The Mission to Enceladus – The ESA L4 mission

The ESA Voyage 2050 Senior Committee in 2021 recommended a mission to the “Moons of the Giant Planets” as one of the upcoming ESA Large missions. The scientific goals for this mission include exploring the habitability of ocean worlds, searching for biosignatures, and studying the connection between moon interiors and near-surface environments, as well as their implications for the overall moon-planet system. This theme follows the breakthrough science from the NASA-ESA Cassini-Huygens mission and the expected scientific return from the ESA JUICE mission in Cosmic Vision. The mission to the “Moons of the Giant Planets” will be ESA’s fourth Large-class mission – L4. 

Following the recommendation by the Voyage 2050 Senior Committee, ESA convened an Expert Committee [1] to define the scientific scope and target for this mission, considering the characteristics of each moon and future planned missions to Jupiter and Saturn's ocean worlds. Scientists identified Saturn’s moon Enceladus as the most compelling target, followed by Saturn’s moon Titan and Jupiter’s moon Europa. 

Figure 1: Summary of the Scientific objectives versus their relevance and whether they will be addressed by planned missions (JUICE, Europa Clipper, and Dragonfly) for each target (Europa, Ganymede, Enceladus, and Titan). Dark blue marked areas indicate most relevant objectives that will not be addressed by other forthcoming missions and are thus of highest interest for the L4 mission (from [1]).

The continued exploration of icy moons in the outer solar system suggests that subsurface oceans are common, although these oceans are hidden under thick ice shells. Their habitability can only be assessed indirectly. However, this changed with the Cassini mission in July 2005, which definitively detected water vapor plumes with jets of ice particles erupting from Enceladus during its third flyby. This was further supported by Magnetometer data returned at Cassini's first flyby in February 2005​[1], [2], [3], [4], [5]​. At its equator, which receives the most direct sunlight, surface temperatures on Enceladus average minus 193 degrees Celsius. The south pole, despite receiving less sunlight, was slightly warmer than the equator, at minus 188 degrees Celsius. The linear fractures, referred to as "tiger stripes," were as warm as minus 163 degrees Celsius in some areas, indicating active geological processes (​[2], [6], [7], [8], [9]​. These observations demonstrate that Enceladus is an active world. More importantly, it implies that Enceladus provides the opportunity to measure ocean water “in-situ” and directly search for biosignatures. Cassini demonstrated this capability by measuring the plume composition while passing through the plumes, despite the spacecraft and instruments not being designed for such measurements. 

It is now generally accepted that Enceladus has the three necessary conditions for a habitable environment capable of supporting life: the presence of liquid water, a source of energy, and a specific set of chemical elements ​[10], [11]​. The L4 mission will build on this legacy with a mission design and payload complement focused on exploring the habitability of Enceladus and searching for biosignatures. The mission will take a significant step forward by not only placing an orbiter around Enceladus but also deploying a lander on the moon's south pole area. No space agency has previously landed on Enceladus, offering substantial potential for new scientific discoveries, particularly concerning habitability. Landing on Enceladus presents unique challenges as its environment is both familiar and alien: The moon’s surface is coated with fine, icy particles originating from active cryovolcanic plumes—effectively “snow” precipitating from the subsurface ocean beneath the ice shell. These particles may continuously fall onto the landing site and the lander [12], [13], potentially delivering material rich in salts, organics, and even biosignatures directly into the scientific instruments 

Since March 2025, the ESA study team has been working with a newly selected Payload Working Group and the Expert Committee to refine the science requirements and identify key technologies to achieve the ambitious goals of ESA’s Mission to Enceladus [15]. 

This new mission will advance European expertise in several scientific and technological fields, including in-orbit assembly, operating in extreme environments, landing technologies, and novel scientific instrumentation. These revolutionary technologies will have wide-ranging applications beyond ESA’s space science programme.