SAN ANTONIO — May 18, 2026 — Looking back at 14 years of Hubble telescope data for Jupiter's moon Europa has given Southwest Research Institute (SwRI) scientists a better understanding of its tenuous atmosphere. The findings have cast doubt on previous evidence suggesting that the icy moon intermittently discharges faint water plumes from a presumed subsurface ocean.
"The evidence for water vapor plumes on Europa isn't as strong as we first understood it," said SwRI's Dr. Kurt Retherford, one of the authors of a 2014 paper initially making that assertion. Retherford and his colleagues have recently published a new paper reanalyzing the data.
The new paper looks at the last 14 years of data from the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS) focused on Europa's Lyman-alpha emissions. Lyman-alpha is a specific wavelength of ultraviolet light emitted and scattered by hydrogen atoms. From 2012-2014, the team was pushing the limits of the Hubble telescope's capabilities.
"One of the difficulties in interpreting the data back then was determining where to place Europa within its context," Retherford said. "The way Hubble works left some uncertainty in terms of placement relative to the center of the image. If Europa's placement was off even just by a pixel or two, it could affect how the data gets interpreted."
As a result, what they thought could be evidence of a water vapor plume could also just be statistical noise.
"Our reanalysis took our original 99.9% confidence in the plumes' existence and reduced it to less than 90% confidence," said Dr. Lorenz Roth (Royal Technical Institute, Sweden), the paper's lead author. "That's simply not enough evidence to support the certainty of claims we made at the time."
Retherford said the current dataset does not rule out the possibility of the water vapor plumes described in the 2014 paper, but it no longer provides concrete evidence of them.
"The description of the phenomena just doesn't hold up the same way anymore," said Retherford. "The new data has made us reconsider the strength of the previous paper's conclusion regarding water vapor plumes. The recent analysis also provides improved information about the neutral hydrogen atom component of Europa's escaping atmosphere, originating from its water ice surface."
SwRI scientists still hope to find water vapor plumes escaping from Europa. Similar water vapor plumes have been confirmed on Saturn's moon Enceladus, and Europa's neighbor Io, another moon of Jupiter, has plumes of sulfur dioxide expanding out into space.
Scientists are particularly interested in Europa because its icy surface is thought to obscure a vast saltwater ocean beneath. Cracks in Europa's icy shell could provide potential pathways for liquid water to rise to the surface and shoot out into space. This remains a distinct possibility that NASA's Europa Clipper mission will investigate when it arrives in the Jupiter system in 2030.
To read the Astronomy & Astrophysics paper titled "Europa's Lyman-alpha emissions from HST/STIS observations," go to https://doi.org/10.1051/0004-6361/202659406 .
