THEO

THEO (Testing the Habitability of Enceladus's Ocean) is a concept for a New Frontiers-class orbiter that would fly to Enceladus and directly sample the water plumes erupting from its south pole. The goal is to assess whether the moon’s subsurface ocean could support life and to look for signs of it. The study was developed during the 2015 Jet Propulsion Laboratory Planetary Science Summer School with help from TeamX. It has not been formally submitted for funding.

The mission would take advantage of the plume—a mix of water vapor and ice—that Cassini data suggests comes from a liquid reservoir under the icy crust. That reservoir appears to contain organic compounds, salts, and products of water-rock interactions, meaning the basic ingredients for life are present. THEO would not directly search for living organisms. Instead, it would look for elevated levels of certain molecules, specific ratios of organic compounds, or chains of amino acids that could serve as biosignatures.

To get to Enceladus, the spacecraft would use one Venus and two Earth gravity assists for speed, then slow down with flybys of Titan, Rhea, Dione, Tethys, and Enceladus before entering orbit around Enceladus. Data collection would happen in three phases at altitudes of 500 km, 100 km, and 30 km. Over a six-month nominal mission, more than 600 science orbits are planned. At the end, the orbiter would be sent on a collision course with Tethys, following planetary protection rules.

The proposed payload—about 80 kg total—includes a mass spectrometer (SWAMP), a sub-mm radiometer-spectrometer (WAVES), a camera (DRIPS), a magnetometer (OSMOSIS), and doppler tracking (GEISER). These instruments are designed to answer four key questions: how the plumes connect to the ocean, whether the ocean’s abiotic conditions are suitable for life, how stable that ocean environment is, and whether any biological processes are occurring.