An ocean moon that is thought to be habitable may be oxygen-depleted

Jupiter’s moon Europa is thought to harbor a salty ocean beneath its bright, icy shell, making it a world that may be one of the most habitable places in our solar system.

But life as we know it needs oxygen. And it is an open question whether the European ocean has this.

Now astronomers have determined how much of the molecule is made on the icy moon’s surface, which could provide a source of oxygen for the water below. Using data from NASA’s Juno mission, the… Resultspublished Monday in the journal Nature Astronomy, suggest that the frozen world generates less oxygen than some astronomers might have hoped.

“It’s at 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 unaffordable” for habitability, he added.

On Earth, photosynthesis by plants, plankton and bacteria pumps oxygen into the atmosphere. But in Europe the process works differently. Charged particles from space bombard the moon’s icy crust, breaking frozen water into hydrogen and oxygen molecules.

“The ice shell is like the lung of Europe,” said Dr. Szalay. “The surface, the same surface that protects the ocean below from harmful radiation, is in a sense breathing.”

Astronomers speculate that this oxygen could enter Europa’s watery underworld. If so, it could mix with volcanic material from the seafloor, creating “a chemical soup that could eventually yield life,” says Fran Bagenal, a planetary scientist at the University of Colorado Boulder.

The Juno orbiter, which was launched in 2011 to discover what lies beneath Jupiter’s thick veil of clouds, is now on an extended mission to explore the planet’s rings and moons. On board the vehicle is an instrument called JADE, short for Jovian Auroral Distributions Experiment. The team of Dr. Szalay studied the data collected by JADE as Juno flew through the plasma engulfing Europe.

But the team wasn’t immediately looking for oxygen; it was counting hydrogen. Because the molecule is so light, all the hydrogen produced on Europa’s surface floats high in the atmosphere. Oxygen, being heavier, is more likely to hang lower or become trapped in the ice.

But both molecules come from the same source: broken down frozen H₂O.

“And so when we measure the hydrogen, we have a direct line to determine how much oxygen is being produced,” said Dr. Szalay.

The team found that Europa’s surface generates about 13 to 40 pounds of oxygen per second. That’s more than 1,000 tons per day, about enough to fill the Dallas Cowboys football stadium 100 times a year.

Although previous studies reported widely varying ranges, up to 2,245 pounds per second, this result shows that the higher end of that range was unlikely. But according to Dr. Bagenal, this does not necessarily harm the habitability of Europe.

“We don’t really know how much oxygen you need to make life,” she said. “So the fact that it’s lower than some previous, wishful-thinking estimates isn’t much of a problem.”

Studying Europa’s atmosphere is “an important piece of the puzzle in learning about the moon as a system,” says Carl Schmidt, a planetary scientist at Boston University who was not involved in the work.

But the findings only confirm the amount of oxygen being born in the ice. The study does not reveal how much of the molecule is lost to the atmosphere, or how it might penetrate the ice to enrich the ocean below.

In other words, said Dr. Schmidt: “We still have no idea how much is decreasing instead of increasing.”

Juno won’t make another short flyby of the global water world, but next-generation missions specifically designed to study Europa could potentially find more answers. The European Space Agency’s Jupiter Icy Moons Explorer, expected to arrive in the Jovian system in 2031, aims to confirm the existence and size of the European ocean. And that of NASA Europa Clipperexpected to launch in October, will investigate how the moon’s icy shell interacts with the water beneath it.

For now, astronomers have their hands full with data from Juno. Although the flight lasted only a few minutes, it was the first time that the composition of plasma near Europa’s atmosphere was measured directly.

“This is just the tip of the iceberg,” said Dr. Szalay. “For years we will dig through just one flight to find all the treasures.”