Europa’s Lyman-α Shadow on Jupiter - A New Way of Searching for Water Plumes
Masters Thesis Presentation
Time: Tue 2020-08-18 13.15
Lecturer: Johan Ferm
Europa is one of the most interesting satellites in the solar system in the search of extra-terrestrial life, as it harbours an interior water ocean under its icy surface. Water vapour in Europa’s atmosphere has been previously observed, suggesting water plume eruptions from the surface. These plumes could potentially originate from the subsurface ocean, and as such contain ocean constituents that can be examined in orbit. Two observations of Europa’s far-ultraviolet shadow on Jupiter were made by the Hubble Space Telescope in 2018 and 2019. It was observed in Lyman-α (1 216 Å), a spectral line of hydrogen. This study investigates the imaged Lyman-α shadow in search of potential plumes at the shadow limb. Examining the shadow instead of the moon itself is a new method of remotely studying the Europan atmosphere. Forward modelling is applied to create artificial images that are compared to the observations. Any anomalies around the shadow limb are then analysed and evaluated for their statistical significance. Two noteworthy outliers are found at the limb (one on each occasion) corresponding to H2O line of sight column densities of 3.07 × 1017 cm−2 and 4.72 × 1016 cm−2 , for the 2018 and 2019 observation, respectively. They are not significant however, as they lie within three standard deviations from the expected value (< 3σ). An upper limit on what column density is detectable in the data is computed, yielding 6.71 × 1016 cm−2 (using only 2019 data due to a weak signal on the 2018 occasion). A constraint on the maximum possible H2O column density at Europa is thus provided. The new method is shown to be useful for the intended purpose and could potentially be applied on other icy moons.