\n<\/p>\n
Under its bright, frosty shell, Jupiter\u2019s 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.<\/p>\n
But life as we know it needs oxygen. And it\u2019s an open question whether Europa\u2019s ocean has it.<\/p>\n
Now, astronomers have nailed down how much of the molecule is made at the icy moon\u2019s surface, which could be a source of oxygen for the waters below. Using data from NASA\u2019s 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.<\/p>\n
\u201cIt\u2019s on the lower end of what we would expect,\u201d said Jamey Szalay, a plasma physicist at Princeton University who led the study. But \u201cit\u2019s not totally prohibitive\u201d for habitability, he added.<\/p>\n
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\u2019s icy crust, breaking down frozen water into hydrogen and oxygen molecules.<\/p>\n<\/div>\n<\/div>\n
\u201cThe ice shell is like Europa\u2019s lung,\u201d Dr. Szalay said. \u201cThe surface, which is the same surface that protects the ocean underneath from harmful radiation, is, in a sense, breathing.\u201d<\/p>\n
Astronomers speculate that this oxygen might move into Europa\u2019s watery underworld. If so, it could mix with volcanic material from the seafloor, creating \u201ca chemical soup that may end up making life,\u201d said Fran Bagenal, a planetary scientist at the University of Colorado Boulder.<\/p>\n
The Juno orbiter, which launched in 2011 to discover what lies beneath Jupiter\u2019s thick veil of clouds, is now on an extended mission exploring the planet\u2019s rings and moons. Aboard the vehicle is an instrument called JADE, short for Jovian Auroral Distributions Experiment. Dr. Szalay\u2019s team studied data collected by JADE as Juno flew through the plasma engulfing Europa.<\/p>\n
But the team wasn\u2019t directly looking for oxygen; it was counting hydrogen. Because the molecule is so light, all of the hydrogen produced at Europa\u2019s surface floats high into the atmosphere. Oxygen, which is heavier, is more likely to hang lower or remain trapped in the ice.<\/p>\n<\/div>\n<\/div>\n
But both molecules come from the same source: broken-down frozen H\u2082O.<\/p>\n<\/div>\n<\/div>\n
\u201cAnd so if we measure the hydrogen, we have a direct line to determine how much oxygen is produced,\u201d Dr. Szalay said.<\/p>\n
The team found that Europa\u2019s surface generates about 13 to 40 pounds of oxygen each second. That\u2019s over 1,000 tons per day, about enough to fill the Dallas Cowboys football stadium 100 times a year.<\/p>\n
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\u2019t necessarily harm Europa\u2019s potential for habitability.<\/p>\n
\u201cWe don\u2019t really know how much oxygen you need to make life,\u201d she said. \u201cSo the fact that it\u2019s lower than some earlier, wishful-thinking estimates is not such a problem.\u201d<\/p>\n
Studying Europa\u2019s atmosphere is \u201can important puzzle piece in learning about the moon as a system,\u201d said Carl Schmidt, a planetary scientist at Boston University who was not involved in the work.<\/p>\n<\/div>\n<\/div>\n
But the findings only confirm the amount of oxygen born in the ice. The study doesn\u2019t reveal how much of the molecule gets lost to the atmosphere, or how it might permeate the ice to enrich the ocean below.<\/p>\n
In other words, Dr. Schmidt said, \u201cwe still have no idea just how much is going down as opposed to going up.\u201d<\/p>\n
Juno won\u2019t make any more close flybys of the global water world, but next-generation missions specifically intended to study Europa might find more answers. The European Space Agency\u2019s Jupiter Icy Moons Explorer, expected to arrive at the Jovian system in 2031, aims to confirm the existence and size of Europa\u2019s ocean. And NASA\u2019s Europa Clipper, scheduled to launch in October, will investigate how the moon\u2019s icy shell interacts with the water below.<\/p>\n
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\u2019s atmosphere was directly measured.<\/p>\n
\u201cThis is just the tip of the iceberg,\u201d Dr. Szalay said. \u201cFor many years, we\u2019re going to be digging through just this one flyby to find all the treasure.\u201d<\/p>\n<\/div>\n<\/div>\n