Concentration of species in the ocean of Enceladus and in the seawater of the oceans on earth.
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
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
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
The geochemical system of the ocean of Enceladus could be composed mainly by
Carbonates and bicarbonates ions
The presence of hydrogen can form linear chains of hydrocarbons like methane
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
Abundance of
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
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
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.
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.
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
Species | Enceladus concentration (g/kg) | Earth concentration (g/kg) |
---|---|---|
0,008 | ||
0,019 | ||
0,067 | ||
0,412 | ||
7076 | 19,353 | |
2867 | 0,016 | |
0,013 | ||
0,031 | 0,107 | |
0,399 | ||
1284 | ||
7343 | 10,784 | |
0,024 | ||
1788 | ||
0,015 | ||
0,008 | ||
0,038 | ||
0,1–0,01 | 2713 | |
0,008 |
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.
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
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
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
Species | Enceladus concentration (g/kg) | Earth concentration (g/kg) |
---|---|---|
7076 | 19,353 | |
2867 | 0,016 | |
0,031 | 0,107 | |
7343 | 10,784 | |
0,1 - 0,01 | 2713 |
Two scenarios can be considered to calculate the amount of sulphate that could be oxidized on the ocean of Enceladus. The lower concentration of
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.
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
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
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.
Acknowledgments
This work was possible thanks to the Technology-Based Company RS3 Remote Sensing SL.
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