The Case for a Return to Enceladus

Morgan L. Cable, NASA Jet Propulsion Laboratory, California Institute of Technology Shannon MacKenzie, Johns Hopkins University/Applied Physics Laboratory Marc Neveu, NASA Goddard Space Flight Center/University of Maryland Tori M. Hoehler, NASA Ames Research Center Amanda R. Hendrix, Planetary Science Institute Jennifer Eigenbrode, NASA Goddard Space Flight Center Frank Postberg, Freie Universitaet Berlin Carolyn Porco, Space Sciences Lab, University of California, Berkeley CA Christopher R. Glein, Southwest Research Institute Linda Spilker, NASA Jet Propulsion Laboratory, California Institute of Technology Amy E. Hofmann, NASA Jet Propulsion Laboratory, California Institute of Technology Alfred McEwen, LPL, University of Arizona J. Hunter Waite, Southwest Research Institute Peter Wurz, Physics Institute, University of Bern, Switzerland Jörn Helbert, German Aerospace Center (DLR) Ariel Anbar, Arizona State University Jean-Pierre de Vera, German Aerospace Center (DLR) Jorge Núñez, Johns Hopkins University Applied Physics Laboratory


An Accessible, Global Ocean
Enceladus is a relatively small (505 km diameter) moon of Saturn, and one of the brightest objects in the solar system, suggesting that its surface is young (see Schenk et al., 2018 and references therein). It is located within Saturn's E ring, which we now understand to be fed by ice grains emanating from Enceladus' plume (Kempf et al., 2018). While Voyager data suggested a link between Enceladus and the E ring, the Cassini Mission provided the first direct evidence for the existence of the plume and is therefore credited with its discovery (Porco et al., 2006;Dougherty et al., 2006;Hansen et al., 2006;Spahn et al., 2006;Tokar et al., 2006;Waite et al., 2006). The plume is formed, in part, by ~100 jets erupting from four main surface fissures, the 'Tiger Stripes', in the South Polar Terrain; material is also likely erupting from these fissures in the form of sheets . The presence of sodium salts in the ice grains ejected in the plume indicates that this material originates from Enceladus' ocean (Postberg et al., 2009). Modeling from Cassini data suggests that the plume is geophysically controlled (Běhounková et al., 2015;Patthoff and Kattenhorn, 2011), and therefore a long-lived phenomenon (Hemingway et al., 2020;Lainey et al., 2020).
Two independent lines of evidence show that the plume's subsurface water reservoir is not a regional sea but a global ocean. First, analysis of gravity measurements (McKinnon, 2015;Čadek et al., 2016;Beuthe et al., 2016) indicate that Enceladus is not in hydrostatic equilibrium, and that it requires isostatic compensation to fit the gravity field and excess topography. The simplest explanation for this is a floating ice shell of variable thickness atop a global ocean. Second, comparison of surface images collected over 7 years of Cassini observations imply that Enceladus' rotation has a forced physical libration (wobble) too large to be consistent with a crust grounded to the rocky core (Thomas et al., 2016). The simplest explanation is a freefloating crust, which is only possible if there is a global ocean.
The implication of a global ocean as opposed to a regional sea is important for life. A regional sea could have been formed by an impact and might only be transient, whereas a global ocean is more likely to be long-lived. This may mean that suitable conditions in the subsurface ocean of Enceladus have persisted long enough for life to gain a foothold.
Enceladus may be the only place in the solar system where we can easily sample fresh material from a global subsurface ocean without the need to dig or drill.

