Alien life in the universe could be close to home, swimming around in Europa's ocean.
The idea has been floating around scientific minds for more than a decade: beneath the icy surface of the Jovian moon could slosh a deep, wide ocean with the perfect environment for life to develop.
The thought is so compelling that NASA has lobbied Congress for 15 years for money to send an expedition to Jupiter's moon and finally procured the funds in 2016 – just in time to test a new study, published Tuesday in the journal Geophysical Research Letters, that offers a compelling new reason why Europa could be a good place for life.
Essentially, NASA scientists have modeled the proposed ocean and found Europa's ability to produce oxygen and hydrogen is comparable to Earth's, a sign that the moon could have available energy for life. And the method of producing that oxygen and hydrogen could be totally different from the process on Earth.
"We predict that Europa's large and persistent chemical fluxes of oxygen and hydrogen.... could balance in a way that is unique among potentially habitable icy worlds in the solar system," researchers write in the study. "For this reason, Europa may have favored the origin and evolution of life."
Since the Galileo spacecraft (1995 - 2003) first detected evidence of an under-the-surface ocean on Europa, debates have raged over whether that ocean would be habitable. At the root of that debate is whether Europa has the right ratio of chemicals to "power biological processes," as NASA states in the press release.
On Earth, the process behind getting the right chemical balance for life relies heavily on volcanic hydrothermal activity – so much so that previous researchers have speculated volcanism is a precursor for a habitable environment in Europa's ocean, as Steve Vance, lead author of the study and planetary scientist at NASA's Jet Propulsion Laboratory, explained in the press release.
But Europa may be able to do it without the volcanic activity, according to the study.
To determine its viability, the researchers addressed the hydrogen-oxygen ratio. Oxygen is the easy part of the equation. Europa is close enough to Jupiter to be "bathed" in radiation, according to the press release. The radiation is able to split apart the molecules in the ice abundant on the surface of the moon and produce oxidants, oxygen, and other chemical compounds capable of reacting with hydrogen. The oxidants could then be carried by the surface into the ocean below.
Finding hydrogen and other chemicals without volcanic activity was trickier. Dr. Vance and his team of researchers drew on data that suggests Europa's rocky interior, below the ice, could be more Earth-like than typically expected.
"We're studying an alien ocean using methods developed to understand the movement of energy and nutrients in Earth's own systems," Vance said.
When saltwater and rock make contact, they have a small chemical reaction called serpentinization that produces hydrogen. The researchers calculated how much hydrogen could be produced through this process, factoring in that cracks in Europa's rocky crust would open over time, exposing more fresh rock to produce hydrogen.
On Earth, fractures in the oceanic crust can penetrate 3 to 4 miles deep, but on Europa, estimates place those fractures at up to 15 miles deep.
The end result: a ratio of oxygen production roughly 10 times higher than hydrogen, very similar to Earth, meaning Europa very likely has all the necessary components for an environment suited to life.
"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. Whether or not life and biological processes complete the circuit is part of what motivates our exploration of Europa," said Kevin Hand, a planetary scientist at JPL who co-authored the study.
NASA is currently planning on launching a probe to explore Europa in the 2020's.