Open access peer-reviewed chapter

Is the Ocean of Enceladus in a Primitive Evolutionary Stage?

Written By

Katherine Villavicencio Valero, Emilio Ramírez Juidías and Aina Àvila Bosch

Submitted: 02 April 2022 Reviewed: 08 April 2022 Published: 18 May 2022

DOI: 10.5772/intechopen.104862

From the Edited Volume

Astronomy and Planetary Science - From Cryovolcanism to Black Holes and Galactic Evolution

Edited by Yann-Henri Chemin

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Abstract

Enceladus has a subsurface ocean in the South Pole that has been inferred due to the presence of water vapor and other molecules like molecular hydrogen and ammonia detected by the Cassini mission from the ejection of material through the plumes in that region. The chemical composition of this ocean could give some information about the evolutionary stage of the icy moon if its components are found to be similar with the aqueous chemistry of the primitive oceans on Earth during glacial periods. Here we present a comparative geochemical analysis between the ocean of Enceladus and the aqueous composition of the oceans on Earth during the Snowball Event, in order to figure out if there are similar species, how the interaction of the metabolic processes between them works and if, in the future, those molecules could evolve making possible the emergence of life.

Keywords

  • ocean
  • snowball event
  • aqueous chemistry
  • species
  • life

1. Introduction

Enceladus, one of the moons of Saturn, presents a global ocean beneath the ice shell [1]. The existence of that ocean was suggested because of the water vapor detected by the Cassini mission, through the ejection of material from the plumes located in the south pole [2, 3]. The expulsion of material from the water plumes could be related to hydrothermal activity [4], where ice particles are heated due to the tidal deformation [5] and expelled to the surface. Evidence that those particles are associated to hydrothermal activity are the silicate salts residues found at the E-ring [6], and the small size of the nanoparticles of that ring. Both characteristics indicate that the possible liquid water within the ocean layer was previously in contact with a hot silicate environment [7].

The Ion and Neutral Mass Spectrometer (INMS) instrument on board of the Cassini mission also detected ammonia and some traces of organic molecules like benzene [8]. Ammonia is one more clue of the presence of liquid water. Residuals from ammonia are nitrogen-bearing and oxygen-bearing molecules that, in combination, could convert into amino acids like it happens on Earth [9]. Other detected species were H2O and CO2 [10]. The metabolic interaction between these latter two species, through methanogenesis, can form methane. It was also detected molecular hydrogenH2 by the Cassini spacecraft [11]. There were also found some species with compounds of carbon, nitrogen, oxygen and sulfur [8, 12, 13]. The interaction between molecular hydrogen and some carbonates within the ocean produce a chemical instability that constitutes an energy source that may support life [11].

The Cosmic Dust Analyzer (CDA) aboard of the Cassini mission detected water ice, organic molecule, and siliceous material [14]. There were also detected concentrations of Na, and some sodium salts like NaCl, NaHCO3 and Na2CO3 indicating the presence of liquid water [15, 16]. To maintain this liquid water into the global ocean, the tidal dissipation could be considered as an energy source that come from inside Enceladus [17]. Tidal heating also acts in the solid core provoking high temperatures into the hydrothermal activity [18]. The hydrothermal activity creates convection columns that produce a dynamic movement in the ocean transporting the heat into the ice shell from the core [19]. The dissipation of the heat linked to the gas ratios present in the water plume could determine the state of the hydrothermal activity [7].

According to Woods [20], a hydrothermal source of gas could explain the distribution of hydrogen in the water plume. In this sense, it must be emphasized that the abundance of hydrogen detected is similar to some traces of volatile compounds like carbon dioxide, methane, and ammonia [8]. Laboratory simulations [7] suggested that, in Enceladus, the molecular hydrogen is a product of internal reactions. Evidence of the internal production of molecular hydrogen is the high ratio of H2/H2O which cannot come from a gas trapped in the ocean, because of its high concentration, and it cannot also come from the remnants of a formation environment, due to the low ratio ofHe/H2 [21].

