Europa\u2019s faint glow in ultraviolet light once looked like a possible water plume punching through ice. After 14 years of Hubble data, that signal now looks far less certain. Meanwhile, a different picture of the moon\u2019s escaping hydrogen atmosphere comes into sharper view.<\/p>\n
Europa has long tempted scientists with the same question. If a salty ocean lies beneath its ice, can any of that buried water break through to space?<\/p>\n
For a time, one set of Hubble observations seemed to hint that the answer might be yes. A bright patch seen in ultraviolet light near the moon\u2019s limb was interpreted in 2014 as possible evidence of a faint water vapor plume. Now, after a much broader reanalysis of Europa observations from 1999 and 2012 through 2020, that case has weakened sharply.<\/p>\n
\u201cThe evidence for water vapor plumes on Europa isn\u2019t as strong as we first understood it,\u201d Southwest Research Institute<\/a> scientist Kurt Retherford said. Retherford was part of the team behind the earlier plume claim and is also an author of the new reanalysis.<\/p>\n The revised work does not say plumes are impossible. It says this particular dataset no longer provides solid evidence that they were there.<\/p>\n A new SwRI study has raised doubts about the existence of water vapor plumes on Jupiter\u2019s moon Europa (shown above), initially reported based on Hubble Space Telescope observations from 2012. A reanalysis of the data reduced the certainty of that initial finding, but scientists are still hopeful that such plumes will be observed at some point in the future. (CREDIT: NASA) A signal that depended on a few pixels<\/p>\n The team revisited 23 Hubble Space Telescope observations made with the Space Telescope Imaging Spectrograph. They focused on Europa\u2019s Lyman-alpha emissions, which is a specific ultraviolet wavelength tied to hydrogen. That glow can help researchers study both Europa\u2019s thin atmosphere and any localized burst of material near the surface.<\/p>\n Back in the earlier analysis, the interpretation rested in part on where Europa was placed within the image. That turned out to matter a great deal.<\/p>\n \u201cOne of the difficulties in interpreting the data back then was determining where to place Europa<\/a> within its context,\u201d Retherford said. \u201cThe way Hubble works left some uncertainty in terms of placement relative to the center of the image. If Europa\u2019s placement was off even just by a pixel or two, it could affect how the data gets interpreted.\u201d<\/p>\n That is exactly what the reanalysis found.<\/p>\n The new model accounted for several sources of ultraviolet light at once. There were foreground emissions from Earth\u2019s own hydrogen exosphere and interplanetary hydrogen. In addition, sunlight reflected from Europa\u2019s icy surface and a broad hydrogen exosphere surrounding Europa itself were included. When those known contributors were fitted together, the once-notable local excess faded into something much less persuasive.<\/p>\n Lead author Lorenz Roth of the Royal Technical Institute in Sweden<\/a> said the team\u2019s earlier confidence dropped steeply. \u201cOur reanalysis took our original 99.9% confidence in the plumes\u2019 existence and reduced it to less than 90% confidence,\u201d Roth said. \u201cThat\u2019s simply not enough evidence to support the certainty of claims we made at the time.\u201d<\/p>\n That shift came from two main changes. There was a revised estimate of Europa\u2019s exact position on the detector. Integration of Europa\u2019s hydrogen exosphere, which earlier work had not yet fully accounted for, was also important.<\/p>\n Overview of the analyzed HST\/STIS observations. Left: orbital positions of Europa from the start of the first exposures to end of the last exposures for each of the 23 HST\/STIS visits. (CREDIT: Astronomy and Astrophysics) What Hubble did find more clearly<\/p>\n Even as the plume case weakened, the same observations sharpened another result. Europa appears to have a more substantial escaping hydrogen exosphere than some earlier estimates suggested.<\/p>\n Europa\u2019s atmosphere is extremely thin and is constantly replenished as charged particles from Jupiter\u2019s magnetosphere strike the moon\u2019s frozen surface. That irradiation helps break apart water ice, producing molecules such as oxygen and hydrogen. Molecular oxygen dominates close to the surface, while lighter hydrogen can spread farther out and escape more easily.<\/p>\n Using the reprocessed Hubble data<\/a>, the researchers found hydrogen exosphere emissions in all but one of the 23 observations. They derived vertical hydrogen column densities of about 1.4 \u00d7 10^12 cm^-2 for observations from 2012 to 2015. Observations from 2018 to 2020 showed about 1.1 \u00d7 10^12 cm^-2.