The search for alien life has shifted dramatically in recent decades. Mars remains the leading nearby target because it is rocky, once held abundant water, and is covered in organic molecules, but the icy moons of Jupiter and Saturn are mounting a compelling challenge.

Jupiter has three large icy moons: Europa, Ganymede, and Callisto. Europa is known to contain an ocean with twice as much water as Earth, while questions remain about the interiors of Ganymede and Callisto.

Saturn offers Enceladus, a smaller world that is now considered one of the most exciting targets in the search for alien life. Reaching it with a dedicated mission, however, may take several decades.

Professor Caroline Smith, Head of Collections and Principal Curator of Meteorites at the Natural History Museum, favors the icy moons. “I would go for one of the icy moons of Jupiter or Saturn,” Smith told IFLScience last year.

“These ocean worlds with a protective icy crust... underneath, there's a good chance you've got liquid water, and liquid water is something you need for life,” Smith said. Yet water alone is not enough, since potentially habitable environments also require useful chemistry and energy.

Enceladus may hold an advantage on those fronts. The Cassini mission observed geysers erupting from the Saturnian moon and detected complex chemicals within them, although major questions remain about how life might form, evolve, and survive in a dark ocean hundreds of kilometers deep.

“If you think about the discoveries that have been made even in the last 40 or 50 years, about the diversity of life in the deepest oceans of the Earth far away from sunlight, you know people never thought there'd be anything life down there. And yet these areas sometimes are teeming with life. You could have the same things happening on the icy worlds,” Smith said.

While enthusiasm for Enceladus has grown, Europa has faced some disappointing scientific developments. New analysis earlier this year suggested that its seabed may not be especially active, with no fractures, active volcanoes, or plumes of hot water.

Such features could be important for supplying the complex chemistry and energy needed by life. Researchers have also retreated from earlier findings that appeared to show water plumes and carbon dioxide emissions coming from Europa.

Confirmed plumes would offer an enticing opportunity because a probe could fly through them and sample the ejected material. However, researchers who originally reported one water plume claim later lowered their confidence in it.

“Our reanalysis took our original 99.9% confidence in the plumes’ existence and reduced it to less than 90% confidence,” lead author Dr Lorenz Roth of KTH Royal Institute of Technology, Sweden, said in a statement. “That’s simply not enough evidence to support the certainty of claims we made at the time.”

Two missions could help resolve some of Europa’s mysteries. NASA’s Europa Clipper will concentrate on the moon and assess its potential habitability, while the European Space Agency’s JUICE will fly past Europa before focusing on Callisto and especially Ganymede.

“[JUICE] is a very important mission because we want to find out whether there are habitable places inside the icy moons, so Europa, Ganymede, or Callisto. That’s the main question of the mission,” Dr Olivier Witasse, a planetary scientist working on the mission’s scientific aspects, told IFLScience at the launch.

Both missions are expected to arrive in the early 2030s, potentially transforming scientists’ understanding of Europa. Enceladus will demand more patience, with the European Space Agency planning its most ambitious examination for the early 2050s.

That plan calls for landing a probe near the geysers at Enceladus’s South Pole and waiting for ocean material to fall onto it. If life exists in the ocean, the probe is intended to find it.

NASA, meanwhile, will return to the Saturn system with Dragonfly, a flying mission that will explore Titan. Titan may contain a deep water ocean, while its surface has methane and other hydrocarbons, rivers, lakes, seas, and an atmosphere rich in organics.

The icy moons will remain compelling targets for decades, but Mars still holds a singular distinction. It is the only place where evidence of something that could have been life has been found, with the Cheyava Falls rock currently described as the leading contender for possible extraterrestrial life detection.