The geochemical system of the ocean of Enceladus could be composed mainly byNa2O -HCl-CO2 -H2O. The concentration of CO2 in the plumes is assumed to be the same that may be found in the dissolved molecules of the subsurface ocean. Its presence suggests a basic pH for the ocean of Enceladus. The estimation of this pH is based on the study of the thermodynamic equilibrium, considering the temperature close to 0°C, the pressure at 1 bar, the carbon dioxide activity, the chloride concentration, and the dissolved inorganic carbon HCO3/CO32. According to Glein et al. [22], the pH is 12.15 ± 1.15.

Carbonates and bicarbonates ions CO32/NaCO3 are also present in the ocean [23] and could come from soluble carbonate minerals, formed through the reaction of a trapped CO2 and silica minerals during water-rock differentiation. If the rocks of the ocean react withCO2, it is feasible the carbonation process for high water-rock ratios [24]. The metal concentration in the seawater is formed by phyllosilicates and hydroxide minerals, that need an acid in order to hydrolyze and being incorporated into carbonate minerals [23].

CO2/H2O ratio in the plume of Enceladus is similar to the ratio found in the seawater on Earth [23], where theCO2 activity is controlled by alteration of minerals, assuming that water-rock interactions are the main driving force of the pH as well as the composition of the ocean. On Enceladus, the serpentinization is assumed to be the result of minerals alteration [25], through a hydrolysis of primary minerals containing iron and magnesium which product is the hydrogen. This process is usually associated with ultramafic rocks (<45% of SiO2 and high Mg - Fe content), which reaction that takes place is the oxidation by water of Fe(II) and Fe(III) in minerals such as olivine and pyroxene. The product of this reaction is the molecular hydrogen.

The presence of hydrogen can form linear chains of hydrocarbons like methane CH4 from the chemical reaction between CO2 and H2. This methane could be present in low concentrations into the ocean of Enceladus [26]. Evidence of this, it is the formation of clathrates that are able to trap certain molecules, which then, would rise to the surface and eventually dissociate and enrich the plume with methane. Methanol was also detected by the Cassini mission; it is possible that this compound has a biological origin [27]. It was found that the CH3OH/H2O ratio has certain correlation with biotic activity around the hydrothermal vents. The concentration of methanol detected in the atmosphere is high, which gives a clue about that this specie is formed beneath the ice shell before being expelled into the atmosphere. These organic compounds detected could be considered as a building block of life or even by-products of life [28].

On Earth, the first signs of life came from the Archean oceans where the oxidative reactions were a product of the interaction between molybdenum and rhenium [29]. There were only traces of oxygen before the Great Oxygen Event but then, after it, the photosynthetic activity led to an increment of this element [30]. The evolution of oxygen in the atmosphere and oceans went through five stages [31]. During the Cryogenian age, the atmosphere and the shallow oceans had an increase of oxygen. The oxygen concentration was stagnant in that era, and subsequently it had an increment that continued after the next million years and might have culminated around the Carboniferous age. During glacial periods, the concentration of CO2 in the atmosphere dropped and, before the emergence of photosynthetic life, the carbon dioxide was more abundant in the atmosphere than nowadays [32].

Abundance of CO2 in the atmosphere during that time would be a consequence of a carbon-silicate cycle during millions of years that after changed the Snowball events conditions [33]. The concentration of oxygen was low in the oceans during the Snowball periods. The water had a high level of acidity due to the high concentration of CO2 in the atmosphere [32]. The ice-covered conditions on Earth were altered because of the melting of the ice crust that took place due to the increase of the temperature by volcanoes activity, which reduced the presence of CO2 in the atmosphere and provoked the emergence of liquid water [34]. The high volcanic activity triggered the extensive presence of hydrothermal vents during the Cryogenian age [35, 36, 37].

