Plumes and Tides on Europa - Implications from the 2015 HST Campaign

Abstract:

Two plumes of water vapor emanating from Europa’s southern leading hemisphere provide the best explanation for UV emissions detected by HST in Dec 2012 (Roth et al., 2014a). The detection occurred when Europa was near its orbital apocenter; there was no indication of plumes in two previous observations that occurred near pericenter. A southern location and cyclic activity peaking at apocenter are characteristic of Enceladus’ plumes (Porco et al., 2006; Spencer et al., 2006; Hedman et al., 2013), which led to the suggestion that Europa also has long-lived, active plumes controlled by tides (Roth et al., 2014a). However, two subsequent observations did not result in a repeat detection, even when Europa was again near apocenter (Roth et al., 2014b). Because tidal stresses caused by eccentricity, the supposed control mechanism of Enceladus’ plumes (Hurford et al., 2007; Nimmo et al., 2014), repeat exactly over each orbit, the lack of a repeat detection challenged the interpretation of tidally-modulated plumes on Europa.

In Rhoden et al. (2015), we considered the role of Europa’s tilted and precessing spin pole in modulating the eruption timing of the plumes. We found that, for a range of plausible precession rates (see Bills et al., 2009), tidal stresses could change substantially over subsequent orbits even for the same fracture orientation, location, and time in Europa’s orbit. We then identified subsets of fractures that would have been in tension when the plumes were detected and in compression during the other four observations.

An additional 15 HST observations of Europa have now been conducted over a wide range of true anomalies, with no plumes detected. Whether we would predict plumes at these times - due to tidal stress - depends on the precession rate we assume for Europa. We use the approach presented in Rhoden et al. (2015) to simultaneously fit all 20 HST observations to determine whether any fractures on Europa would be compatible with long-lived, tidally-modulated plumes. Furthermore, the 4-fold increase in the number of observations from the 2015 HST cycle can provide new insight into the conditions that may control plume behavior, such as the availability of liquid water and the number of active faults. We will discuss these implications and how they relate to the upcoming mission to Europa.