Hydrothermal Activity and Chemical Energy for Metabolic Processes
Life as we know it requires liquid water and energy as either light or redox disequilibrium. While light energy is not available in the subsurface oceans of Enceladus or any of the icy moons, Enceladus' plume composition is consistent with hydrothermal activity that could sustain redox disequilibrium over long durations. On Earth, hydrothermal activity supports chemosynthetic microbial communities and is proposed by some to have enabled the origin of life (Martin et al., 2008). Three key discoveries strongly support the occurrence of hydrothermal processes at the ocean-core interface of Enceladus. The first is that the Cassini Cosmic Dust Analyzer (CDA) found nanometer-scale dust particles consisting of silica (SiO2) (Hsu et al. 2015). Particles of this specific composition and limited size range (2-8 nm radius) are most likely to have been formed from ongoing high-temperature (>90 °C) hydrothermal leaching of silicate minerals. Once formed, the nanoparticles are transported through thermal-induced convection streams from the seafloor to the near-surface plume sources, where they are incorporated into ejected ice grains and subsequently released by sputter erosion in Saturn's E ring. A second line of evidence for hydrothermal activity comes from Cassini Ion and Neutral Mass Spectrometer (INMS) plume gas measurements. The relative proportions of measured volatiles are unlikely without a gas input, as methane would be preferentially trapped in clathrate hydrates (water-ice cages) and therefore depleted in the plume (Bouquet et al. 2015). One plausible explanation is hydrothermal activity (or microbes supported by such activity), which could generate sufficient methane to match observed levels. The third piece of evidence is identification by INMS of molecular hydrogen (H2) in the plume in a 'deep dive' flyby (E21) at an altitude of only 48 km (Waite et al., 2017). Molecular hydrogen is a product of serpentinization, a reaction process between ultramafic rocks (igneous rocks with high Mg and Fe contents) and liquid water that occurs in hydrothermal systems. This evidence further suggests that the ocean of Enceladus is in contact with, and reacting with, the rocky core through hydrothermal processes.
The flux of hydrothermally-sourced H2 and other reduced species into a CO2-containing ocean appears to provide a redox chemical disequilibrium that, on Earth, is known to support microbial metabolism. The pH of the Enceladus ocean is alkaline, likely in the range of 8.5-9, based on geochemical interpretations of Cassini mass spectrometry data . This is consistent with long-term buffering by a seafloor rich in quartz, talc and carbonate minerals. Together with the silica-rich nanograins and detection of molecular hydrogen, these results suggest a heterogeneous rocky core featuring chemical gradients between a carbonate-rich upper layer and serpentinized interior. Alkaline hydrothermal systems on Earth such as Lost City in the Mid-Atlantic Ocean (Kelley et al. 2005) are also rich in H2, and may represent the closest analogue to fluids associated with serpentinization at Enceladus' seafloor. These hydrothermal sites on Earth host teeming ecosystems based on chemosynthetic organisms.

Chemically Diverse, High Mass Organics
Cassini's CDA and INMS instruments characterized the composition of ice grains and vapor, respectively, emanating from the plume. Intriguingly, a subset of plume particles (~4%) contain complex organic molecules with masses greater than 200 u (u = atomic mass units). These Cassini evidence suggests that conditions in the ocean of Enceladus are consistent with hydrothermal systems on Earth, where redox chemistry is available to support rich communities of life and perhaps may have been where life first emerged.
organic molecules are found in relatively high abundance (>1% by mass) in these ice particles and are consistent with unsaturated (i.e., containing double and triple carbon-carbon and carbonnitrogen bonds) and partially aromatic (i.e., containing benzene substructures) molecules (Khawaja et al. 2019). Sources of biologically available nitrogen (NH3/NH4 + and amines) have also been found by these instruments. Importantly, analysis of CDA spectra collected at different velocities indicates that the detected organics are probably fragments of even larger organic molecules (Postberg et al., 2018); this interpretation is supported by detection of diverse, highmass organics in the vapor phase at the fastest flyby velocities (~18 km/s) (Waite et al., 2009). However, these instruments lacked the mass range and resolution to characterize these fascinating organics any further, so their sources (abiotic or biotic) are currently unknown.

Enrichment of Organics
Processes may be occurring in the ocean to increase the concentration of organic molecules in plume particles, which could improve the likelihood of detecting trace species (such as biomolecules). In the ocean, volatile exsolution or turbulence in the conduits leading to the surface can create bubbles that rise through the water column and could scavenge organic material, including microorganisms, by cells attaching to the bubbles' surfaces and being released as a spray when the bubbles burst at the water's upper surface (Porco et al., 2017). This process is most efficient with hydrophobic molecules and surfactants, and has been shown to increase organic and microbial concentrations by orders of magnitude on Earth (Carlucci and Williams, 1965;Blanchard and Syzdek, 1972). Bubble scrubbing may also lead to an organicrich film at the ocean surface of Enceladus, similar to the sea surface microlayer seen on Earth's oceans. On Earth, the wind-induced bursting of air bubbles at the sea surface microlayer in the polar regions generates a sub-micron sea spray aerosol containing ice particles (Burrows et al., 2014). These ice particles are similar in organic and salt content to the organic-rich plume grains detected by Cassini (Postberg et al., 2018); they are also enriched 10 to 1000-fold in organic material compared to Earth ocean concentrations. If aerosols forming at the water-ice interface at Enceladus emerge from a boundary layer of organic material, this process could concentrate hydrophobic biomarkers such as certain amino and fatty acids, if present (Cable et al., 2020).