Nowadays, hydrothermal systems can be classified as black smokers and lost city systems. The first one, are characterized by the black smoke that rises from the chimney-like rocky formations, where seawater is in contact with the magma chambers and emerges with an acid pH 2–3, a high content of dissolved metals such as Fe (II) and Mn (II), a variety of gases originated from volcanic activity like CO2, H2S, H2, CH4 and also with high temperatures up to 405°C. In contrast, in the lost city systems, the water that circulates trough the vents is not in contact with the magma, instead, it is heated by convection from the mantle and by exothermic chemical reactions between the fluid and the surrounding rocks reaching temperatures of 200°C [35]. The rock that interacts with the fluid is dominated by low-silica iron and magnesium rich minerals, provoking the methanogenesis by serpentinization of the hydrogen and the reduction of carbon dioxide in the ocean. In this case, the pH of the fluid is basic 9–11, it has dissolved gases likeH2, CH4, low-mass of hydrocarbons, and a low dissolved CO2.

Similarities could be found along with the ancient oceans on Earth during the Snowball Events and the current conditions of the ocean on Enceladus. Here we present a comparative geochemistry analysis of both oceans. We also describe a chemical metabolic process based on numerical simulations that could take place within the global ocean of Enceladus, in order to infer if the current conditions of that ocean could evolve to create the building chains of life. During glaciations ages, the ice-covered Earth allowed for maintaining the liquid water beneath the ice crust, and subsequently that liquid water emerged to the surface by the hot spots or hydrothermal vents once the high concentration of CO2 started changing the conditions of the atmosphere [38]. On Enceladus, there are hints that indicate the presence of liquid water, such as the hydrated sodium salts detected by the Cassini mission. The molecular hydrogen found also gives clues about a hydrothermal activity beneath the ice shell. We aim to infer a possible evolutionary stage of the ocean of Enceladus that could make possible the emergence of life.

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2. Study area

Models of the internal structure of Enceladus reveals an ocean on average 26–31 km in depth below an ice layer between 21 and 26 km of thickness [39]. Salinity geochemistry simulations of the ocean of Enceladus show values nearly similar to Earth, around 20 g/kg [40, 41]. The quantity of water vapor ejected from the plumes is around 150–300 kg/s [42]. This ejection of particles supply the composition of the ring E of Saturn, with <10% of the material catching into it, also suggesting a liquid origin [43, 44]. Figure 1 shows the distribution of the plumes along the south pole of Enceladus.

Figure 1.

Digital elevation model (DEM) of the plumes called “Tiger stripes” located in the south pole of Enceladus. It was used the images taken by the Cassini Mission. This DEM was developed using the software TopoCal 2022 v.9.0.811.

Beneath the south pole the composition of the particles is mainly salt rich, implying that those salts are larger than salt-poor grains and they are expelled with lower escape velocity. The escape speed of particles from the plumes in Enceladus is on average 1.85–2.25 km/s, according to the measures from the dusty plume by the flyby of the Cassini spacecraft [45]. Figures 2 and 3 show the longitudinal (y axis) and transversal (x axis) height profiles of the plumes of Enceladus from Figure 1. The longitudinal axis of Figure 2 presents a radius in the central plume of 190 m, besides, the transversal axis of Figure 3 shows a radius of 90 m. The distribution of the fissures along the plumes seems to be aligned in the y axis.

Figure 2.

Elevation profile of the DEM from top to bottom.

Figure 3.

Elevation profile of the DEM to the center from left to right.

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3. Materials and methods

In this research, we used the data of the molecular species detected by the INMS instrument on board of the Cassini Mission. The spectral signatures were encoded according to Ramírez-Juidías et al. [46]. Then, there were selected the common spectral lines present in the ocean of Enceladus and in the seawater of the oceans on Earth. Based on the spectral lines, it was applied data mining in order to extract the concentration of species detected in the material ejected from the plumes. Table 1 shows some of the species present in the ocean of Enceladus and the seawater of the oceans on Earth [47] with their concentration in g/kg. Each specie was extrapolated to the geochemical processes associated to the activity of CO2 and H2O within the ocean [22].

SpeciesEnceladus concentration (g/kg)Earth concentration (g/kg)
BOH40,008
BOH30,019
Br0,067
Ca2+0,412
Cl707619,353
CO3228670,016
F0,013
HCO30,0310,107
K+0,399
Mg2+1284
Na+734310,784
NaCl0,024
NaCO31788
NaHCO30,015
NaOH0,008
OH0,038
SO420,1–0,012713
Sr2+0,008

Table 1.

