Saturn’s moon Enceladus continuously ejects vast quantities of ice particles into space, originating from its subsurface ocean. Researchers at the University of Stuttgart and Freie Universität Berlin have now chemically analyzed freshly emitted particles originating directly from Enceladus’ subsurface ocean. To do this, they used data from the Cassini space probe. They were able to detect intermediates of potentially biologically relevant organic molecules, which were thus discovered for the first time in ice particles from an ocean outside Earth. The results of the study have been published in the journal Nature Astronomy, available at https://doi.org/10.1038/s41550-025-02655-y.

Enceladus measures approximately 500 kilometers in diameter; its surface is covered by an ice shell with an average thickness of approximately 25‒30 kilometers. “In 2005, NASA’s Cassini spacecraft discovered a huge cloud of gas and ice particles above its south pole,” explains Dr. Nozair Khawaja, who conducted the study at the Institute of Space Systems (IRS) at the University of Stuttgart and the Institute of Geological Sciences at Freie Universität Berlin.

Measurements taken by Cassini later revealed that there must be an ocean of liquid water inside the satellite. They also showed that the ice particles contain organic molecules. “However, the particles examined were not fresh, but had already been orbiting the moon for some time,” emphasizes Khawaja, who has since moved to the Freie Universität Berlin as a research group leader.

In 2008, Cassini flew twenty-one kilometers above the moon’s surface to the edge of the “ice and gas plume.” The data collected in this process therefore originates from fresh ice particles that had been in the satellite’s interior just a few minutes earlier. Khawaja, who led the study, has now evaluated these measurements together with doctoral student Thomas R. O’Sullivan, his Stuttgart colleague Professor Ralf Srama from the IRS, and the Planetary Sciences and Remote Sensing Working Group led by Professor Frank Postberg from Freie Universität Berlin. Researchers from the USA and Japan also participated in the study.

“Our analysis confirms the results obtained from the analysis of other Cassini data,” says Khawaja. “We can now be fairly certain that both the simple and complex compounds discovered in older ice grains in Saturn’s E ring also originate from the ocean of Enceladus. We suspect that these molecules potentially synthesized at the so-called hydrothermal fields on Enceladus – these are vents at the bottom of the ocean from which hot water rises. In Earth’s oceans, there is evidence of life around similar types of hydrothermal vents.

The scientists involved also identified molecules that had never before been detected in ice particles from an ocean outside Earth. These include those that can serve as building blocks for complex compounds. Such molecules, which include pyrimidines, for example, have also been detected on the asteroids Bennu and Ryugu. On Earth, pyrimidines are among the essential components of DNA.

Some of these compounds were not found in the ice particles analyzed previously. One reason for this discrepancy is probably that Cassini was traveling at a particularly high speed on its journey to the plume. This made it easier for a central measuring instrument onboard the space probe to collect reliable data on organic particles. The instrument is called a “Cosmic Dust Analyzer” (CDA), which is a type of sensor operated by the IRS under the direction of Ralf Srama. Upon impact, the fragments of chemical species in ice grains lose electrons and become positively charged. They are drawn to a negatively charged electrode and reach it more quickly the lighter they are.

Measuring the transit time of all positively charged fragments yields a so-called mass spectrum. From this, conclusions can then be drawn about the original molecule. “If the speed at the moment of collision is too low, there may in some cases be interference in CDA mass spectra,” explains Khawaja. “The signatures left behind by the molecules can then no longer be interpreted clearly – they are masked, so to speak.”

However, Cassini was traveling at a very high speed when it flew past the ice and gas plume in 2008: The probe was traveling at almost 65,000 km/h instead of the usual 40,000 km/h or less. Due to the high energy released by the collision with the ice particles at this speed, certain interfering influences in some of the CDA of the mass spectra can be ruled out.

Cassini is, in fact, history – the probe was intentionally crashed in 2017. “But even today, the data recorded by its measuring instruments many years ago still provide new insights into the ocean inside Saturn’s moon,” explains Frank Postberg from Freie Universität Berlin. The research findings on Enceladus are so promising that the European Space Agency (ESA) is already planning a follow-up mission there for 2040. On board will be measuring instruments that will be able to extract significantly more information from the ice particles inside the satellite. “Our results will help to design these instruments and plan the space mission,” says Khawaja.

Scientists from the University of Stuttgart, Freie Universität Berlin, the University of Colorado, Boulder (USA), the University of Washington, Seattle (USA), and the Earth-Life Science Institute (ELSI) at the Institute of Science in Tokyo (Japan) participated in the study.

Khawaja, N., Postberg, F., O’Sullivan, T.R. et al. “Detection of Organic Compounds in Freshly Ejected Ice Grains from Enceladus’s Ocean.” Nature Astronomy (2025). https://doi.org/10.1038/s41550-025-02655-y.