Ocean worlds are a promising environment for harboring extraterrestrial life. Their oceans, however, are often enclosed under a thick layer of ice. The current best estimate of the Europan ice crust thickness, for example, is 24 km. Thus, a key challenge to in situ exploration of these potentially life-rich waters is developing effective and efficient ways to penetrate the ice. Analytical and numerical thermal models of ice penetrators in cryogenic ice are available in the literature, but experimental validation of these models has been limited. To help close this gap, we have built scaled Model Validation Probes (MVPs) and evaluated their performance in the Europa Tower—a cryogenic vacuum chamber hosting an ice column of 0.75 m diameter and 2 m height, capable of maintaining ice at 90 K and surface pressure at near-vacuum (10-3 torr), similar to the conditions found on Europan surface. The tests monitored the fundamental probe performance variables of power and penetration speed, and additional variables such as ice and meltwater temperatures, and rough measurement of melt-hole shape. Seven tests were performed, ranging from 500 W to 1,200 W of input power. Four probes reached ~1 m of cryogenic ice penetration, and three probes reached the bottom of the 2 m ice column. All probes had confirmation of hole closure and the presence of liquid water inside the closed hole. This is the first realistic Europa-environment testing (cryogenic and vacuum) with subsurface penetration, hole closure, and liquid water. Comparison of our experimental results to the seminal Aamot model modified to handle temperature dependence of ice thermal properties shows that probe efficiency was below expectations, especially for high power probes. A detailed comparison of the experimental results to full finite volume numerical models results in better agreement and reveals the parameter range over which the Aamot model is valid. These results validate the core of the Aamot model while giving insight into its regime of applicability and the important factors governing deviations from its assumptions. The result of these endeavors is a deeper understanding of the dynamics of cryogenic ice penetration, directing future technological development and mission planning to enable direct exploration of environments that may harbor extraterrestrial life.

Starting in 2010 with the VALKYRIE cryobot project (NASA ASTEP), Stone Aerospace has been investigating methods to penetrate thick layers of ice. The focus of these methods is to develop cryobot vehicles capable of transporting payloads representative of an Ocean Worlds sub-ice mission (including on-board sensing and deployable swimming rovers). Critical to these types of carrier vehicles—and in fact any efficient ice-penetrating probe—is a detailed understanding of the thermal and physical dynamics of ice penetrators under the wide variety of conditions that may be encountered in the five operating regimes of such a mission: starting (under cryogenic temperatures and vacuum), brittle ice transit, ductile ice transit, dirty ice, and breakthrough. Early work on terrestrial ice-penetrating probes generated initial closed-form models which remain powerful for first-cut analyses. Further work on refining these models for more exotic environments (cryogenic or/and impure ices as will be encountered on Europa and other Ocean Worlds) has resulted in varying levels of success. Above all, the field suffers from very sparse, limited experimental validation. We review the current understanding of the thermodynamics of ice-penetrating vehicles in a variety of ice environments, both terrestrial and those of other Ocean Worlds, and present new models for ice regimes expected in both terrestrial and extraterrestrial applications. In addition, to begin to address the limited empirical understanding of these penetration dynamics—particularly in very cold environments—we present initial results and planned further work on validation tests in the Stone Aerospace Europa Tower cryogenic vacuum chamber. Validated thermodynamic models for cryobots operating in multiple regimes will allow for the assessment of feasibility of designs, prediction of full mission times, and enable optimal design of critical top-level parameters such as required power, vehicle shape, and internal heat distribution mechanisms.

Ocean Worlds are strong candidates for the first discovery of extraterrestrial life as they may provide liquid water, energy, and biologically essential elements. These bodies are characterized by large volumes of water under a layer of ice, often in contact with a rocky core and powered by tidal heating. Jupiter’s moon Europa is believed to have at least twice as much water as Earth. A key remaining challenge is reaching the oceans of Europa: the thickness of the ice crust may range from 3 km to 30 km. Initial steps have been taken to develop analytical and numerical models of the thermal and physical dynamics of ice penetrators in cryogenic environments, but experimental validation of these models has been limited. We have built and experimentally tested the performance of a set of melt probes in the Europa Tower located at Stone Aerospace. The Europa Tower is a cryogenic vacuum chamber with an internal diameter of 0.75 m and an ice column height of 2 m, capable of maintaining ice at approximately 90 K and surface pressure at near-vacuum (10-3 torr), allowing for the testing of probes designed for the surface of Europa. The Model Validation Probes (MVPs) used are designed to test the fundamental thermal and kinetic properties of melt probes in cryogenic ice. They include monitoring of power, temperature, and penetration depth, with wires stored and released via spools internal to the probe, allowing continued connection after hole closure. MVP1 was tested in January 2020, was powered by a 500 W cartridge heater, and reached a total depth of approximately 1.1 m after about 10 h of test, resulting in an average steady state penetration velocity of roughly 12 cm/h, which is within 10% of theoretical model predictions. This test also confirmed two important aspects of hole initialization: the capability of the probe to start in cryogenic, vacuum conditions where the lack of liquid water limits heat transfer (the “starting problem”), and the rapid melt hole closure following penetration, which allows the probe to continue penetration in a pressurized bubble that contains liquid water. We will also present results of tests with other MVPs, which validate the modeled dependency between the average steady state velocity and the heater power input level. Future work will investigate the effects of ice density and impurities on penetrator performance.