In the solar system, the body with the highest likelihood of hosting life beyond Earth is Saturn's moon Enceladus. That is because organic matter has been observed in plumes of water erupting from its subsurface. This suggests there could be an ocean where life lives beneath thick ice. A study has dashed scientists' hopes that had continued for more than 20 years.
On the 9th (local time) at the EPSC-DPS 2025 conference in Helsinki, Finland, Grace Richards of Italy's National Institute for Astrophysics–Institute for Space Astrophysics and Planetology (INAF) said, "The organic molecules seen in plumes erupting from fissures on the surface of Enceladus may not have originated in a deep subsurface ocean, but could have been produced by radiation exposure on Saturn's icy moon."
EPSC-DPS is a joint scientific conference that combines the European Planetary Science Congress (EPSC) and the American Astronomical Society's Division for Planetary Sciences (DPS). The findings were also published in the international journal Planetary and Space Science. Does this mean the dream of scientists to find extraterrestrial life in the solar system is fading?
◇Simulating the surface of an icy moon through experiments
In 2005, the joint U.S.-Europe spacecraft Cassini observed plumes erupting from so-called "tiger stripes," cracks in the surface at Enceladus' south pole. Based on magnetic and gravitational measurements, scientists estimated that Enceladus has an ocean up to 10 kilometers deep about 40 kilometers beneath the surface. In other words, an ocean lies between the surface ice shell and the rocky core.
Enceladus' subsurface ocean is heated by tidal forces from Saturn and vents plumes to the surface. Cassini flew through the plumes and analyzed their molecules. Remarkably, the plumes were rich in salts and contained a variety of organic compounds. Organic compounds can form the building blocks of life.
Richards' team has now demonstrated the opposite possibility in experiments simulating Enceladus' environment. The idea is that organic compounds did not come from a subsurface ocean but were produced at the surface under cosmic radiation. The team first cooled ice containing carbon dioxide, methane, and ammonia to minus 200 degrees Celsius. They then bombarded the ice with cations that had lost one electron to reproduce the radiation environment around Enceladus.
The ions reacted with the ice components to produce various molecules, including cyanates combining nitrogen, oxygen, and carbon, as well as carbon monoxide and ammonium. In particular, they formed precursor molecules that can become amino acids, the building blocks of proteins. Some of the compounds synthesized in the experiment had been detected on Enceladus' surface, while other substances were identified in the plumes.
Richards said, "Molecules regarded as the origin of life may have been generated in situ through radiolysis, rather than originating in a subsurface ocean," adding, "We do not entirely rule out the possibility that life could exist in Enceladus' ocean, but this means we should be cautious about making such assumptions based on the composition of the ejecta alone." In other words, organic molecules in the plumes cannot serve as evidence of subsurface life.
◇Ocean world exploration in the solar system tracking life
Enceladus is 504 kilometers in diameter, only 4% the size of Earth. It is just one-seventh the size of the moon. Scientists focus on this small moon because, along with Jupiter's moon Europa, it is considered one of the most likely bodies in the solar system to have water. In particular, unlike other places where water may be in slushy or icy form, Enceladus is estimated to have an ocean warm enough for life to emerge.
Before searching for life that may exist beyond Earth, scientists first look for oceans—so-called "ocean worlds." Liquid water is required for life to exist. Water is the best solvent, dissolving numerous substances and supplying materials needed for life. Water also has a high specific heat, meaning the heat required to raise the temperature of 1 gram of water by 1 degree is greater than for other substances. As a result, it can protect life from heat and cold. Oxygen and hydrogen, which make up water, generate energy flows and also form the framework of life.
Scientists say it is too soon to give up on the dream of finding other life in the solar system. That is because the plume signals Enceladus is sending to scientists are growing stronger. The Enceladus plume first observed by Cassini in 2005 was 100 kilometers long, but in 2023 the James Webb Space Telescope observed a plume 10,000 kilometers long. That is comparable to the distance from Los Angeles in the United States to Buenos Aires, Argentina.
A team at Freie Universität Berlin in Germany reported in 2023 in the international journal Nature that they confirmed the presence of phosphates—key building blocks of life—in Enceladus' subsurface ocean. Phosphorus had never been detected in oceans beyond Earth. Phosphorus in phosphates is essential for all life on Earth. It forms ATP, the molecule that carries energy, and DNA, which stores genetic information. It also makes cell membranes and the bones and teeth of humans and animals.
◇Life exploration at Europa in 2030
The United States and Europe have sent a series of probes to the solar system's ocean worlds. Jupiter orbiter Galileo (launched in 1987) and Saturn orbiter Cassini (1997) led ocean world exploration from the 1990s to the early 2000s. Following them, in 2011 NASA's Juno was launched and arrived in Jupiter orbit in 2016.
Juno explored Jupiter's moon Ganymede in 2021 and the following year closely observed Europa's surface valleys and craters. Juno's findings at Europa will provide useful data for NASA's Europa Clipper, which NASA launched in Nov. 2024. Clipper is scheduled to arrive in Jupiter orbit in 2030. Its goal is to approach as close as 26 kilometers above Europa's surface to take high-resolution images and analyze chemical compositions. Through this, it will assess whether life could exist.
References
EPSC-DPS 2025, DOI: https://doi.org/10.5194/epsc-dps2025-264
Planetary and Space Science (2025), DOI: https://doi.org/10.1016/j.pss.2025.106179
Nature Astronomy (2023). DOI: https://doi.org/10.1038/s41550-023-02009-6
Nature (2023), DOI: https://doi.org/10.1038/s41586-023-05987-9