Concentration of species in the ocean of Enceladus and in the seawater of the oceans on earth.

According to the method patented by Ramírez-Juidías et al. [46], the data mining process was carried out through the application of modified genetic algorithms, iteratively analyzing a large amount of data through a process similar to genetic mutation, in order to extract the variables that are then used to obtain the concentrations (g/kg) of species in the ocean of Enceladus, using the wavelengths between 0.35 and 1 μm from the spectral data taken by the VIMS instrument.

The encoding model developed to obtain these concentrations consists in building a vector of size equals to the number of iterations to execute. The kth-order of the vector represents the work that is done in the kth-position. In this case, a population of alternative solutions is settled for a certain number of chromosomes, that represent the natural sequence in which the variables (spectral signatures) are programmed.

The process of planning and programming required for the extraction of the concentrations of species is usually conducted by applying a three-level model called respectively Strategic Approach, Tactical Approach and Operational Approach. This model can be replicated using machine learning.

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4. Results and discussion

Sodium ion and Chlorine are the most abundant species in the ocean of Enceladus. Figure 4 shows the concentration of both species in mol per kg of H2O. The quantity of mol is calculated in function of the CO2 activity. Na+ is present in concentrations around 0.800 mol/kg while the concentrations of Cl are constant, around 0.400 mol/kg. Figure 5 also shows that there are present some carbonates CO32, bicarbonate HCO3 and sulfate SO42. CO32 has a concentration on average 0.075–0.030 mol/kg which tends to decrease with the activity of CO2. HCO3 presents an increment from 0.020 to 0.070 mol/kg and SO42 has a concentration between 0.01 and 0.1 mol/kg.

Figure 4.

Sodium ion and chlorine present in the ocean of Enceladus. The concentration of species are calculated in function of the CO2 activity.

Figure 5.

Some carbonates, bicarbonates, and sulfate present in the ocean of Enceladus. The concentration of species are calculated in function of the CO2 activity.

The concentrations of salinity and chlorinity are relatively constant in the current terrestrial oceans. The average concentration from the seawater with a pH of 8.1 and temperature of 25°C are detailed in Table 1. Geological information extracted from sedimentary layers reveals that deep oceans were in a reduced state till the end of the Paleoproterozoic era. Iron and calcium sulfate probably played as reduced agents with the oxygen converting FeO into Fe2O3, and precipitating CaSO4. During the Cryogenian era the concentration of sulphate could rise to levels similar to the recent ones, around 23 mol/kg of H2O [47].

Table 2 shows few key species present in the ocean of Enceladus and in the seawater of the oceans on Earth. Sodium ion and Chlorine are the most abundant species in both oceans. The oceans on Earth are saltier with a pH of 8.1 on average, while the ocean of Enceladus is more basic, around pH 12.2. The ocean of Enceladus has more dissolved inorganic carbon than the ocean on Earth. On Enceladus, the predominant carbonate is CO32 while on Earth is the bicarbonateHCO3. The abundance of CO32in the ocean of Enceladus could be due to the serpentinization of the molecular hydrogen. The concentration of sulfur in the ocean of Enceladus is variable compared to the one present on Earth.

SpeciesEnceladus concentration (g/kg)Earth concentration (g/kg)
Cl707619,353
CO3228670,016
HCO30,0310,107
Na+734310,784
SO420,1 - 0,012713

Table 2.

Concentration of species present in the seawater of the oceans on earth and in the ocean of Enceladus.

Two scenarios can be considered to calculate the amount of sulphate that could be oxidized on the ocean of Enceladus. The lower concentration of SO42, 0.01 g/kg, displayed in Table 2, takes place only in aqueous reductants environments where HS reacts with the oxidants, while in the larger concentration 0.1 g/kg, some minerals are considered as a source for reductants. The concentration of sulphate in the ocean of Enceladus is below to the current amount of sulphate found on the oceans on Earth but, this concentration could have been smaller during the Snowball events, being close to the current quantities on the ocean of Enceladus.

