Scientists are strongly speculating (backed by the 1995 Galileo Spacecraft mission's findings) that Europa, the smallest of the four Galilean satellites and the 6th closest moon of the planet Jupiter house a liquid ocean underneath its solid icy crust. That cosmic mission discovered that underneath the icy crust of Europa, lies salty ocean which is kept warm by tidally generated heat and volcanic activities. It is believed that the biggest of Europa's craters are surrounded by concentric rings and these rings are likely to be filled with ice. And there is a possibility that this outer crust of ice is approximately 100km thick where only the top 10km are frozen solid which ushers the great possibility of the existence of a global ocean in a liquid form and at least 62 miles deep beneath these icy crust. And chances are, wherever there is water, there will be life. But what kind of life it would be in Europa that is a matter of great research. But some ideas are already there.
Since Europa's ocean lies quite a few miles beneath the icy crust, it is perceived that the way oxygen influences our existence here on Earth, is perhaps not the case with Europa due to liquid water's separation from atmospheric oxygen by several miles of chilling ice. But it has been proved that without oxygen, life could conceivably exist at hot springs deep in the ocean floor. Life in Europa could exist in its under-ice ocean, perhaps in a similar fashion to that of Earth's deep-ocean hydrothermal vents or the Antarctic Lake Vostok, the largest of more that 140 sub-glacial lakes found under the surface of Antarctica. Up until 1970's, it was believed that Sun was absolutely essential for the existence of life. But in 1977, during a deep-sea exploration in the Galapagos Rift, scientists discovered flocks of giant tube worms, clams, crustaceans, mussels, and other various creatures gathered around undersea volcanic features known as black smokers and these aquatic creatures were found to have thrived despite having no access to sunlight, depending on an entirely independent food chain! Instead of usual plants, it was found that these unique species depended on a form of bacterium that itself gains its energy from oxidization of reactive chemicals, such as hydrogen o hydrogen sulfide, that bubbled up from the Earth's interior. And all of these provide a great deal of idea regarding how life could survive in Europa's ocean. If life can thrive here on Earth, without the aid of sunlight and in harshest of environments, then why not in Europa?
According to experts, life on Europa could exist clustered around hydrothermal vents on the ocean floor, or below the ocean floor, where endoliths (an organism, e.g. lichen, alga or amoeba that lives inside rock, coral, animal shells or in the pores between mineral grains of a rock) are known to inhabit on Earth. Alternatively, it could exist clinging to the lower surface of the moon's ice layer, much like algae and bacteria in Earth's Polar Regions, or float freely in Europa's ocean. On the other hand, if Europa's ocean were too cold, biological processes similar to those known here on Earth, perhaps won't take place. Volcanic activity provides some of the heat necessary to keep the water on Europa from freezing and provides key dissolved chemicals required by the living organisms. If the water is too salty, only extreme halophiles (organisms that thrive in environments with very high concentrations of salt) could survive in its environment.
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