Re-analysis of limb anomaly detections in three HST/STIS transit images of Europa: No evidence for plumes

After evidence for present-day geological activity on Jupiter’s moon Europa remained elusive for decades, several recent studies derived the existence of plumes on various locations. We have re-analyzed the three HST/STIS transit images in which Sparks et al. (2016) identified limb anomalies as evidence for Europa’s plume activity. After reproducing the results of Sparks et al. (2016), we find that positive outliers are similarly present in the images as the negative outliers that were attributed to plume absorption. A physical explanation for the positive outliers is missing. We identify two factors that affect the significance of the measured outliers in the region above Europa’s limb: The exact location of Europa on the detector and the description of the statistical fluctuations in the images. When accounting for these factors, the statistical significance of the plume candidate features is about 3 sigma or lower in the three images. The resulting positive and negative outliers are consistent with random statistical occurrence in a sample size given by the number of pixels in Europa's limb region.