The predominant concentration of inorganic carbonate species found in the ocean of Enceladus, set the ocean as not compatible with life except for the methane detected that can be a product of the methanogenesis of the carbon dioxide and the hydrogen. Would it be possible that the species detected in the ocean of Enceladus evolve to create the chains of life? how were the chemical conditions of the primitive terrestrial oceans before rising life? In order to figure out which similarities could be found between the terrestrial oceans and the ocean of Enceladus, it is necessary to understand the evolution of the ancient aqueous geochemistry of the oceans in the primitive Earth.

During the first stage of formation of Earth, it was bombarded by hydrous asteroids mainly type Cl chondrites bringing water, organic molecules, and chondritic minerals. Tectonic activity facilitated to diversity the mineralogy along the crust, increasing the mafic content of the top layers through the eruption of hot basaltic lavas. Chondritic material has been also detected in the plumes of Enceladus [14, 21], that is why, it could be possible to infer that this material can be settled in the seafloor of its ocean [48].

Organisms cannot devise chemical processes by themselves, they must copy natural reactions, adapt them, and optimize them through time. Phosphorylation is the addition of a phosphate group into a protein, being the main mechanism of biochemistry. This mechanism participates in some proteins regulation like ATP formation, fatty acids metabolization, and citric acid cycle. Prebiotic phosphorylation of biological molecules is a reaction that represent a challenge for the study of the origin of life. It has been proven that using diamidophosphate (DAP) instead of phosphates, thermodynamic barriers decreased for this reaction in water, and different organic building blocks were able to be assembled [49]. Based on that analysis, it was demonstrated that is possible to generate DAP and other amino - phosphor compounds when P-bearing molecules are mixed with aqueous ammonia solutions. The sources of phosphor could come from iron P-bearing minerals, condensed phosphates which contain salts and metals, or reduced phosphorus compounds. Those reactions probably took place in the aqueous conditions of the early Earth. If the concentration of ammonia in the hydrothermal vents of Enceladus would be similar to the prebiotic oceans on Earth, that phosphate reaction could happen in the ocean of Enceladus.

Although the currents anaerobic sulfur-reducing hyperthermophiles are associated to the first forms of life on Earth, the supply of sulfur in early times is supposed to have been more limited than now. However, due to geochemical evidence, it was proposed that iron could has been the first external electron acceptor in microbial metabolism [50]. Table 2 shows a low concentration of sulfur in the ocean of Enceladus, and for this reason, the hydrothermal activity inferred by the molecular hydrogen detected from the plumes suggests that the iron could play a similar role in the geochemical reactions in the ocean of Enceladus.

Life not only came from hydrothermal vents but also, it could have risen on fresh-water accumulations from geysers, precipitations, and hot spots, which could have linked to hydration-dehydration cycles. In hydrothermal vents, the thermal gradient allows for the concentration of solutes in the vents through the polymerization of minerals and sources of chemical energy like serpentinization. In the second system, the extreme concentration of chemical species took place due to the wetting-drying cycles, and the energy derived from evaporation provided the conditions of polymerization [51].

Enceladus looks like a potentially habitable world due to the similar current concentration of some key species present in the ocean to the ones that were present in the seawater of the oceans on Earth. There were detected traces of organic elements that could come from the water-rock interaction which can be also filled by minerals like iron, sodium, potassium, and calcium. There have been also detected the presence of biological consumable energy that on Earth, this energy is supplied by photosynthetic organisms like chemoautotrophs from a methanogenesis activity.

The environmental condition into the ocean of Enceladus could be in accord with life due to similarities with the oceans on Earth (pressures from 0.5 to 600 bar, which can be also found in some terrestrial environments [52], temperatures of 0–90°C, salinity calculated from the plumes between 0.5 and 2%. These values are lower than the ones on Earth which salinity is 3.5%). According to Porco et al. [53], the concentration of biological compounds could be potentially higher in the plume than in the seawater if the bubble scrubbing were allowed. These structures rise through the fluid while the organic material is attached to the water-gas interface until the eruption of the bubble through the jets. The addition of these organic compounds depends on their solubility and the surface activity. Surfactants like amphiphilic molecules would be instantly attached to the interface as they are able to reduce the surface tension, then the hydrophobic compounds would also be quickly attached.

