Europa is a premier target for advancing both planetary science and astrobiology, as well as for opening a new window into the burgeoning field of comparative oceanography. The potentially habitable subsurface ocean of Europa may harbor life, and the globally young and comparatively thin ice shell of Europa may contain biosignatures that are readily accessible to a surface lander. Europa’s icy shell also offers the opportunity to study tectonics and geologic cycles across a range of mechanisms and compositions. Here we detail the goals and mission architecture of the Europa Lander mission concept, as developed from 2015 through 2020. The science was developed by the 2016 Europa Lander Science Definition Team (SDT), and the mission architecture was developed by the preproject engineering team, in close collaboration with the SDT. In 2017 and 2018, the mission concept passed its mission concept review and delta-mission concept review, respectively. Since that time, the preproject has been advancing the technologies, and developing the hardware and software, needed to retire risks associated with technology, science, cost, and schedule.

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Highly disordered insulating materials exposed to high electric fields will, over time, degrade and fail, potentially causing catastrophic damage to devices. Step-up to electrostatic discharge (ESD) tests were performed for two common polymer dielectrics, low density polyethylene and polyimide. Prebreakdown transient current spikes or arcs were observed, using both slow and high speed detection. These pre-ESD discharge phenomena are explained in terms of breakdown modes and defect generation on a microscopic scale. The field at which prebreakdown arcing begins was compared to the onset field for electrostatic discharge at which complete breakdown occurs for each material studied. We present evidence that these two threshold fields are the same. Thus, the important parameter to consider in design may not be the maximum field for breakdown, as much as the defect structure in the materials and the field where pre-breakdown arcing begins in a material.

Electrostatic discharge (ESD) continues to pose significant risks to space missions despite decades of intense study. Tabulated values of material breakdown strength used in spacecraft charging models are often based on cursory measurements that may not be relevant to a given mission. Materials physics offers insight into the relevant variables that affect breakdown and how to address them experimentally for spacecraft applications. Measured distributions of ESD data across several test configurations, taken together, begin to provide an understanding of how to estimate the likelihood of ESD events as a function of acquired charge over a spacecraft's mission lifetime. We discuss how consequences of these results apply to spacecraft charging modelling and design considerations.

This work provides physical insight into common statistical models for DC dielectric breakdown field strengths. Voltage step-up tests were performed on low density polyethylene films. The merits of generalizations to widely-used empirical Weibull models are discussed. The cumulative probability distributions of the breakdown fields were fit to standard two-and three-parameter Weibull distributions. Mixed two-parameter Weibull distributions, sometimes used in the literature to model multiple breakdown modes, were found to yield the best fits to the data. In addition, the same data were fit to a physically-motivated dual-defect mean field model incorporating both low-and highenergy defect modes with different defect densities; this produced a much better fit than single-defect mean field models. Values obtained for the mean defect energies and densities were within the ranges expected from independent determinations of these intrinsic materials properties. By incorporating these physicsbased concepts into traditionally empirical models, their accuracy and utility can be extended. The mixed Weibull distribution and the dual-defect model predicted very similar cumulative distributions of LDPE breakdown data, suggesting that mixed Weibull distributions may reflect similar multiple defect modes used in dual-defect models. Theories of DC breakdown, based on distributions of microscopic defects in disordered insulating materials may provide improved guidance in understanding the physical origins of empirical parameters used in statistical methods to characterize breakdown properties.