By David Rothery for The Conversation
NASA recently announced $600,000 (£495,000) in funding for a study into the feasibility of using swarms of miniature swimming robots (known as independent microswimmers) to explore the oceans beneath the icy shells of the many ‘sea worlds’ of ours to send to the solar system. But don’t imagine metal humanoids swimming underwater like frogs. They will likely be simple, triangular wedges.
Pluto is an example of a probable ocean world. But the worlds with oceans closest to the surface, making them most accessible, are Europa, a moon of Jupiter, and Enceladus, a moon of Saturn.
life in sea worlds
These oceans are of interest to scientists not only because they contain so much liquid water (Europa’s ocean probably has about twice as much water as all of Earth’s oceans), but because chemical interactions between rocks and ocean water could support life. In fact, the environment in these oceans can be very similar to that on Earth at the time life began.
These are environments where water that has penetrated the rocks of the seafloor becomes hot and chemically enriched—water that is then expelled back into the ocean. Microbes can feed on this chemical energy and in turn be eaten by larger organisms. No sunlight or atmosphere is actually needed. Since their discovery in 1977, many such warm, rocky structures of this type, called “hydrothermal vents,” have been documented on Earth’s seafloor. In these locations, the local food web is actually supported by chemosynthesis (energy from chemical reactions) rather than photosynthesis (energy from sunlight).
In most of the ocean worlds of 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 warming of the earth’s interior. But the chemistry of water-rock interactions is similar.
The ocean of Enceladus has already been sampled by flying the Cassini spacecraft through clouds of ice crystals erupting through cracks in the ice. And there’s hope that Nasa’s Europa Clipper mission could find similar feathers to sample when it begins a series of near Europa flybys in 2030. However, going into the ocean to explore would potentially be a lot more informative than just sniffing at a freeze-dried sample.
This is where the concept of independent microswimmer (swim) detection comes into play. The idea is to land on Europa or Enceladus (which would be neither cheap nor easy) in a place where the ice is relatively thin (not located yet) and use a radioactively heated probe to melt a 25 cm wide hole to the ocean — hundreds or thousands of meters below.
Once there, it would release up to four dozen 12 cm wedge-shaped microswimmers to go exploring. Their endurance would be much less than that of the 3.6 m long autonomous underwater vehicle famously named Boaty McBoatface with a range of 2,000 km, which has already achieved a journey of more than 100 km under the Antarctic ice.
At this point, Swim is just one of five “Phase 2 studies” in a series of “advanced concepts” funded in the 2022 round of NASA’s Innovative Advanced Concepts (NIAC) program. So there’s still a big chance that Swim will become a reality, and a full mission hasn’t been fleshed out or funded.
The microswimmers would communicate acoustically (through sound waves) with the probe, and the probe would transmit its data to the lander on the surface via wire. The study will test prototypes in a test tank with all subsystems integrated.
Each microswimmer might be able to explore just ten meters from the probe, limited by their battery power and the range of their acoustic data link, but as a herd they could map changes (in time or space) in temperature and salinity. They may even be able to measure changes in the water’s turbidity, which could indicate the direction to the nearest hydrothermal vent.
However, performance limitations of microswimmers may mean that none could carry cameras (which would require their own light source) or sensors that could specifically sniff out organic molecules. But at this stage nothing is excluded.
However, I think finding signs of hydrothermal vents goes a long way. The sea floor would eventually be many kilometers below the microswimmer’s release point. But to be fair, the location of vents isn’t specifically suggested in the Swim proposal. To locate and examine the vents themselves, we’ll probably need Boaty McBoatface in space. That said, swimming would be a good place to start.