Jupiter's icy moon Europa may host life in an ocean of liquid water hidden under its 10 kilometre-deep ice crust, scientists say.
Researchers from the University of Sao Paulo (USP) in Brazil conducted a theoretical research to evaluate microbial habitability of Europa using data collected from similar environments on the Earth.
"We studied the possible effects of a biologically usable energy source on Europa based on information obtained from an analogous environment on Earth," said Douglas Galante, a researcher at Brazil's National Synchrotron Light Laboratory (LNLS).
In the Mponeng gold mine near Johannesburg, South Africa, at a depth of 2.8 km, researchers not only found traces of major changes linked to history of life on Earth, but also a terrestrial context similar to Europa.
It was recently discovered that the bacterium Candidatus Desulforudis audaxviator survives inside the mine without sunlight by means of water radiolysis, the dissociation of water molecules by ionising radiation.
"This very deep subterranean mine has water leaking through cracks that contain radioactive uranium. The uranium breaks down the water molecules to produce free radicals," Galante said.
The free radicals attack the surrounding rocks producing sulphate. The bacteria use the sulphate to synthesise and store energy. This is the first time ecosystem has been found to survive directly on the basis of nuclear energy, Galante said.
According to the researchers, the environment colonised by bacteria in the Mponeng mine is an excellent analogue of the environment assumed to exist at the bottom of Europa's ocean.
Although the temperature in Europa's surface is next to absolute zero, there is an enormous amount of thermal energy in its core.
Jupiter's powerful gravitational attraction causes the Europa's orbit to be extremely elliptical, meaning the latter finds itself either to close or too far from the gas giant.
This makes the icy moon suffer geometrical deformation as it moves at the mercy of Jupiter's immense tidal force.
The energy released by the alternating states of elongation and relaxation makes Europa's subsurface capable of hosting an ocean of liquid water.
"However, it's not enough for there to be heated liquid water," said Galante.
The basis for all biological activity known to Earth are the differences in concentrations of molecules, ions or electrons in distinct regions which produce a flow in a certain direction, allowing the occurrence of cellular respiration, photosynthesis, energy storage and other processes common to living beings.
Researchers evaluated how chemical imbalance in Europa could be initiated through the emanation of water leading to chain reactions between water and chemical elements found in Europa's crust.
However, a total lack of empirical data prevents scientists from unequivocally presuming any of these events.
"That's why we looked for a more universal physical effect that was highly likely to occur. That effect was precisely the action of radioactivity," Galante said.
Celestial bodies in the solar system with rocky cores share the same radioactive materials, ejected in space by the Supernova explosion that originated the Sun and the planets.
Uranium, thorium and potassium are the radioactive elements considered by the research, which estimated the concentrations for these materials in Europa, based on the quantities already observed and measured on Earth, in meteorites and in Mars.
"From these amounts, we were able to estimate the energy released, how this energy interacts with the surrounding water, and the efficiency of the water radiolysis resulting from this interaction in generating free radicals," Galante said.
According to the study published in the journal Scientific Reports, pyrite is a crucial ingredient whose presence is indispensable for life in Europa.
"Traces of pyrite should be looked for as part of any assessment of the habitability of a celestial body," said Galante.
Chances for finding pyrite in a hypothetical mission to Europa are good, since sulphur (S) and iron (Fe) are elements found in abundance across the solar system.
"The ocean bed on Europa appears to offer very similar conditions to those that existed on primitive Earth during its first billion years. So studying Europa today is to some extent like looking back at our own planet in the past," Galante said.