Astronomers using 14 years of Hubble Space Telescope observations have lowered their confidence in water vapor plumes on Jupiter’s icy moon Europa from 99.9 percent to below 90 percent, according to a study published in Astronomy & Astrophysics. The reanalysis led by Lorenz Roth indicates that what was once interpreted as south-polar water-vapour aurora may actually be indistinguishable from background noise when accounting for detector positioning and the moon’s extended atomic-hydrogen exosphere.
Reevaluating 14 Years of Hubble Data on Europa’s Plumes
The original evidence for plumes emerged from a December 2012 Hubble Space Telescope Imaging Spectrograph observation. According to Roth and his colleagues, that image contained excess hydrogen Lyman-alpha and oxygen emission near Europa’s southern limb. The team initially interpreted the feature as energetic electrons striking water vapour, suggesting geysers reaching 200 kilometers into space. However, a broader archive covering 23 observations between 1999 and 2020 has changed those statistical conclusions.
According to Kurt Retherford of the Southwest Research Institute, who co-authored both the 2014 study and the new evaluation, the original work pushed Hubble to its limits. When researchers shifted the assumed position of Europa’s disk on the detector by just one or two pixels, counts once classified as emission beyond the limb fell onto the disk or background. Furthermore, the new forward model incorporated a diffuse hydrogen exosphere that scatters solar Lyman-alpha light. When those factors were applied, no localized enhancement appeared in any image, dropping statistical confidence below 90 percent.
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Europa’s potential plumes were initially compared to those of Saturn’s moon Enceladus, where NASA’s Cassini spacecraft directly flew through active geysers and analyzed their chemical composition. Unlike Enceladus, Europa has no equivalent confirmed plume.
How Alternative Evidence Keeps the Search Alive
While the primary Hubble ultraviolet data no longer strongly supports active venting, other instruments have produced suggestive clues over the years. Separately, a 2018 reanalysis of Galileo spacecraft data from a 1997 flyby reproduced a brief plasma-wave disturbance consistent with passing through a plume.
Despite these hints, researchers emphasize that none of these observations definitively tie detected vapor to Europa’s global subsurface ocean. As Retherford noted in a Southwest Research Institute release, the evidence is no longer strong enough to support past certainty, though intermittent or smaller geysers cannot be fully ruled out.
The Path Forward for NASA’s Europa Clipper Mission
The question of whether Europa actively vents material into space may finally be resolved later this decade. NASA’s Europa Clipper mission is scheduled to reach the Jovian system in April 2030, where it will perform dozens of close flybys of the moon. According to mission planning details, the spacecraft carries radar, magnetometry, thermal imaging, and surface spectroscopy to evaluate Europa’s habitability.
While flying directly through fresh plume ejecta would aid analysis of the subsurface ocean, Europa Clipper’s science objectives do not depend on active geysers. The spacecraft is designed to investigate the ice shell and hidden ocean regardless of whether Europa remains quiet during its encounters.
Frequently Asked Questions
Are there confirmed water plumes on Europa?
No. While scientists previously reported 99.9 percent confidence in Hubble observations from 2012, a 2026 reanalysis reduced that confidence below 90 percent, meaning the original data is now considered compatible with background noise.
Does Europa have a subsurface ocean?
Yes. The existence of a global salty ocean under Europa’s ice sheet is strongly supported by magnetic induction, gravity measurements, and tidal behavior, independent of whether water plumes exist.
When will we know for sure if plumes exist?
NASA’s Europa Clipper mission is scheduled to arrive at Jupiter in April 2030 to conduct close flybys and investigate the moon’s ice shell and habitability.
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