Towards a Global Unified Model of Europa’s Tenuous Atmosphere

Abstract

Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions

Plasma Interaction Signatures of Plumes at Europa

“…The asymmetry between leading and trailing hemisphere is controlled through the parameter A, where A = 0 results in a symmetric (Runs ".2") and A = 10 (adopted from Arnold et al, 2019) in an asymmetric density distribution (Runs ".3" and ".4"). In agreement with the model of Arnold et al (2019) and HST observations (Hall et al, 1995;Plainaki et al, 2018;Saur et al, 1998), we have chosen a neutral scale height of h 0 = 100 km and a surface density of n 0 = 5 · 10 13 m −3 .…”

Energetic Ion Dynamics in the Perturbed Electromagnetic Fields Near Europa

“…Another major contribution to these plasma currents arises from the interaction between the thermal magnetospheric flow and Europa's dilute atmosphere (e.g., Plainaki et al, ). The moon's gas envelope is partially ionized, mainly through electron impacts (Saur et al, ).…”

“…For instance, Paranicas et al () suggested that asymmetries in the flux of energetic electrons onto Europa's surface are the major cause of the H 2 SO 4 patterns seen in the moon's trailing hemisphere by Galileo. In addition, sputtering of Europa's surface by energetic ions was found to be the principal agent for the generation of the moon's dilute atmosphere (e.g., Plainaki et al, , ), consisting mainly of molecular oxygen (e.g., McGrath et al, ). Ultimately, the fraction of sputtered surface molecules that escapes Europa's gravity also forms the extended neutral gas torus along the moon's orbit; that is, Europa's surface serves as a source of particles for the Jovian magnetosphere (Johnson et al, ).…”

“…Only in the megaelectron volt regime, the precipitation patterns are qualitatively similar for uniform and draped fields. However, the precipitation of megaelectron volt ions onto Europa is still not homogeneous, since the strong magnetospheric field keeps ion gyroradii much smaller than the moon's radius.Another major contribution to these plasma currents arises from the interaction between the thermal magnetospheric flow and Europa's dilute atmosphere (e.g., Plainaki et al, 2018). The moon's gas envelope is Key Points: • A combination of hybrid simulations and particle tracing is applied to study the dynamics of energetic magnetospheric ions near Europa • Energetic ion dynamics near Europa are strongly affected by the Alfvénic field perturbations near the moon • In the kiloelectron volt regime, field line draping due to ionospheric mass loading effectively shields Europa's surface against energetic ions…”

On the in-situ detectability of Europa's water vapour plumes from a flyby mission

“…These products react chemically to produce mostly O 2 and H 2 . Molecular oxygen is thought to be the most abundant species in the atmosphere, because it doesn't stick to the surface as much as other species (such as H 2 O) or escapes Europa's gravity easily (such as H 2 ) (Plainaki et al 2018). While sputtering is responsible for the extended, large scale height atmosphere, sublimation could also cause an additional low altitude confined atmospheric component with a much smaller scale height (Smyth and Marconi 2006).…”