On the in-situ detectability of Europa's water vapour plumes from a flyby mission
Introduction
Jupiter's moon Europa is thought to harbour a subsurface ocean of salty liquid water that could potentially support life (Hand et al., 2009). The release of material from below the surface of Europa, water molecules for example, has been hypothesized to explain geological features on the surface (Hoppa et al., 1999, Fagents et al., 2000, Nimmo et al., 2007; Quick et al., 2013). However, past optical observations from Galileo and New Horizons have failed to prove the existence of any such releases (Phillips et al., 2000, Hurford et al., 2007, Roth et al., 2014a). On the other hand, during the E12 Galileo flyby of Europa a significantly raised electron density (Kurth et al., 2001) was observed together with anomalously strong magnetic fields (Kivelson et al., 2009). These atypical plasma conditions might be linked to active processes at Europa's surface, although they could possibly be explained the passing of a cold, dense blob of iogenic plasma as well (Bagenal et al., 2015).
Hubble Space Telescope observations in December 2012 of ultraviolet emission lines of oxygen (at the 130.4 nm and 135.6 nm emission line) and hydrogen (Lyman-α) at Jupiter's moon Europa were interpreted as the existence of water vapour plumes near the south pole (Roth et al., 2014a). Plume activity persisted during the 7 h of observation time. The height of the plume derived from these observations is ∼200 km. Furthermore the observations indicate a mass flux of 7000 kg/s and source kinetic temperature of the gas of 230 K.
During similar Hubble observations in November 2012 and 1999 no plumes were observed. If a plume was present during these observations the density would have been two to three times lower (Roth et al., 2014a). Results were ambiguous for Hubble observations made in June 2008 (Saur et al., 2011). Also, during repetitions of the successful observation in January and February 2014 (Roth et al., 2014b) and in the period between November 2014 until April 2015 (Roth et al., 2016) no plume signatures were detected. Such observations could imply that the plumes are not persistent or that the successful detection from December 2012 corresponded to an exceptionally strong event.
Nevertheless, the successful plume observation in December 2012 (Roth et al., 2014a) implies the existence of a localized source of neutral water molecules. The presence of the neutrals will result in the production of ions, mainly via electron impact ionization. The plume-originating neutrals and ions might be a potential source of the hypothetical Europa torus (Lagg, 2003, Mauk et al., 2003, Mauk et al., 2004) or plasma plumes (Intriligator and Miller, 1982, Russell et al., 1998 and Eviatar et al., 2005) in addition to the exospheric particles sublimated or sputtered from the surface.
At Enceladus, one of Saturn's moons, ongoing plume activity is taking place with a global production of about 93 kg/s (Waite et al., 2006). In-situ plume sampling indicates the plumes are linked to Enceladus’ subsurface ocean (Postberg et al., 2009, Postberg et al., 2011) and has allowed the study of Enceladus’ subsurface ocean (see for example Bouquet et al., 2015). This suggests that if Europa's plumes are connected to its interior, sampling of the plume gasses could allow the study of the subsurface ocean. Some predictions have been made about the spreading of dust (Southworth et al., 2015) and bacteria-sized particles by Europa plumes (Lorenz 2015).
In this work we investigate the feasibility of in-situ measurements of Europa's plumes, by modelling the trajectories of neutral and ionized plume particles and the respective measurements by neutral and ion mass spectrometers. First, we use a Monte Carlo particle tracing method (also called test-particle method) to model the trajectories. Then, we simulate the first planned Europa flyby of the JUICE spacecraft that will take place on the 13th of February 2031. For this flyby we simulate the measurements conducted by the instruments for ion and neutral detection, and determine if they can detect the particles originating from the plume. We show that these instruments can provide a sufficiently high signal-to-noise ratio for plume-originating particles, even if we assume a plume that is three orders of magnitude less dense than the one reported in Roth et al. (2014a).
Section snippets
Reference mission: JUICE
As a reference mission case we investigate the feasibility of plume particle detections by the Particle Environment Package (PEP) on-board of the ESA mission JUpiter ICy moon Explorer (JUICE). JUICE is scheduled to be launched in 2022 and will make two flybys of Europa in early 2031 (Grasset et al., 2013). During the flybys JUICE will approach Europa up to a height of ∼400 km.
The PEP experiment is composed of six particle instruments that will study the particle environment at Jupiter. In this
Neutral plume model
We employed a Monte Carlo particle tracing method to model the neutral environment due to the Europa plume. In our model neutral plume particles are represented by super particles, a super particle (sometimes also called macro or meta particle) is a virtual particle that represents many (water) molecules travelling together (see for example Holmström 2007). The trajectories of the super particles are calculated under the influence of Europa's gravity by numerical integration. Particle
Ionized plume model
From the simulated spatial profile of the neutral density, we launch ionised super particles from each grid cell. The ion super particles represent many ions travelling together. By calculating the trajectories of the ions under the influence of the Lorentz force, we determine the distribution of ion density and ion flux in each grid cell in a three dimensional space. The directional and energy distribution of the ions at each grid cell are determined as well, which are later used for the ion
Neutral particles
Fig. 2 shows the spatial distribution of the neutral H2O density from the plume. The plume source in this case is located at the south pole of Europa. Fig. 2a shows the spatial density distribution on the scale of Europa, Fig. 2b shows a close-up of the source region. In Fig. 2a a local elongated increase in density can be observed at the north pole, exactly on the opposite side of the plume source. In this simulation Europa functions as a ‘gravity lens’. The gravity lens focusses particles
Detectability of the plume particles
It can be seen in Fig. 3 that the signal of neutral plume particles is well above the noise level of the NIM instrument (Table 1) for the wide range of plume positions considered for this plot. In Fig. 3a it can be seen that the count rate is around 2×105/5 s. Assuming 5 s integration time and that the background is about 35 counts (Table 1), the signal to noise ratio is ∼5700 at this point, therefore also allowing the detection of trace chemical species contained in the water plume. Thus, we
Conclusion
We simulated the detection of H2O and H2O+, by respectively NIM and JDC. The signal-to-noise ratios in the optimal cases, in terms of plume position with respect to JUICE, are ∼5700 for NIM and ∼33 for JDC, if only a background due to penetrating radiation is assumed. The results show that the particles of the low mass flux plumes (1 kg/s) can be detected with large margins. By comparing the density distributions from a point source and a 1000 km crack, we conclude that the difference does not
Acknowledgements
The contribution of Karl-Heinz Glassmeier is financially supported by the German Bundesministerium für Wirtschaft und Energie and the Deutsches Zentrum für Luft und Raumfahrt under grant 50QJ1501.
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