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.