Measurements in situ will be necessary to probe the feasibility of the ocean of Enceladus to harbor life. The information taken from the plumes by the Cassini mission provided data about the composition of the material expelled by the jets. The possibility to analyze samples from the plumes could bring a better understanding in how to make a characterization of the seawater and also, distinguish if there are residual elements that come from the interaction between living organisms and the environment.

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5. Conclusions

The Earth had been through three different periods of time totally covered by ice, while maintaining a liquid ocean beneath the crust. The first Snowball event took place around 2.5 billion years ago, during the Paleoproterozoic age, and it was closely related to the Great Oxidation Event. The second and third Snowball events came about the Cryogenian era during the Neoproterozoic age, from 720 to 630 million years ago. Those Earth stages could be similar to the current physical and chemical conditions on Enceladus. The composition of the ocean on Enceladus is theorized through geochemical models, using the data taken by the Cassini mission. The concentration of species present in the material expelled from the plumes has been also calculated, allowing for the estimation of the pH of the ocean.

The pH is more basic on Enceladus than it was on Earth. The ratio of the carbonate equilibrium HCO3/CO32 was lower on Earth than on Enceladus. The most abundant ionic species are Na+ and Cl in both oceans. The release of molecular hydrogen by serpentinization of the seafloor is also possible in both oceans. Sulphate species SO42 appear to be scarcer on Enceladus but the role of electron acceptor could be taken by other elements like Iron, as it happened on Earth. The possible hydrothermal activity on Enceladus could be considered as a hint to biological activity, if it is compared with the hot spots from the deep oceans on Earth, where life arose.

The data calculated and compared in this research show a slightly similarity between the ocean on Enceladus and the oceans on Earth during the Snowball events, but it will be necessary to analyze some samples taken from the material expelled by the plumes. Previous research emphasized that the traces of organic material detected on Enceladus could come from biotic sources due to the few amino acids detected, that are known to be essential for the presence of life. Methane detected could also have a biotic origin, since there is a methanogenic bacterium called Methanothermococcus okinawensis which should be capable of thriving under the physical and chemical conditions of Enceladus. These organisms were found in a deep-sea hydrothermal vent, and they are able to survive in an environment with high temperatures and high pressures, up to 50 bar. The production of molecular hydrogen by serpentinization allows for them to survive in these extreme conditions. It is possible that these lifeforms can spread inside the hydrothermal activity that is present on Enceladus [54].

To probe the presence of biological activity on Enceladus and to infer the possible evolutionary primitive stage of its ocean, it is necessary to consider some bioindicators, such as the isotope carbon rates in organic and inorganic molecules, the ratio of simple hydrocarbons and amino acids in function of more complex molecules, and how the amino acids detected from the plumes could evolve. This research shows that the inorganic carbonates species are higher than the organic ones and the presence of sulphates are low, yet similar to the ones present in the oceans on Earth during the glaciation stages. Answering the question about the evolutionary stage of the ocean, these results allow us to speculate that, instead of having some keys species that could change the global conditions of Enceladus through time, it will be essential a global geological event that allows for the release of these species from the ocean to the surface, leading to an increase in the mass flow of species in the atmosphere and, therefore, an enrichment of it over time.

Furthermore, because of the presence of methane and some aminoacids, it could be possible to infer that, in the future, those molecules could evolve to more complex ones and ignite the chains of life. If more glaciations on Enceladus would happen in the future, it will allow the oxygenation of the atmosphere and the releasing of carbon dioxide into the atmosphere, leading to a change of the global conditions of Enceladus. It would be also important to analyze samples taken from the plumes, to have a better understanding of the seafloor conditions and to figure out which kind of extreme lifeforms could thrive on Enceladus.

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Acknowledgments

This work was possible thanks to the Technology-Based Company RS3 Remote Sensing SL.

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Katherine Villavicencio Valero, Emilio Ramírez Juidías and Aina Àvila Bosch

Submitted: 02 April 2022 Reviewed: 08 April 2022 Published: 18 May 2022