New Study Challenges Previous Signs of Water Vapor on Europa

An extensive review of fourteen years’ worth of Hubble Space Telescope observations has raised uncertainties about earlier claims that Jupiter’s frozen moon Europa releases intermittent streams of water vapor. Published in Astronomy & Astrophysics, this new investigation suggests that previous evidence for these elusive plumes may have been overinterpreted, leaving the question open for further research.

Reevaluating Europa’s Faint Atmosphere

Europa has captivated scientists for nearly 15 years due to its icy exterior and possible underlying ocean. Initial observations from Hubble in 2012 indicated that the moon might sporadically emit subtle vapor plumes, fueling excitement over studying its ocean without surface landing. If verified, these emissions could provide crucial insights into Europa’s potential for supporting life.

“Our proof of water vapor plumes on Europa is not as conclusive as previously thought,” remarked SwRI’s Dr. Kurt Retherford, a contributor to the 2014 study that first announced the plume detections. Retherford and his team’s recent paper reexamines the dataset and offers a more measured interpretation of Europa’s sparse atmosphere.

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Summary of the HST/STIS data examined for this study. Left: Europa’s orbital trajectory tracked from initial to final exposures across 23 HST/STIS visits. Note: intervals between exposures within a session are omitted. Visit 21 includes exposures before and after transit. Right: full STIS detector spectral image from visit 22. Geocoronal and interplanetary hydrogen produce broad Lyα emission (yellow-green area). Reflected solar continuum from Europa appears near row y=200. Atmospheric oxygen emissions at 1304 Å and 1356 Å exceed surface reflections. Credit: Astronomy & Astrophysics

The investigation concentrated on Lyman-alpha radiation, ultraviolet light scattered by hydrogen atoms that can signify escaping water molecules. By refining Hubble’s spatial positioning and instrument calibration, the researchers identified uncertainties that may have skewed earlier data interpretations.

“One of the difficulties in interpreting the data back then was determining where to place Europa within its context,” Retherford explained. “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 by just a pixel or two, it could affect how the data gets interpreted.”

Lower Confidence Sparks New Questions

The initial research asserted a near-perfect 99.9% certainty that water vapor plumes were present. The recent evaluation, as published in Astronomy & Astrophysics, lowered this confidence to under 90%, a level deemed insufficient to claim firm proof.

“Our reanalysis took our original 99.9% confidence in the plumes’ existence and reduced it to less than 90% confidence,” said Dr. Lorenz Roth of the Royal Technical Institute, Sweden, and lead author of the reanalysis. “That’s simply not enough evidence to support the certainty of claims we made at the time.”

While the possibility of plume activity is not excluded, the findings highlight that previous interpretations may have been overly optimistic. Retherford added,

“The description of the phenomena just doesn’t hold up the same way anymore. 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.”

Modeling and data examination results. Top: brightness along the Lyα slit’s y-axis averaged across x-axis width. Middle: zoomed section of the top panel. Bottom: brightness along the x-axis averaged over a y range covering Europa’s visible disk. Black histogram marks STIS data; dashed lines show modeled components — green for background, blue for background plus hydrogen exosphere, red for full model including reflection and IPH adjustments. Profiles are smoothed due to averaging and instrumental response. Vertical dotted lines mark disk center. Model fits were made around disk center and gray shaded regions (top panel). Credit: Astronomy & Astrophysics

Context From Icy Worlds with Confirmed Activity

Europa is one of several moons displaying possible geological phenomena. Saturn’s moon Enceladus has confirmed water vapor jets, while Jupiter’s Io releases sulfur dioxide gas. These examples provide comparative cases showing plumes may arise in differing chemical contexts.

Interest in Europa remains strong due to its expansive saltwater ocean concealed beneath ice. Cracks along the icy surface might offer a route for water escape, but definitive verification depends on direct measurements. NASA’s forthcoming Europa Clipper mission, aiming to reach the moon in 2030, promises critical data to resolve enduring mysteries about Europa’s atmosphere and potential plume activity.