The space agency is studying the idea of sending robots swimming under the icy crusts of Europa and Enceladus.
NASA recently announced $600,000 in funding for a study on the feasibility of sending swarms of miniature swimming robots (known as independent micro-swimmers) to explore the oceans beneath the icy shells of the many ” oceanic worlds” of our solar system. But don’t imagine metallic humanoids swimming like frogs underwater. They will likely be simple triangular wedges.
Pluto is an example of a probable ocean world. But the worlds whose oceans are closest to the surface, making them the most accessible, are Europa, a moon of Jupiter, and Enceladus, a moon of Saturn.
Life inside ocean worlds
These oceans are of interest to scientists not only because they contain a lot of liquid water (the ocean of Europe probably contains twice as much water as all the oceans on Earth combined), but also because the chemical interactions between rock and ocean water could support life. In fact, the environment of these oceans could be very similar to that of Earth when life began.
These are environments where water that has seeped into the rock of the ocean floor becomes warm and chemically enriched – water that is then released back into the ocean. Microbes can feed on this chemical energy, and in turn can be eaten by larger organisms. In fact, no sunlight or atmosphere is needed. Many such hot rock structures, called “hydrothermal vents”, have been documented on the Earth’s seabed since their discovery in 1977. In these places, the local food web is indeed fueled by chemosynthesis (energy from chemical reactions) rather than photosynthesis (energy from sunlight).
On most ocean worlds in our solar system, the energy that warms their rocky interiors and keeps the oceans from freezing to the base comes primarily from the tides. This contrasts with the largely radioactive heating of the Earth’s interior. But the chemistry of the interactions between water and rock is similar.
Enceladus’ ocean has previously been sampled by flying the Cassini probe through plumes of ice crystals that erupt through cracks in the ice. It is also hoped that Nasa’s Europa Clipper mission will find similar plumes to sample when it begins a series of close flybys of Europe in 2030. However, getting into the ocean to explore it would potentially be much more informative than simply sniff a freeze-dried sample.
This is where the concept of detection by independent micro-swimmers (Swim) comes in. The idea is to land on Europa or Enceladus (which wouldn’t be cheap or easy) at a place where the ice is relatively thin (not yet located) and use a radioactively heated probe to melt a hole 25 cm wide to the ocean – located hundreds or thousands of meters deep.
Once there, it would release up to four dozen 12cm-long wedge-shaped micro-swimmers to explore. Their endurance would be much less than that of the 3.6 m long autonomous underwater vehicle, baptized Boaty McBoatface, whose autonomy is 2,000 km and which has already reached a cruise of more than 100 km under the ice of Antarctica.
At this point, Swim is just one of five “Phase 2 studies” of a series of “advanced concepts” funded under the 2022 cycle of NASA’s Innovative Advanced Concepts (NIAC) program. There is therefore still little chance that Swim will become a reality, and no full mission has been defined or funded.
The micro-swimmers would communicate with the probe acoustically (through sound waves), and the probe would send its data by cable to the lander on the surface. The study will consist of testing prototypes in a test tank with all subsystems integrated.
Each micro-swimmer could explore only a few tens of meters from the probe, limited by the power of its battery and the range of its acoustic data link, but acting as a herd, it could map changes (in time or space) temperature and salinity. They might even be able to measure changes in water cloudiness, which could indicate the direction of the nearest hydrothermal vent.
Due to the power limitations of micro-swimmers, none of them may be able to carry cameras (which would need their own light source) or sensors capable of specifically detecting molecules organic. But at this stage, nothing is excluded.
However, I think it is unlikely that we find traces of hydrothermal vents. The ocean floor would, after all, be several miles below the micro-swimmer’s drop point. But, to be fair, the identification of chimneys is not explicitly suggested in the Swim proposal. To locate and examine the vents themselves, we likely need Boaty McBoatface in space. That said, Swim would be a good start.