Constraints and Outstanding Questions
The age of Enceladus and its liquid water ocean are still debated; Enceladus could be only ~100 Myr old (Ćuk et al., 2016), although recent evidence supports an older age (Lainey et al., 2020). Whether this influences the probability that life has taken hold there depends on how long it takes for life to emerge given suitable conditions, and this is also weakly constrained. Should life exist on Enceladus, the quality and abundance of evidence thereof will depend on the balance of factors that control the production, transport, and destruction of cells and their remnants. It has The Enceladus ocean appears to support a range of organic chemistry, including high-mass compounds diverse in CHON, that could serve as building blocks for, or be the byproducts of, life in this habitable environment.
If enrichment processes are occurring on Enceladus, the potential for detecting organic biosignatures in the plume is much higher than may be expected based on estimates of bulk concentrations in the ocean alone.
been argued that energy flux may be the chief limitation on the productivity of an enceledan biosphere. Existing estimates of energy flux span 5 orders of magnitude and propagate into even larger uncertainty in the possible abundance of cells and rate of production of biomolecules (Cable et al., 2020). Research to better constrain both energy fluxes and biomass-energy relations (Ray et al., 2020) would better inform the expected detection limits of biosignatures. Laboratory and field work studying the limits of life as we know it on Earth can also provide context for astrobiological investigations and challenging real-world samples to test instrument thresholds (e.g., Klenner et al., 2020a,b). Finally, investment in strategies and technologies to minimize the risk of contamination would help in two areas: first, reducing contamination risk for ultrasensitive life-detection payloads could lead to increased confidence that analytical results would reflect true biosignature detections; second, minimizing contamination to the ocean world environment would open to exploration regions previously declared off-limits in the interest of planetary protection, ultimately enabling more capable and wider-reaching astrobiology mission architectures.

Strategies to Search for Life
While remote sensing techniques can provide valuable information about surface composition, geology, topography, etc., ultimately in situ sampling is required to fully address the life question. Current remote sensing instruments (such as UV or NIR spectrometers) cannot unambiguously identify biomolecules (e.g., fatty acids or a biopolymer) from orbit at the subppm limits of detection expected in this potentially energy-constrained environment (Cable et al., 2020), although we note that flight instruments in development may be able to achieve ppb limits of detection by the end of the decade. Thanks to NASA investments in the last decade, the available technology and community expertise are ready to tackle one of the biggest questions in NASA's portfolio, and one that reaches beyond planetary science to impact us all: Are we alone?

Conclusions
The Enceladus plume provides direct access to recently ejected ocean-derived materials in the plume or as surface deposits. In response to the seminal discoveries of the Cassini mission (many made after the last decadal survey), the next strategic step is to search for biosignatures in the ocean materials. This is a key objective of the 2018 NASA Strategic Plan, tackling an overarching question of the Vision & Voyages Decadal Survey: "Beyond Earth, are there contemporary habitats [...] and do organisms live there now?". The subsurface ocean of Enceladus is the only confirmed modern environment beyond Earth with all ingredients necessary for life: liquid water, chemical building blocks, and energy sources (e.g., Des Marais et al., 2008;McKay et al., 2008McKay et al., , 2014Hendrix et al., 2019;Cable et al., 2020). Current mature instrumentation and measurement strategies can enable the detection of trace quantities of key features expected to be universal to life, while also measuring the possible breadth of abiotic or prebiotic compositions if life is not present. The search for recent and extant life in Enceladus' ocean materials is possible with today's technology, and NASA investments are continuing to improve the maturity and sensitivity of these instruments. It is recommended that Enceladus receive strong consideration for New Frontiers and Flagship missions in the coming decade.