Jupiter's satellite has conditions that could allow life. Slightly smaller than the Moon, Europa is mainly composed of silicates with a crust made up of frozen water, probably inside there is an iron-nickel core and is externally surrounded by a tenuous atmosphere, composed mainly of oxygen.
Unlike Ganymede and Callisto, its surface is streaked and slightly cratered and is smoother than that of any known object in the solar system. In 1997, the passage of the Galileo probe through a plume of water emerging from a surface geyser proved beyond any doubt the existence of an ocean of water present under the crust, which could be home to extraterrestrial life.
In this hypothesis it is proposed that Europa, internally heated by the tidal forces caused by its proximity to Jupiter and by the orbital resonance with its neighbors Io and Ganymede, releases the heat necessary to maintain a liquid ocean below the surface and at the same time stimulate an activity geological similar to plate tectonics.
On September 8, 2014, NASA reported that it had found evidence of plate tectonic activity on Europa, the first such geological activity on a world other than Earth. In December 2013, NASA identified some clay minerals on the crust of Europa, more precisely, phyllosilicates, which are often associated with organic material.
NASA itself announced, based on observations made with the Hubble Space Telescope, that geysers of water vapor similar to those of Enceladus, the satellite of Saturn, have been detected. Under the surface of Europa there is thought to be a layer of liquid water maintained by the heat generated by the tides caused by the gravitational interaction with Jupiter.
The surface temperature of Europa is about 110 K at the equator and only 50 K at the poles, so that the surface ice is permanently frozen. The first clues to a liquid ocean below the surface came from theoretical considerations related to gravitational warming.
Galileo project imaging team members analyzed Europa images from the Voyager spacecraft and the Galileo spacecraft to say that Europa's surface features also demonstrate the existence of a liquid ocean below the surface.
Unlike the oxygen present in the Earth's atmosphere, that of Europa has no biological origin: it is most likely generated by the interaction of sunlight and charged particles with the icy surface of the satellite, which leads to the production of water vapor.
Following the dissociation into oxygen and hydrogen always caused by radiolysis, the latter, which is lighter, easily escapes the gravitational attraction of the body and disperses into space. Europe is regarded as one of the worlds with the highest probability that extraterrestrial life has developed.
It has been speculated that life could exist in this ocean beneath the ice, in an environment similar to that of hydrothermal springs found on Earth in the depths of the ocean or on the bottom of Lake Vostok, Antarctica. At present there is no evidence that life forms exist on Europa, but the presence of liquid water is so likely that it reinforces requests to send probes to investigate.
Until 1970 it was thought that life needed solar energy to develop, with plants capturing solar energy on the surface and, through photosynthesis, producing carbohydrates from carbon dioxide and water, releasing oxygen in the process.
, which are then consumed by animals creating a food chain. Even in the deep ocean, far below the reach of sunlight, nourishment was thought to come only from the surface, directly or indirectly, particularly via the organic debris that descends from it. Access to sunlight was therefore considered essential in order to sustain life in a given environment.