An RF Concept for Communicating with a Subsurface Cryobot on an Ocean World

Introduction: A successful mission to the ocean of an icy ocean world will require penetrating the ice shell with instrumentation robust to thermal and mechanical conditions that are significantly variable, down to depths of kilometers to 10s of km, while maintaining communication with the surface. A robust communication strategy and hardware that can provide data transmission rates adequate to achieve science and exploration objectives are critical for any future mission to access an ocean world’s subsurface. Here, we explore the potential of RF communications to provide that capability

Background: The propagation of radio waves through ice is strongly dependent on both the wavelength chosen and the temperature, composition (including the nature and concentrations of any salts/other impurities), and texture of the ice. This has been the subject of extensive evaluation via interest in radar sounding of ice sheets on Earth and other planetary bodies [e.g. 1-4]. Early proposals for through-ice communication on Europa [e.g., 5,6] have advocated the use of RF relay modules and a recent NASA Compass concept study [7] discussed the use of tethers with coupled radio frequency (RF) repeater ‘pucks’. However, with the exception of analysis like that done by [6], work to determine if/how an RF communication system would operate reliably in an extreme environment like that expected at Europa or Enceladus has been lacking.

We recently evaluated potential antenna designs and performance for a through-ice RF communication system in a modeled Europa-like environment and chose a dual resonant loop antenna design with a circumference-to-wavelength ratio between 0.8 and 1 [8]. Considerations for choosing an antenna and operating wavelength included: maximizing gain in the vertical direction; balancing choice of operating frequency against background noise sources (galactic and planetary) and acceptable attenuation in ice (across expected temperature range, composition, and texture); and providing sufficient bandwidth to channelize communication by frequency while yielding sufficient data rate. In addition, available volume for housing and deploying RF relay modules at Europa, based on current cryobot concept studies [5,7], was used as a constraint.

Current Work: We are currently evaluating an RF module design (i.e., antenna, electronics, power source, etc.) that can function in a Europa- or Enceladus-like ice shell environment, for the anticipated lifetime of a cryobot mission. Previous RF relay module designs for cryobot architectures [5-7] have been conceptual in nature and have not adequately addressed fundamental thermo-mechanical constraints on RF module operation in an ocean world ice-shell. Primary among these are the need to operate for long durations at temperatures that range from < 80K to > 200K, under large hydrostatic pressures, and in the presence of periodic (tidal) or instantaneous (fracturing) forcing. We intend to model, build, and test a RF relay module notional design (Figure 1) to characterize its thermal and mechanical performance, under Europa- and Enceladus-like conditions. The notional design is cylindrical to maximize available volume in a cryobot and to help ensure a stable up-down antenna orientation when the module is deployed. Within the cylinder is a central core consisting of batteries, power/heating elements, electronics, and antennas. This is surrounded by insulating foam and encased in a ‘roll-cage’ outer structure.

Modeling of the relay module notional design is underway and is using relevant thermo-mechanical characteristics of currently available, high-TRL components. Analysis of that modeling effort will be used to inform the manufacture of 2 RF module mock-ups for testing. The modules used in testing will be manufactured with space-qualified RF-transparent materials and adhesives and will utilize ‘flight-like’ assembly seams/joint construction techniques. One or two styles of access panel and a representative attachment area to the cryobot will also be included. Structural components of the module and the thermal insulating material will be manufactured as intended for a ‘flight-like’ build. All other internal components (e.g. power, antennas, electronics, etc.) will be represented in the manufactured modules with mock-ups that simulate their placement and thermo-mechanical characteristics within the module (e.g., mass, strength, thermal dissipation, conductivity). This choice was made to reduce the cost, complexity, and time required for the manufacturing process and because active internal components are not required to address the thermal and mechanical performance characteristics that are being tested.

Figure 1. Notional design for a RF relay module concept that incorporates previous antenna design work [8] and considers thermal and mechanical needs for function in an ocean world ice shell.

References: [1] Martinez et al., 2009, In International conference on GeoSensor Networks, pp. 131-137, Springer, Berlin, Heidelberg; [2] Blankenship et al., 2009, in Europa, pp. 631–654, Univ. of Ariz. Press, Tucson, Ariz; [3] Miller et al., 2012, Icarus, 220, 877-888; [4] Kasoulová et al., 2017 Journal of Geophysical Research: Planets, 122(3), 524-545 [5] Zimmerman et al., 2001; [6] Bryant, 2002, In Proceedings, IEEE Aerospace Conference (Vol. 1, pp. 1-356); [7] Oleson et al., 2019, NASA/TP—2019-220054.; [8] Lorenz et al., 2022, in prep.