No abstract

As an initial step toward in situ exploration of the interiors of Ocean Worlds to search for life using cryobot architectures, we test how various communication tethers behave under potential Europa-like stress conditions. By freezing two types of pretensioned insulated fiber optic cables inside ice blocks, we simulate tethers being refrozen in a probe’s wake as it traverses through an Ocean World’s ice shell. Using a cryogenic biaxial apparatus, we simulate shear motion on preexisting faults at various velocities and temperatures. These shear tests are used to evaluate the mechanical behavior of ice, characterize the behavior of communication tethers, and explore their limitations for deployment by a melt probe. We determine (a) the maximum shear stress tethers can sustain from an ice fault, prior to failure (viable/unviable regimes for deployment), and (b) optical tether performance for communications. We find that these tethers are fairly robust across a range of temperature and velocity conditions expected on Europa (T = 95–260 K, velocity = 5 × 10−7 m s−1 to 3 × 10−4 m s−1). However, damage to the outer jackets of the tethers and stretching of inner fibers at the coldest temperatures tested both indicate a need for further tether prototype development. Overall, these studies constrain the behavior of optical tethers for use at Ocean Worlds, improve the ability to probe thermomechanical properties of dynamic ice shells likely to be encountered by landed missions, and guide future technology development for accessing the interiors of (potentially habitable ± inhabited) Ocean Worlds.

<div> <p><strong>Introduction: </strong>After orbiting and landing on an ocean world such as Europa or Enceladus, the next step will be to “rove”. However, rather than traveling along the surface, a cryobot probe would instead vertically traverse in order to reach the ocean and explore for signs of life. Many challenges exist for technologies to function within the harsh conditions at ocean worlds, including extreme temperatures, pressures, faulting, and the presence of potentially corrosive chemicals. Yet, these must be overcome. In order to return valuable science and engineering data, cryobot communication devices must be capable of transmitting data after deployment through ice shells of a few to 10s of km thick, and over mission lifetimes of several years.</p> <p>A strategy for cryobot communication that employs both optical tethers coupled with radio frequency (RF) relays has been suggested as part an ocean world cryobot mission architecture (e.g. Oleson et al., 2019). Here we present work on a NASA Concepts for Ocean worlds Life Detection Technology (COLDTech) project called Signals Through the Ice (STI) Tech for Ocean Worlds (OW), that builds on previous NASA SESAME STI work, to develop communication hardware solutions (e.g. Craft et al., 2022).</p> <p><strong>Technology developments:</strong> Building on previous characterizations of optical tether robustness shear testing (Singh et al, 2022<em>& in review; </em>McCarthy et al., <em>this meeting</em>), RF relay antenna design (Lorenz et al, 2022), and modeling efforts to characterize Europa’s ice shell conditions (Walker et al., 2020), the OW STI Tech team is expanding efforts to consider conditions at Enceladus and other factors affecting cryobot communication technologies.</p> <p><em>Optical Tethers</em>. A main hazard for tethered communication is the potential faulting activity inducing strain on the tether and causing data transmission reduction, or even severance the tether. Numerical models have been performed to simulate fault slip resulting from tidal forcing at multiple time points during one Europa tidal cycle. Calculations at multiple time points allow consideration of motion throughout the orbital cycle and evaluates the accumulation of strain on a tether crossing a fault at a range of different strikes, angles, and depths (Lien et al., 2022 & <em>in prep</em>). Calculations have also been performed to constrain Europa's deformation induced by tidal forcing and how it affects the thermal equilibrium state of the ice shell (Walker and Rhoden, 2022). Improving our understanding of the temperature and heat profiles within ice shells provides important estimates of the specific temperature and pressure environments both tethered and RF technologies will encounter on their journey through an ice shell.</p> <p>Also, a factor not yet constrained for ocean worlds is how a tether’s jacket material would potentially adhere to the ice. This ice-tether behavior would influence the resulting strain induced on the optical fibers during fault motion. Adhesion tests and design trades on candidate jacket and strength layer materials are in progress with a focus on making improvements for Europa and Enceladus temperatures, strain, and chemical conditions. Additionally, tethers are undergoing environmental chamber “soaks”, with tethers embedded in ice of different compositions for months. Material degradation and transmission performance is measured during and upon removal.</p> <p><em>RF relays.</em> An initial RF relay thermo-mechanical design is undergoing evaluation to assess if the essential elements that must be housed and thermally insulated in an Europa or Enceladus ice shell can be kept at the necessary operational conditions. Essential components include batteries, power/heating elements, electronics, and antennas. Prototypes are being built and will undergo thermal and mechanical environmental tests (including thermal balance and rapid & cyclic loading).</p> <p><strong>Summary: </strong>Development of optical tethers and RF relay devices for communication through ice on ocean worlds is underway by the OW STI Tech team. Through modeling of the extreme conditions at Europa and Enceladus, design and build of prototypes, and thermo-mechanical testing, these technologies will be robust to the environmental conditions and will therefore enable the search for extraterrestrial life in the subsurface of ocean worlds.</p> <p><strong>Acknowledgments:</strong> We would like to acknowledge funding from grants 80NSSC19K0613 and 80NSSC21K0995 for this work.</p> <p><strong>References: </strong></p> <div> <p>Craft et al. (2022) <em>53<sup>rd</sup> LPSC</em>, Abstract #2753<em>. </em></p> <p>Lien et al. (2022) <em>AbSciCon</em>, Abstract #518-05.</p> <p>Lorenz et al. (2022) <em>NETS, </em>May, 2022, Cleveland.</p> <p>Oleson et al. (2019) Europa Tunnelbot, <em>NASA/TP</em>—2019-220054.</p> <p>Singh et al. (2022) <em>AbSciCon</em>, Abstract #518-02.</p> <p>Walker and Rhoden (2022),<em> PSJ, </em>accepted<em>.</em></p> <p>Walker et al. (2020) <em>51<sup>st</sup> LPSC</em>, Abstract #2448.</p> </div> </div>

