New Study Identifies Organic Molecules Spewing from Saturn's Icy Moon Enceladus

Enceladus chemistry -JH Final DRAFT 10-14-25, includes author comments

New Study Identifies Organic Molecules Spewing from Saturn's Icy Moon Enceladus

A better chance for a habitable Enceladus

Twenty years after the Cassini spacecraft discovered an ocean under the icy surface of Saturn's moon Enceladus, a new study of previously collected data hints at a much better chance for habitability. The change comes from recording how a plume of ice grains hit Cassini just minutes after it erupted from the moon. The close encounter exposed organic compounds not previously seen on Enceladus.

The discovery is not clear evidence of life nor even a sign Enceladus might be habitable, says planetary scientist Nozair Khawaja (Free University Berlin). But it is highly suggestive and has encouraged an ambitious European Space Agency mission called the L4 project, which will search for evidence of life on Enceladus. To be launched between 2040 and 2042, L4 would reach the Saturn system around 2050, with goals to land on and orbit Enceladus and sample plumes from its underground ocean as well as fly by other Saturnian moons. 

Cassini spent 13 years exploring the Saturn system, including a series of flybys of Enceladus as well as of Saturn's E ring, which collects debris from Enceladus’s icy plumes. Khawaja began working on his PhD in 2012, analyzing data collected by Cassini’s Cosmic Dust Analyzer (CDA) mass spectrometer to infer the composition of the moon’s ocean.

Khawaja started out his research using data freshly gathered by the CDA and transmitted by Cassini. At the time, most of the dust being collected came from the E ring, hitting the sensor at no more than 12 kilometers per second (27,000 mph). When Cassini passed near particles vented directly from the ocean, those particles hit the CDA at a much higher velocity, up to 18 km/s.

Using impact speed to compare measurements from different locations showed an important difference: Dust grains from the E ring included water features, which obscured signals from the organic compounds that Khawaja was trying to detect. However, the particles just ejected from a plume didn’t have that interference.

He also found that the impact speeds revealed how long the particles had been in space. Slower grains had been vented from the ocean and captured into the E ring, spending years in the harsh conditions of space before reaching Cassini. But the faster grains that collided with Cassini had been in space only minutes after being vented from within Enceladus. Thus the molecular structures that had formed in the frozen-over ocean were thus still intact when collected by the mass spectrometer.

After the Cassini mission ended on September 15, 2017, no more new observations were possible. However, Cassini had accumulated a large archive of unanalyzed data, and Khawaja's group searched them to find measurements made when the spacecraft was closest to the plume and most likely to produce the widest variety of organic molecules.

Finding data collected in 2008 just minutes after a plume ejection, “we found records of organic compounds we had never seen before,” says Khawaja, lead author of a paper in the October 1st Nature Astronomy.

His team identified several types of molecules, including chains of carbons as well as complex carbon rings, including rings that contain oxygen. These are precursors to more complex organic molecules and can react to form, for example, pyrimidines, a class of organic materials recently found in non-biological material in the asteroid Bennu. The conditions under which these molecules formed could be similar to those found near hydrothermal vents deep in Earth’s oceans, Khawaja speculates. 

The results provide "further evidence” that there might be complex chemistry going on within the rocky or sandy floor of Enceladus’s underground oceans, says Andrew Coates (University College London), who was not involved in the study. "I think Enceladus and Europa have high chances of habitability now,” he adds.

Two missions are currently on their way to Jupiter to probe its icy moon Europa which has its own interior ocean. ESA launched the Jupiter Icy Moons Explorer (JUICE) in April 2023. A year later, NASA launched the Europa Clipper mission, which is expected to begin gathering data around 2030. Both carry a new generation of technology, with mass spectrometers updated from the one on Cassini. Likewise, the L4 mission will deliver even more capable technology to Enceladus when it arrives in 2050, aided by the laboratory models that Khawaja and others are preparing.