Europa and Enceladus are two of the most fascinating moons in the Solar System. Both have active geysers and oceans of water containing minerals from tidal forces or radioactive decay. The minerals are critical for the existence of life and missions to sample the plumes are in the works, which include landing on the surface to study deposits near the plumes.

Europa is one of the four large moons of Jupiter. The Voyager and Galileo spacecrafts found evidence that the surface crust floats on a subsurface ocean. By analyzing the data, it is possible to model the composition and physical properties of the core, silicate layer, and ocean. Different minerals lose water and volatiles at different depths and temperatures. Heating and increased pressure caused by radioactive decay and tidal flexing cause the breakdown of water-containing minerals and release the trapped water. The resulting ocean would be mildly acidic with high concentrations of carbon dioxide, calcium, and sulfate. Data from the Hubble Space Telescope reveals chloride on the surface indicating that the water is chloride rich. This means that the ocean beneath Europa is more like Earth’s oceans and could be habitable for life. Europa is one of the best chances for finding life, but more understanding is needed such as how fluids migrate through Europa’s rocky interior. It remains unclear if icy moons can generate enough heat to melt rock beneath the ocean floor. There could be some interesting chemistry occurring within the ocean depths of Europa, Enceladus, and other icy worlds.

There is a new concept to move around the surface of other worlds being investigated by the Jet Propulsion Laboratory that uses power that may seem antiquated. Steam power can be associated with locomotives of the Wild West and the Titanic, but it might be possible to design a probe that can hop across the surface using steam power. The surfaces of Europa, Enceladus, and other icy worlds may seem quite smooth, but very little is known about the surfaces, which could make navigating them a challenge. The surfaces could be porous, riddled with crevasses, or there might be towering icy spikes. This kind of terrain would make roving very treacherous if not impossible. It would be nice to have a spacecraft able to hop from one location to another.

The Steam Propelled Autonomous Retrieval Robot for Ocean Worlds, or SPARROW, is a probe about the size of a soccer ball and consists of a system of thrusters, avionics, and instruments encased in a protective spherical cage. SPARROW would travel through the rarified atmosphere using short bursts of steam produced from melted ice. This would keep the environment pristine for study by not contaminating it with rocket fuel. The low gravity of icy worlds with no atmospheric drag to slow the hopper down would make it easy to hop several miles over landscapes that other robots would have difficulty navigating. The mission would consist of a stationary lander serving as the home base for SPARROW. It would mine the ice and melt it before loading the water onto the hopper robot. SPARROW would then heat the water inside its engines creating bursts of steam to give a boost off the surface. When it runs low on fuel, the hopper would return to the lander for more water, a true refueling station. The hopper would also drop off samples for the lander to analyze. The lander would carry several hoppers that could be sent together to swarm around a specific location or split up to explore as much of the terrain as possible. SPARROW received funding from NASA in 2018 to explore the use of steam propulsion and to develop and test different water-based propellant systems. The study will also further increase the understanding of how a spherical probe might tumble when landing which would aid in determining the most efficient angle of launch and speed of hop. It has already been found that a single long hop would be more efficient than several smaller hops.

There are exciting ways to explore the Solar System which now include sample return spacecraft, rovers, helicopters, and soon a drone on Titan. A hopper probe will pave the way to exploring the icy, airless moons of Jupiter, Saturn, Uranus, Neptune, and perhaps Pluto and Charon. Wherever water ice is available for fuel, the possibilities of exploring are endless.