When considering locations in the solar system beyond Earth where life could have been sparked, you'd be hard pressed to find a more suitable world than Jupiter's moon Europa.
This moon is thought to be awash with a subsurface ocean protected from space radiation by a thick shell of ice. The ocean is also believed to be salty, containing much of the same chemicals that facilitated life on Earth. Some hypotheses even suggest there's enough oxygen and nutrients cycling through the water to support not only microbial life, but also multi-cellular life.
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But how Earth-like is Europa's ocean? Could it truly be a biological oasis in orbit around the most massive planet in the solar system?
In new research published in the journal Geophysical Research Letters, NASA scientists studied how the chemical composition of the Europan ocean may have evolved and what chemicals it possibly contains, assuming similar geochemical processes as on Earth are at play.
"We're studying an alien ocean using methods developed to understand the movement of energy and nutrients in Earth's own systems," said planetary scientist Steve Vance, of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "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."
Europa is thought to possess a rocky core fractured with deep cracks that have filled with water. Since the formation of the moon, the core has continued to cool, creating more cracks and exposing more rocks to chemical processes with this water.
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On Earth, interactions between water and minerals in rock is called serpentization; a reaction that forms new minerals, releasing copious quantities of hydrogen in the process. As more cracks form over the billions of years since formation, more reactions can take place, loading the salty water with hydrogen. This is the first half of Europa's life-giving equation.
Assuming Europa has these complex geochemical processes, for life to be a possibility there needs to be some kind of oxidizing agent injected into the ocean to undergo key chemical reactions with all this hydrogen fizzing from below. And it just so happens that the surface of Europa may be the ideal supplier. As the intense radiation surrounding Jupiter continuously barrages the icy surface, the chemical bonds of water ice are broken to produce an oxididant factory. Oxidants are molecules of oxygen atoms that combine with other chemicals.
It is well known that Europa has a fractured surface that is constantly renewed. The lack of impact craters proves that some form of icy tectonic activity is constantly rejuvenating the surface allowing these oxidants to be dissolved into the ocean below.
"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," said planetary scientist Kevin Hand, also from JPL. "Whether or not life and biological processes complete the circuit is part of what motivates our exploration of Europa."
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These chemical processes do not require any form of volcanic activity to help the reactions along - in fact, the serpentization reactions with the rocky core could be perfect for a surprisingly Earth-like chemistry.
"... if the rock is cold, it's easier to fracture," said Vance. "This allows for a huge amount of hydrogen to be produced by serpentinization that would balance the oxidants in a ratio comparable to that in Earth's oceans."
It certainly seems that Europa has the right ingredients for life, but if these geochemical processes are at play, the Jovian moon could be more ideal than we ever dreamed.
These are incredible questions we're asking of Europa, but as we realize there's even more subsurface oceans inside other moons, it begs the question: How many more have just the right chemistry for life (at least "life as we know it")? Life's potential suddenly becomes boundless throughout our solar system and beyond.