Jupiter's moon Europa is thought to harbor a salty ocean beneath its bright, icy crust, making it a world that could be one of the most habitable places in our solar system.
But life as we know it needs oxygen. It is an open question whether Europe's periphery has it or not.
Now, astronomers have determined how much of the molecule is being made on the icy moon's surface, which could be a source of oxygen for the water below. Using data from NASA's Juno mission, resultsA new study published Monday in the journal Nature Astronomy suggests that the frozen world is generating less oxygen than some astronomers had hoped.
“It's at the lower end of what we expected,” said Jami Szalay, a plasma physicist at Princeton University who led the study. He added that it is “not completely prohibited” for housing.
On Earth, photosynthesis by plants, plankton, and bacteria pumps oxygen into the atmosphere. But the process works differently in Europe. Charged particles from space bombard the moon's icy crust, breaking down the frozen water into hydrogen and oxygen molecules.
“The ice crust is like the lungs of Europe,” Dr. Salai said. “The surface — the same surface that protects the ocean beneath it from harmful radiation — is, to some extent, breathing.”
Astronomers speculate that this oxygen may be transported to Europa's watery underworld. If so, it could mix with volcanic material from the sea floor, creating a “chemical soup that could eventually lead to the formation of life,” said Fran Bagnall, a planetary scientist at the University of Colorado Boulder.
The Juno spacecraft, launched in 2011 to discover what lies beneath Jupiter's thick veil of clouds, is now on an expanded mission to explore the planet's rings and moons. On board is an instrument called JADE, short for the Jovian Auroral Distributions Experiment. Dr. Salai's team studied data collected by JADE during Juno's flyby through the plasma sweeping across Europa.
But they weren't looking directly for oxygen, they were counting hydrogen. Because the molecule is so light, all the hydrogen produced on Europa's surface floats high into the atmosphere. Oxygen, which is heavier, is more likely to get stuck or remain trapped in the ice.
But both molecules come from the same source: the decomposition of frozen H₂O.
“Thus, if we measure hydrogen, we have a direct line to determine how much oxygen is being produced,” Dr. Szalay said.
The team found that about 13 to 40 pounds of oxygen are generated every second by Europa's surface. That's more than 1,000 tons per day, enough to fill the Dallas Cowboys' football stadium 100 times a year.
While previous studies have reported widely varying ranges of up to 2,245 pounds per second, this result shows that the upper limit of that range is unlikely. But according to Dr. Bagnall, this does not necessarily harm Europe's habitability.
“We don't really know how much oxygen we need to form life,” she said. “So the fact that it's lower than some of the previous wishful estimates isn't a big deal.”
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 results only confirm the amount of oxygen generated 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, “we still have no idea how much is down compared to up,” Dr. Schmidt said.
Juno won't make any more close flybys of the global water world, but next-generation missions specifically designed to study Europa may find more answers. ESA's Jupiter Icy Moons Explorer, expected to arrive in the Jovian system in 2031, aims to confirm the existence and size of Europa's ocean. And NASA Europe ClipperScheduled for launch in October, it will investigate how the moon's icy crust interacts with the water below.
Right now, astronomers are busy with data coming 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. Salai said. “For many years, we will dig through this single flight to find all the treasure.”
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