<p>Understanding the rheology of ice in the outer shells of ocean worlds is important for a variety of reasons. The mechanical behavior has long been studied to make sense of intriguing surface morphologies, some of which bear striking resemblance to rocky features on Earth and other bodies, some of which are totally unique. The mechanical behavior of planetary ice also is used to provide possible answers to questions related to transport and conveyance of melt and/or chemical species, which in turn addresses potential habitability. Finally, in the era of ocean world exploration, knowledge of the mechanical behavior of very cold and moderately impure ice provides critical information needed to manage expectations and inform protocols for future missions. In this presentation I will share results from experiments conducted in our lab on ice and ice-mixtures that are designed to measure the mechanical behavior at planetary conditions. In particular we use a custom cryogenic, servo-controlled biaxial apparatus to measure frictional response of ice sliding on ice to explore faulting behavior in the upper, brittle portion of an icy shell. Moving beyond simple Coulomb analysis, we measure fault stability as a function of temperature and velocity, which point to a predicted source region for icequakes at depth, a so-called “seismogenic zone” for icy worlds. We employ rate- and state- dependent frictional properties determined from experiments in numerical models to describe a variety of fault and boundary types, including thrust, subduction, and strike-slip faulting. We explore the nature of unstable frictional behavior by analyzing laboratory stick-slip events for frequency-magnitude, or recurrence, relations. Such information can inform expected seismic activity and be useful for planning instrument sensitivity and can be used ultimately to interpret results from a lander-based seismometer. Additionally we will present results from performance testing of communications hardware that may be deployed in the vicinity of such active faults. We will share mechanical and optical results from a series of shear-testing experiments on fiber optic communication tethers embedded in ice and will articulate the feasibility of their use in future probe missions that will descend into an icy shell. The experimental work coming out of our lab seeks to unravel the dynamic properties of ice on icy moons, identify challenges that may arise when spacecraft interact with it, and develop the technology required for assessing the habitability of ocean worlds.</p>