**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 �/CO3 <sup>2</sup>� was lower on Earth than on Enceladus. The most abundant ionic species are Na<sup>þ</sup> and Cl� in both oceans. The release of molecular hydrogen by serpentinization of the seafloor is also possible in both oceans. Sulphate species SO4 2� 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.
