Is the Ocean of Enceladus in a Primitive Evolutionary Stage?

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Katherine Villavicencio Valero, Emilio Ramírez Juidías and Aina Àvila Bosch 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 1 species, through methanogenesis, can form methane. It was also detected molecular hydrogenH2by 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, NaHCO3and Na2CO3indicating 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 productionofmolecularhydrogenisthehighratioofH 2=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 CO2in 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 CO2and 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 theCO2activity 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 SiO2and 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 CH4from the chemical reaction between CO2and H2. This methane could be present 2 Astronomy and Planetary Science - From Cryovolcanism to Black Holes and Galactic Evolution 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 CO2in the atmosphere dropped and, before the emergence of photosynthetic life, the carbon dioxide was more abundant in the atmosphere than nowadays [32]. Abundance of CO2in 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 CO2in 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 [35–37]. 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,H 2S, H2, CH4and 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,CH 4, 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 3 Is the Ocean of Enceladus in a Primitive Evolutionary Stage? DOI: http://dx.doi.org/10.5772/intechopen.104862 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 CO2started 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. 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. 4 Astronomy and Planetary Science - From Cryovolcanism to Black Holes and Galactic Evolution 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 3show 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. 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 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. 5 Is the Ocean of Enceladus in a Primitive Evolutionary Stage? DOI: http://dx.doi.org/10.5772/intechopen.104862 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 CO2and H2O within the ocean [22]. 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. Species Enceladus concentration (g/kg) Earth concentration (g/kg) BOHðÞ 40,008 BOHðÞ 30,019 Br 0,067 Ca2þ0,412 Cl7076 19,353 CO322867 0,016 F0,013 HCO30,031 0,107 Kþ0,399 Mg2þ1284 Naþ7343 10,784 NaCl 0,024 NaCO31788 NaHCO30,015 NaOH 0,008 OH0,038 SO420,1–0,01 2713 Sr2þ0,008 Table 1. Concentration of species in the ocean of Enceladus and in the seawater of the oceans on earth. 6 Astronomy and Planetary Science - From Cryovolcanism to Black Holes and Galactic Evolution 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 CO2activity. Naþis 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.CO 32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. 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 Figure 4. Sodium ion and chlorine present in the ocean of Enceladus. The concentration of species are calculated in function of the CO2activity. Figure 5. Some carbonates, bicarbonates, and sulfate present in the ocean of Enceladus. The concentration of species are calculated in function of the CO2activity. 7 Is the Ocean of Enceladus in a Primitive Evolutionary Stage? DOI: http://dx.doi.org/10.5772/intechopen.104862 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 CO32in 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. 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 Species Enceladus concentration (g/kg) Earth concentration (g/kg) Cl7076 19,353 CO322867 0,016 HCO30,031 0,107 Naþ7343 10,784 SO420,1 - 0,01 2713 Table 2. Concentration of species present in the seawater of the oceans on earth and in the ocean of Enceladus. 8 Astronomy and Planetary Science - From Cryovolcanism to Black Holes and Galactic Evolution [44] Juhász A, Horányi M. 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DOI: 10.1038/s41467-018-02876-y 15 Is the Ocean of Enceladus in a Primitive Evolutionary Stage? DOI: http://dx.doi.org/10.5772/intechopen.104862