Europe’s Search for Life Around Saturn

Few places in our Solar System are as promising in the search for extraterrestrial life as Saturn’s icy moon, Enceladus. Ever since the Cassini mission flew through the plumes of salted water erupting from its south pole, revealing a remarkable array of organic molecules and molecular hydrogen (see my article here), scientists have been eager to return.

Yet, despite numerous mission proposals and Enceladus being identified as the second-highest-priority target in NASA’s 2023–2032 Planetary Science Decadal Survey (see my article here), the U.S. agency has no plan at present to return back to the moon.

The European Space Agency (ESA), meanwhile, has been considering a mission to Enceladus for many years. Indeed, ESA's next generation of large scientific missions is being developed under its long-term Voyage 2050 programme. Previous L-class missions include JUICE (L1), currently on its way to Jupiter's moon Ganymede (which I covered here), NewAthena (L2), the X-ray space telescope planned for launch in 2039, and LISA (L3), the space-based gravitational-wave observatory planned for launch in 2035.

After studying several possibilities, ESA's expert committee on the next L-class mission (L4) identified Enceladus as the leading target. The mission is currently known simply as L4—the mission to Enceladus. Just to be clear, this is no longer just an interesting proposal produced by a group of planetary scientists. ESA has selected Enceladus as the target for the first large-class mission of Voyage 2050 and is now working through the detailed studies and developing the technologies required to make it possible.

And the current plan is extraordinarily ambitious. The L4 spacecraft would launch around 2042, reach the Saturn system in the early 2050s. The precise schedule will depend on the final mission design and future programme decisions, but ESA is already carrying out detailed industrial studies to determine what the spacecraft will need to do. And it seems the mission designers are determined to make the most of this opportunity, as Enceladus is not necessarily the first stop.

The spacecraft will spend time touring the Saturn system, observing several of its moons and studying the environment around the planet. Titan and Mimas are among the worlds that could be investigated during this phase.

Titan is particularly interesting because it is the only world beyond Earth known to have stable liquid on its surface, although that liquid consists of methane and ethane rather than water, with NASA's quadcopter Dragonfly mission planning to explore its surface from 2034 (my latest article on Titan is here). Mimas is intriguing for another reason. It is mainly composed of water ice, and with tidal forces acting on it, there might be subsurface pockets of liquid water. So even before L4 reaches its principal target, it could add significantly to our understanding of Saturn's remarkable collection of ocean worlds. And for that, this is already going to be a truly fascinating mission.

But eventually the spacecraft will turn its attention to Enceladus, and that is where the mission becomes particularly exciting. Of course, the great advantage of Enceladus is that we do not have to guess what is beneath the ice. The moon is effectively venting out its liquid water into space (thus forming Saturn's E ring). Following Cassini's sampling of the frozen water droplets, it found evidence consistent with hydrothermal activity within Enceladus. That does not mean that life has been discovered, but it indicates that it possesses an environment in which life could exist.

The ESA mission being considered will consist of an orbiter and a lander. The orbiter will first conduct a detailed reconnaissance of the moon, particularly the south polar region. One of its most important tasks will be finding somewhere safe for the lander to touch down. This will be no mean feat, given that the region is a complex landscape riddled with fractures in the ice. The very geological activity that makes Enceladus so scientifically fascinating also makes landing there considerably more challenging. The spacecraft will therefore need to understand the surface in great detail before committing to the descent.

So, once on the surface, what will the lander actually do? The details of the scientific payload are still being worked out. ESA is currently bringing together scientists and engineers to determine which instruments should fly on the mission. According to the current planning described by the mission team, a workshop later in 2026 will help refine the scientific requirements before the process of selecting instruments moves forward. Mission adoption is currently envisaged around 2034. But the basic scientific objectives are already clear: the mission needs to characterise the surface, investigate the plumes and their relationship to the subsurface ocean. As importantly, it needs to search for chemical or physical evidence that could indicate biological activity. In other words, the presence of alien life.

Alas, it will have precious little time to do all this. With so little sunlight reaching Enceladus, the L4 lander will have to rely on batteries, limiting its expected lifetime on the surface to just two to four weeks. Unlike NASA, ESA does not currently have access to the nuclear-powered radioisotope thermoelectric generators (RTGs) that have powered many of its deep-space missions over the years.

Nevertheless, the L4 lander would be sitting on the surface of a world that nobody has ever explored directly. For the first time, we could analyse Enceladus' surface material with instruments designed specifically for the job. And all the while, the orbiter would continue its work overhead, directly sampling the plumes for biosignatures.

I don't know about you, but I think ESA may just have a winning mission on its hands. For more information about the L4 mission and its progress, you can visit ESA's page here.

As always, onwards and upwards. Image credit: NASA/JPL/Space Science Institute