One of the greatest discoveries of the to Voyager missions and the subsequent Galileo and Cassini missions was the discovery of vast quantities of liquid water under the ice of the the Jupiter moons Europa and Enceladus. For astrobiologists, water is a key component for the search of life and attention is slowly turning from dry Mars to these icy moons.
Since 2015, Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California, has been working with Nasa headquarters to develop new technologies and specific systems for use on future missions to these ocean worlds.
The technologies were developed as part of the Ocean Worlds Mobility and Sensing study, a research project funded by Nasa's Space Technology Mission Directorate in Washington.
At this stage, they are prototypes only - and the experiments conducted with them on Earth are starting points that may lead to further development.
"Although several missions are planned to both Europa and Enceladus the technological and engineering challenges are significant," Mark Galloway, an astronomer at the University of Hertfordshire, told PE.
"These tests by Nasa are a first step in trying to develop a robot capable of making its way through the ice and swimming in these vast, dark and cold seas."
Ice diggers
The developments include a subsurface probe that could burrow through miles of ice, taking samples along the way; robotic arms that unfold to reach faraway objects; and a projectile launcher for even more distant samples.
"Robotic systems would face cryogenic temperatures and rugged terrain and have to meet strict planetary protection requirements," said Hari Nayar, who leads the robotics group that oversaw the research. "One of the most exciting places we can go is deep into subsurface oceans, but doing so requires new technologies that don't exist yet."
Brian Wilcox, an engineering fellow at JPL, has designed a prototype inspired by so-called "melt probes" – used to melt through snow and ice to explore subsurface regions.
However, they use heat inefficiently. Europa's crust could be 6.2 miles deep or it could be 12.4 miles deep; a probe that doesn't manage its energy would cool down until it stopped frozen in the ice.
So Wilcox invented a capsule insulated by a vacuum, the same way a thermos bottle is insulated. Instead of radiating heat outwards, it would retain energy from a chunk of heat-source plutonium as the probe sinks into the ice.
A rotating sawblade on the bottom of the probe would slowly turn and cut through the ice. As it does so, it would throw ice chips back into the probe's body, where they would be melted by the plutonium and pumped out behind it. Removing the ice chips would ensure the probe drills steadily through the ice without blockages. The ice water could also be sampled and sent through a spool of aluminium tubing to a lander on the surface. Once there, the water samples could be checked for biosignatures.
"We think there are glacier-like ice flows deep within Europa's frozen crust," Wilcox said. "Those flows churn up material from the ocean down below. As this probe tunnels into the crust, it could be sampling waters that may contain biosignatures, if any exist."
Robotic arms
The probe would heat to more than 482 degrees Celsius which would kill any residual organisms and decompose complex organic molecules that could affect science results.
The Nasa researchers also looked at the use of robotic arms, which are essential for reaching samples from landers or rovers. On Mars, Nasa's landers have never extended beyond 2-2.5m from their base, but the scientists trialled a folding boom arm which can extend almost 10m.
For targets that are even farther away, a projectile launcher was developed that can fire a sampling mechanism up to 50m.
The arm and the launcher could be used in conjunction with an ice-gripping claw. According to Nasa researchers, this claw could someday have a coring drill attached to it - so that the robot could dig out samples from underneath Europa's surface ice. The ice is believed to shield complex molecules from Jupiter's radiation.
The claw would anchor itself with the help of heated prongs that melt into the ice, and a drill's bit would then penetrate and collect a sample.
JPL researchers also looked at potential vehicles and concluded that building a rover for an icy moon would require a rethink.
Tests found that granular ice in cryogenic and vacuum conditions behaves more like sand dunes, with loose grains that wheels can sink into. The engineers turned to designs first proposed for crawling across the moon's surface. They tested lightweight commercial wheels fixed to a rocker bogey suspension system that has been used on a number of JPL-led missions.