Abstract: Many moons in the outer solar systems are known to have a subsurface ocean under the ice shell, which may harbor extraterrestrial life. The exploration of such “ocean worlds” would be a focus of NASA’s scientific Solar System exploration in the coming decades. In particular, Enceladus, a small icy moon of Saturn, is of substantial interest because Cassini found active geysers that eject high-speed water jets, which are believed to be originated from the subsurface ocean.