Jupiter's moon Europa - strongly believed to hide a deep ocean of salty liquid water beneath its icy shell - can have the necessary balance of chemical energy for life even if the moon lacks volcanic hydrothermal activity, researchers have revealed.
Scientists at Nasa's Jet Propulsion Laboratory (JPL), Pasadena, California, compared Europa's potential for producing hydrogen and oxygen with that of Earth through processes that do not directly involve volcanism.
The balance of these two elements is a key indicator of the energy available for life.
The study found that the amounts would be comparable in scale. On both worlds, oxygen production is about 10 times higher than hydrogen production.
The work draws attention to the ways that Europa's rocky interior may be much more complex and possibly Earth-like than people typically think.
"We're studying an alien ocean using methods developed to understand the movement of energy and nutrients in Earth's own systems. The cycling of oxygen and hydrogen in Europa's ocean will be a major driver for Europa's ocean chemistry and any life there, just it is on Earth," explained Steve Vance, planetary scientist at JPL and lead author.
As part of the study, the researchers calculated how much hydrogen could potentially be produced in Europa's ocean as seawater reacts with rock in a process called serpentinisation.
In this process, water percolates into spaces between mineral grains and reacts with the rock to form new minerals, releasing hydrogen in the process.
New cracks expose fresh rock to seawater, where more hydrogen-producing reactions can take place.
In Earth's oceanic crust, such fractures are believed to penetrate to a depth of five-six kms.
On present-day Europa, the researchers expect water could reach as deep as 25 kms into the rocky interior, driving these key chemical reactions throughout a deeper fraction of Europa's seafloor.
The other half of Europa's chemical-energy-for-life equation would be provided by oxidants - oxygen and other compounds that could react with the hydrogen - being cycled into the Europan ocean from the icy surface above.
"The oxidants from the ice are like the positive terminal of a battery, and the chemicals from the seafloor, called reductants, are like the negative terminal," noted Kevin Hand, planetary scientist at JPL.
"Whether or not life and biological processes complete the circuit is part of what motivates our exploration of Europa," he added in a new paper published in Geophysical Research Letters, a journal of the American Geophysical Union.
Europa's rocky, neighbouring Jovian moon "Io" is the most volcanically active body in the solar system.
Scientists have long considered it possible that Europa might also have volcanic activity, as well as hydrothermal vents, where mineral-laden hot water would emerge from the sea floor.
Nasa is currently formulating a mission to explore Europa and investigate the moon's potential habitability.