<\/p>\n That points to roughly a 25% drop between the two observing periods.<\/p>\n The team also estimated an effective hydrogen temperature of about 1000 kelvin for the earlier period. Higher values are still possible within the uncertainties. Those numbers matter because they shape how easily hydrogen escapes Europa\u2019s weak gravity. Based on the inferred density and temperature, the researchers estimated a hydrogen source rate of about 1.1 \u00d7 10^27 atoms per second.<\/p>\n That is a large escape flow.<\/p>\n It is also awkward to explain.<\/p>\n Data analysis and model fitting. Top: brightness for the entire Ly\u03b1 slit along the y-axis, averaged over the x-axis (i.e., over the slit width). Middle: Zoom-in of the top panel. Bottom: brightness along the x-axis, averaged over a y range covering the disk diameter. (CREDIT: Astronomy and Astrophysics) A harder puzzle under the ice<\/p>\n The researchers argue that simple photodissociation, where sunlight breaks apart atmospheric molecules, is unlikely to be the main source of Europa\u2019s atomic hydrogen. That process would tend to produce hydrogen atoms much hotter than the relatively modest temperatures inferred here.<\/p>\n Electron impacts on atmospheric hydrogen-bearing molecules may be more efficient than sunlight. However, the analysis suggests that route also struggles to supply enough hydrogen. In particular, it appears difficult to reconcile the needed production rate with the behavior expected for molecular hydrogen. Much of this should escape before it can be dissociated.<\/p>\n That leaves open the possibility that some hydrogen<\/a> is produced more directly from Europa\u2019s surface ice through sputtering or photolysis. Yet the authors note that those pathways are still poorly constrained. This is especially true when it comes to the amount of hydrogen they can produce and the energies involved.<\/p>\n The observed difference between the earlier and later Hubble periods adds another wrinkle. One possible explanation is that small changes in Europa\u2019s surface temperature could alter radiolysis yields enough to change atmospheric production rates. The paper notes that even a 3% rise near Europa\u2019s average surface temperature could, under some laboratory-based assumptions, produce an increase of about 28% in radiolytic yield. This is close to the shift seen in the hydrogen data.<\/p>\n Still, that remains more suggestion than proof.<\/p>\n The team also found that some of the measured signal was affected by Earth itself. Hydrogen in Earth\u2019s exosphere can attenuate Europa\u2019s Lyman-alpha signal, especially when Europa\u2019s velocity relative to Earth is small. Accounting for that effect helped explain why some observations looked weaker than others.<\/p>\n Analysis of the spectral aspects of the Ly\u03b1 signals: (a) Typical Ly\u03b1 spectral fluxes from the Sun (solid) and from the IPH (dash-dotted) at the heliocentric distance of Jupiter. (b) Ly\u03b1 transmission of the Earth H exosphere in the rest frame of Earth for an assumed column density of 1 \u00d7 1013 cm\u22122 and temperature of 1000 K (solid). (CREDIT: Astronomy and Astrophysics) Why plume claims remain unsettled<\/p>\n The new work does not erase all past interest in Europa plumes. Other studies have proposed signs of intermittent activity using different methods and at different locations. But the broader picture remains patchy.<\/p>\n That is part of what makes Europa so compelling. The moon\u2019s surface appears to cover a vast saltwater ocean. Cracks in the ice could, in principle, provide pathways for subsurface water to rise. Similar plume activity has been confirmed on Saturn\u2019s moon Enceladus<\/a>. Jupiter\u2019s moon Io throws sulfur dioxide high into space as well.<\/p>\n Europa, though, is still holding back its answer.<\/p>\n Retherford said the current dataset does not rule out the earlier plume idea, but it no longer supports it as concrete evidence. \u201cThe description of the phenomena just doesn’t hold up the same way anymore,\u201d he said. \u201cThe new data has made us reconsider the strength of the previous paper\u2019s conclusion regarding water vapor plumes.\u201d<\/p>\n For now, the safest conclusion is narrower and more interesting than a simple retraction. Hubble\u2019s ultraviolet record seems to say less about a dramatic jet of water and more about a global, dynamic hydrogen envelope that scientists still do not fully understand.<\/p>\n That mystery will not last forever.<\/p>\n