Abstract
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.