond, direct observation of energy sources capable of fueling life has, to this point, remained elusive. A report on recent flybys of the ice-covered saturnian moon Enceladus by the Cassini spacecraft reveal the presence of molecular hydrogen (H2) in jets of vapor and particles ejected from a liquid water ocean through cracks in the ice shell. The abundance of H2 along with previously observed carbonate species suggests a state of chemical disequilibria in the Enceladus ocean that represents a chemical energy source capable of supporting life.

The Enceladus Life Finder mission was proposed in 2015 for Discovery Mission 13 funding. If selected, a launch readiness date of December 31, 2021 may be possible.

The ELF mission would search for biosignature and biomolecules in the geysers of Enceladus. The south polar jets loft water, salts and organic molecules dozens of miles over the moon’s surface from an underground regional ocean. The theory is that the water is warmed by thermal vents similar to features found deep in Earth’s oceans. ELF’s instruments would measure amino acids — the building blocks of proteins — analyze fatty acids, and determine whether methane (CH4) found in the plumes could have been produced by living organisms.

The current mission concept would have the ELF orbiter fly 8 to 10 times over a period of 3 years through plumes of water launched above the south pole of Enceladus. The geysers could provide easy access for sampling the moon’s subsurface ocean, and if there is microbial life in it, ice particles from the sea could contain the evidence astrobiologists need to identify them.

Objectives

The goals of the mission are derived directly from the most recent decadal survey: first, to determine primordial sources of organics and the sites of organic synthesis today; and second, to determine if there are current habitats in Enceladus where the conditions for life could exist today, and if life exists there now. To achieve these goals, the ELF mission has three objectives:

1. To measure abundances of a carefully selected set of neutral species, some of which were detected by Cassini, to ascertain whether the organics and volatiles coming from Enceladus have been thermally altered over time.
2. To determine the details of the interior marine environment — pH, oxidation state, available chemical energy, and temperature — that permit characterization of the life-carrying capacity of the interior.
3. To look for indications that organics are the result of biological processes through three independent types of chemical measurements that are widely recognized as diagnostic of life.

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