Saturn’s moon Enceladus has a rocky core, surrounded by a global ocean encrusted by a sheet of ice. The moon may not look very hospitable, but data from NASA’s Cassini spacecraft suggest that Enceladus has all the raw ingredients necessary for life as we know it. Fly-bys of Enceladus previously revealed liquid water, hydrogen gas, and small carbon compounds, which together could potentially support microbial cells.
Cassini’s mission ended in September 2017, when scientists sent it plummeting into Saturn, but researchers are still poring over the data it beamed back to Earth. A new analysis of those data finds that Enceladus not only has small organic molecules, but also larger, more complex carbon compounds bursting from its ocean. Chris Glein (Southwest Research Institute, USA), a member of DCO’s Deep Energy Community and Extreme Physics and Chemistry Community, and an international team of researchers report these findings in a new paper in the journal Nature .
“This finding completely changes our perspective on the organic chemistry of Enceladus,” said Glein. “We’ve gone from simple to complex.”
While Glein and his colleagues were digging through the archive of Cassini data, they noticed something strange. During Cassini’s early, fast fly-bys of Enceladus, its Ion and Neutral Mass Spectrometer (INMS), which identifies compounds based on mass-to-charge ratio, detected small organic compounds, like benzene and butene. But in the later, slower fly-bys of Enceladus, the INMS did not detect these small organic compounds.
Puzzled, the group consulted with the team responsible for Cassini’s other mass spectrometer, the Cosmic Dust Analyzer (CDA), which can detect compounds that are more than 10 times more massive than the INMS can. The CDA had detected large organic molecules in particles from the plume, and these organics most likely contain dozens of carbon atoms. The researchers realized that their fast fly-bys had pulverized these molecules in the INMS, which appeared more clearly during a slower fly-by. “Our initial discovery of organics was just the small bits that were broken off of much larger molecules,” said Glein.
Based on their findings, the researchers hypothesize that hydrothermal fluids rising from the seafloor deliver insoluble organic compounds to Enceladus’ ocean. Currents carry them to the surface, where they float as a film. Eruptions then force the organic molecules, along with seawater, out into space. The organic compounds become incased in water ice, emerge from Enceladus’ plume (see image below), and finally escape to form Saturn’s ghostly E ring.
Now that the researchers have found these larger organic compounds, the big question is, where did they come from? “The short answer is we don’t know,” said Glein. “Everyone is enchanted by the possibility that this could be related to some form of extraterrestrial life. But from exploring deep carbon on Earth and exploring outer space, we’ve come to appreciate that there are many ways to make organic molecules independent of life.”
It is possible that these organic molecules come from life forms. Alternatively, high-temperature and high-pressure chemical reactions occurring in the rocky core may have formed them. Or they might even have originated in the primordial building blocks that coalesced to form the moon.
“There are so many questions that we’re dying to try to answer,” said Glein. “I think this could be the tip of the ‘organic iceberg,’ because our instruments did not have the capability to detect other organics such as amino acids.”
Glein and his colleagues have discussed the possibility of sending a spacecraft to Enceladus in the future, armed with more sophisticated mass spectrometers that could discern the full range of individual compounds. Such an investigation could even determine isotopic ratios, which would provide a powerful means to probe sources of the organic molecules. Or, a lander could visit the moon and scoop up a sample from its snowy surface to scan for potential biomolecules like proteins.
Unfortunately, a new mission to Enceladus is yet to be approved. “It’s a shame, because Enceladus is such a scientifically compelling target and it provides a great opportunity to understand organic chemistry in an ocean beyond Earth’s. This might give us some new clues about the origin of life and the potential for life elsewhere in the Universe.”
Main image: This artist's rendering shows Cassini as the spacecraft makes one of its final five dives through Saturn's upper atmosphere in August and September 2017. Credit: NASA/JPL-Caltech