The south pole of Enceladus—a tiny moon orbiting Saturn—is a volatile place. In this region, the moon’s subsurface ocean spews jets of water through four “tiger stripe” cracks in the icy crust, culminating in a single plume of ice particles that stretches hundreds of miles into space.
The Cassini spacecraft spent two decades studying these particles to search for evidence of habitability on Enceladus. In 2008, the probe flew straight through the icy plume to study particles that were ejected only minutes before they hit the spacecraft’s Cosmic Dust Analyzer (CDA). More than 15 years later, scientists have finally deciphered this data, finding that the particles contained organic molecules never seen in Enceladus’s ejections before.
The study, published Wednesday in the journal Nature Astronomy, explains that the newly detected molecules include those involved in chains of chemical reactions that ultimately give rise to more complex molecules that are essential for life on Earth, according to the researchers.
“There are many possible pathways from the organic molecules we found in the Cassini data to potentially biologically relevant compounds, which enhances the likelihood that the moon is habitable,” said lead author Nozair Khawaja, a researcher at Freie Universität Berlin, in a statement from the European Space Agency (ESA).
The search for signs of habitability on Enceladus
Enceladus’s subsurface ocean has captivated astrobiologists ever since the Cassini mission, a joint endeavor between NASA, ESA, and the Italian Space Agency (ASI), first discovered evidence of it in 2014. Life as we know it can’t exist without water, so a moon with a vast reserve of the stuff is a pretty good place to look for life-giving molecules.
Cassini orbited Saturn from 2004 to 2017 before dramatically plunging into the ringed planet. During this time, the probe detected many organic molecules—including phosphorus and precursors for amino acids—as it flew through Saturn’s E ring, which is largely made of water-ice ejected from Enceladus.
However, grains of ice in the E ring can be hundreds of years old. As they age, they may lose some traces of organic molecules present in Enceladus’s subsurface ocean. To get a better understanding of what’s really going on down there, Khawaja and his colleagues set out to analyze data taken from a fresher source.
Getting closer to the source
The researchers specifically looked at data Cassini gathered during its foray into Enceladus’s icy plume. The freshly ejected particles slammed into the spacecraft’s CDA instrument at high speed—roughly 11 miles (18 kilometers) per second.
The speed of impact proved to be equally important as the particles’ freshness. “At lower impact speeds, the ice shatters, and the signal from clusters of water molecules can hide the signal from certain organic molecules,” Khawaja explained. “But when the ice grains hit CDA fast, water molecules don’t cluster, and we have a chance to see these previously hidden signals.”
This would explain why he and his colleagues uncovered new organic molecules in this data. They also detected some that had already been found in the E ring, confirming that they come from Enceladus’s ocean. This contradicts recent evidence that suggests these molecules may actually stem from radiation-driven chemistry on the moon’s surface and inside its plumes.
While the findings strengthen the case for Enceladus’s habitability, there is still much work to be done to confirm whether this icy wasteland can support life. ESA aims to launch another mission to explore this distant moon, this time searching for signs of habitability on the surface.
“Even not finding life on Enceladus would be a huge discovery, because it raises serious questions about why life is not present in such an environment when the right conditions are there,” Khawaja said.