Beneath its bright, icy shell, Jupiter’s moon Europa is believed to harbor a salty ocean, making it a world that could 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 determined how much of the molecule is produced on the moon’s icy surface, which could be a source of oxygen for the waters below. Using data from NASA’s Juno mission, the findings, published Monday in the journal Nature Astronomy, suggest that the icy world generates less oxygen than some astronomers had hoped.
“This is lower than we would expect,” said Jamey Szalay, a plasma physicist at Princeton University who led the study. But it’s “not completely prohibitive” to habitability, he added.
On Earth, photosynthesis by plants, plankton and bacteria pumps 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 below from harmful radiation – breathes, in a sense.”
Astronomers hypothesize that this oxygen could drift into Europa’s watery underworld. If so, it could mix with volcanic material on the sea floor, creating “a chemical soup that could end up creating life,” said Fran Bagenal, a planetary scientist at the University of Colorado Boulder.
The Juno orbiter, 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. On board the vehicle is an instrument called JADE, short for Jovian Auroral Distributions Experiment. Dr. Szalay’s team studied the data collected by JADE as Juno flew through the plasma engulfing Europa.
But they weren’t looking for oxygen directly: they were counting hydrogen. Because the molecule is so light, all the hydrogen produced on Europa’s surface floats into the atmosphere. Oxygen, which is heavier, is more likely to hang lower or become trapped in the ice.
But both molecules come from the same source: frozen, decomposed H₂O.
“And so if we measure hydrogen, we have a direct line to determine how much oxygen is produced,” Dr. Szalay said.
The team found that Europa’s surface generates 13 to 40 pounds of oxygen every second. That’s more than 1,000 tons per day, enough to fill the Dallas Cowboys football stadium 100 times a year.
While previous studies reported highly variable ranges up to 2,245 pounds per second, this result shows that the upper limit of that range was unlikely. But according to Dr Bagenal, this does not necessarily harm Europa’s habitability potential.
“We really don’t know how much oxygen is needed to create life,” he said. “So the fact that it is lower than some previous, illusory estimates is not a big deal.”
Studying Europa’s atmosphere is “an important piece of the puzzle to 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 results only confirm the amount of oxygen present in the ice. The study does not reveal how much of the molecule is 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 how much the price will go down versus how much it will go up.”
Juno will no longer fly flybys of the global water world, but next-generation missions designed specifically to study Europa may find more answers. The European Space Agency’s Jupiter Icy Moons Explorer, due to arrive in 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 are very busy with data from Juno. Although the flyby lasted only a few minutes, it was the first time that the composition of plasma near Europa’s atmosphere had been directly measured.
“This is just the tip of the iceberg,” Dr. Szalay said. “For many years, we will only dig in this flyby to find all the treasure.”