Under its bright, frosty shell, Jupiter’s moon Europa is thought to harbor a salty ocean, making it a world that might be one of the most habitable places in our solar system.
But most life as we know it needs oxygen. And it’s an open question whether Europa’s ocean has it.
Now, astronomers have nailed down how much of the molecule gets made at the icy moon’s surface, which could be a source of oxygen for the waters below. Using data from NASA’s Juno mission, the results, published on Monday in the journal Nature Astronomy, suggest that the frozen world generates less oxygen than some astronomers may have hoped for.
“It’s on the lower end of what we would expect,” said Jamey Szalay, a plasma physicist at Princeton University who led the study. But “it’s not totally prohibitive” for habitability, he added.
On Earth, the photosynthesis of plants, plankton and bacteria pump oxygen into the atmosphere. But the process works differently on Europa. Charged particles from space bombard the moon’s icy crust, breaking down frozen water into hydrogen and oxygen molecules.
“The ice shell is like Europa’s lung,” Dr. Szalay said. “The surface — which is the same surface that protects the ocean underneath from harmful radiation — is, in a sense, breathing.”
Astronomers speculate that this oxygen might move into Europa’s watery underworld. If so, it could mix with volcanic material from the seafloor, creating “a chemical soup that may end up making life,” said Fran Bagenal, a planetary scientist at the University of Colorado Boulder.
The Juno orbiter, which launched in 2011 to discover what lies beneath Jupiter’s thick veil of clouds, is now on an extended mission exploring the planet’s rings and moons. Aboard the vehicle is an instrument called JADE, short for Jovian Auroral Distributions Experiment. Dr. Szalay’s team studied data collected by JADE as Juno flew through the plasma engulfing Europa.
But they weren’t directly looking for oxygen — they were counting hydrogen. Because the molecule is so light, all of the hydrogen produced at Europa’s surface floats high into the atmosphere. Oxygen, which is heavier, is more likely to hang lower or remain trapped in the ice.
But both molecules come from the same source: broken down frozen H₂O.
“And so if we measure the hydrogen, we have a direct line to determine how much oxygen is produced,” Dr. Szalay said.
The team found that about 13 to 40 pounds of oxygen is generated each second by Europa’s surface. That’s over 1,000 tons per day, about enough to fill the Dallas Cowboys football stadium 100 times a year.
While earlier studies reported widely varying ranges up to 2,245 pounds per second, this result shows the higher end of that range was unlikely. But according to Dr. Bagenal, this doesn’t necessarily harm Europa’s potential for habitability.
“We don’t really know how much oxygen you need to make life,” she said. “So the fact that it’s lower than some earlier, wishful-thinking estimates is not such a problem.”
Studying Europa’s atmosphere is “an important puzzle piece in learning about the moon as a system,” said Carl Schmidt, a planetary scientist at Boston University who was not involved in the work.
But the findings only confirm the amount of oxygen born in the ice. The study doesn’t reveal how much of the molecule gets lost to the atmosphere, or how it might permeate the ice to enrich the ocean below.
In other words, Dr. Schmidt said, “we still have no idea just how much is going down as opposed to going up.”
Juno won’t make any more close flybys of the global water world, but next-generation missions specifically designed to study Europa might find more answers. The European Space Agency’s Jupiter Icy Moons Explorer, expected to arrive at the Jovian system in 2031, aims to confirm the existence and size of Europa’s ocean. And NASA’s Europa Clipper, scheduled to launch in October, will investigate how the moon’s icy shell interacts with the water below.
For now, astronomers have their hands full with data from Juno. Though the flyby lasted only a few minutes, it was the first time the composition of plasma near Europa’s atmosphere was directly measured.
“This is just the tip of the iceberg,” Dr. Szalay said. “For many years, we’re going to be digging through just this one flyby to find all the treasure.”