Probing for Evidence of Plumes on Europa with HST/STIS

Abstract: Roth et al (2014a) reported evidence for plumes of water venting from a southern high latitude region on Europa - spectroscopic detection of off-limb line emission from the dissociation products of water. Here, we present Hubble Space Telescope (HST) direct images of Europa in the far ultraviolet (FUV) as it transited the smooth face of Jupiter, in order to measure absorption from gas or aerosols beyond the Europa limb. Out of ten observations we found three in which plume activity could be implicated. Two show statistically significant features at latitudes similar to Roth et al, and the third, at a more equatorial location. We consider potential systematic effects that might influence the statistical analysis and create artifacts, and are unable to find any that can definitively explain the features, although there are reasons to be cautious. If the apparent absorption features are real, the magnitude of implied outgassing is similar to that of the Roth et al feature, however the apparent activity appears more frequently in our data.

• The paper presents evidence of potential plumes on Europa using HST/STIS observations during transits, consistent with previous findings but showing some geographic variations.
• The methodology involved analyzing far ultraviolet absorption signatures detected beyond Europa's limb against Jupiter during ten transit events.
• While statistically significant, the findings require further corroboration due to potential systematic errors and uncertainties, highlighting the need for future observations.

Evidence of Plumes on Europa from HST/STIS Observations

The document under review presents an investigation by Sparks et al. that utilizes the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in the far ultraviolet (FUV) range to examine potential plume activity on Europa, an icy moon of Jupiter. Previous findings by Roth et al. (2014a) suggested the presence of plumes based on spectroscopic detection of emission lines near the southern pole. This study seeks to corroborate those findings with a different methodological approach, employing direct imaging as Europa transited the face of Jupiter, allowing for a search for absorption signatures indicative of plume activity.

The primary technique involved capturing a series of direct images during ten transit events, searching for evidence of gas absorption beyond Europa's limb against the backdrop of Jupiter. Of these events, three images produced statistically significant features potentially indicative of plumes. Notably, the characteristics of the detected features in these images are coherent with those reported by Roth et al., although discrepancies such as geographic discrepancies and detection failures in other observations necessitate a cautious interpretation.

Key Findings

1. Detection and Consistency with Previous Studies:
   • The paper reports the identification of three transit images with potential plume activity. Two of these correlate with the latitude previously identified by Roth et al. (2014a). The significant statistical features are not aligned perfectly geographically, raising ambiguity in conclusive results.
   • A notable finding is the observation on March 17, 2014, indicating a compact region suggesting plume activity at a more equatorial location. These statistical findings are significant but require further corroboration due to potential systematic errors and uncertainties in modeling and measurement.
2. Approach and Methodology:
   • The methodology involves analyzing the transit images using a model of Europa's expected appearance under the transit conditions and a comparison to observed data. The process involves sophisticated modeling of Europa's surface and incorporating different PSF (point spread function) scenarios to account for optical aberrations.
   • The approach of overlaying Europa's image as it crossed Jupiter provided a unique perspective, potentially unveiling the off-limb activity that would suggest plumes.
3. Statistical and Systematic Considerations:
   • Through rigorous statistical tests, including generating mean and deviation analyses for the data versus model comparisons, the study identifies points of off-limb absorption with statistically significant deviations.
   • However, the report also explicitly addresses potential sources of systematic errors, like detector non-linearities and inaccuracies in atmospheric modeling for Jupiter, emphasizing the need for caution in the interpretation of the results.
4. Implications and Speculations:
   • The results point towards the intriguing possibility of active water plumes on Europa, which, if confirmed, could have profound implications for understanding the moon's geology and potential habitability.
   • The findings propose the need for further observations with both existing and future telescopic assets like the upcoming James Webb Space Telescope or dedicated missions to Europa, to resolve systematic concerns and firmly validate these observations.

Overall, while the analysis by Sparks et al. provides tantalizing evidence in support of potential plume activity at Europa, further confirmatory studies are required to ascertain these claims unequivocally. The research underscores the complexities associated with detecting and characterizing transient and dynamic extraterrestrial phenomena using remote sensing technologies, highlighting both the capabilities and current limitations of such investigative techniques. The implications of these studies stretch beyond the immediate detection of plumes, holding potential insight into Europa’s subsurface ocean, geological activity, and the broader understanding of icy solar system bodies.