**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 *=*CO3 <sup>2</sup>. According to Glein et al. [22], the pH is 12.15 1.15.

Carbonates and bicarbonates ions CO3 <sup>2</sup>*=*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 [35–37].

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.
