Cassini Scientists Find Complex Organic Molecules in Geyser-Like Plumes of Enceladus

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Using data from NASA’s Cassini spacecraft, a research team led by University of Heidelberg scientists Frank Postberg and Nozair Khawaja found that complex, carbon-rich molecules with molecular masses above 200 atomic mass units are ejected from cracks in the surface of Saturn’s icy moon Enceladus.

This artist’s painting of the south polar region of Enceladus shows massive jets of water ice being blasted into space. Image credit: Karl Kofoed / NASA.

This artist’s painting of the south polar region of Enceladus shows massive jets of water ice being blasted into space. Image credit: Karl Kofoed / NASA.

Very little was known about Enceladus prior to 2005 — the year when NASA’s Cassini orbiter first flew close. Since then, it has become a continuous source of surprises.

During Cassini’s incredible career, planetary researchers discovered that Enceladus has a sub-surface ocean hidden underneath an icy crust, with evidence pointing to powerful hydrothermal vents on the seabed that mix up material from the moon’s water-filled, porous core with the ocean water.

Cassini also detected geysers releasing a mixture of water vapor and ice grains from the ocean into space through cracks — known as ‘tiger stripes’ — in the moon’s icy shell, providing material for Saturn’s E-ring.

Now, the scientists have identified fragments of complex organic molecules in these ejected ice grains.

“It is the first ever detection of complex organics coming from an extraterrestrial water-world,” Dr. Postberg said.

“We found large molecular fragments that show structures typical for very complex organic molecules,” Dr. Khawaja added.

“These huge molecules contain a complex network often built from hundreds of atoms of carbon, hydrogen, oxygen and likely nitrogen that form ring-shaped and chain-like substructures.”

The detection was made using data from two instruments aboard Cassini: the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Mass Spectrometer (INMS).

“We are, yet again, blown away by Enceladus. Previously we’d only identified the simplest organic molecules containing a few carbon atoms, but even that was very intriguing,” said team member Dr. Christopher Glein, from the Southwest Research Institute.

“Now we’ve found organic molecules with masses above 200 atomic mass units. That’s over ten times heavier than methane. With complex organic molecules emanating from its liquid water ocean, this moon is the only body besides Earth known to simultaneously satisfy all of the basic requirements for life as we know it.”

Left: the interior of Saturn’s moon Enceladus. The graphic shows the icy crust, which is thinner in the polar regions, below which sits an ocean. The moon has a porous rocky core. Water percolating into the core is warmed by contact with rock in the tidally heated interior. The heated water enters the ocean at hydrothermal vents located beneath the poles. Complex organics and rocky particles are entrained in the hydrothermal flow. Gas bubbles rising through the ocean collect organic material at their surface and transport them upward to the ice shell. Center: the oceanic water table lies inside cracks in the south polar ice crust. Bubbles of gas help bring organic material to the ocean surface, where it creates a thin film in the icy vents. Right: when the bubbles burst at the surface they disperse some of the organics, along with a spray of salty ocean water. Droplets of the dispersed organic material become ice-coated when water vapor freezes on their surface, and along with the frozen spray of salty ocean water, are ejected in the plumes and then detected by Cassini. Image credit: ESA / F. Postberg et al.

Left: the interior of Saturn’s moon Enceladus. The graphic shows the icy crust, which is thinner in the polar regions, below which sits an ocean. The moon has a porous rocky core. Water percolating into the core is warmed by contact with rock in the tidally heated interior. The heated water enters the ocean at hydrothermal vents located beneath the poles. Complex organics and rocky particles are entrained in the hydrothermal flow. Gas bubbles rising through the ocean collect organic material at their surface and transport them upward to the ice shell. Center: the oceanic water table lies inside cracks in the south polar ice crust. Bubbles of gas help bring organic material to the ocean surface, where it creates a thin film in the icy vents. Right: when the bubbles burst at the surface they disperse some of the organics, along with a spray of salty ocean water. Droplets of the dispersed organic material become ice-coated when water vapor freezes on their surface, and along with the frozen spray of salty ocean water, are ejected in the plumes and then detected by Cassini. Image credit: ESA / F. Postberg et al.

The molecular fragments are created as the ice grains hit Cassini’s CDA instrument at speeds of about 19,000 mph (30,000 km/hour), but the researchers believe that, prior to the collision, the grains contain the original, even larger molecules, which could have molecular weights of thousands of atomic mass units.

Such large molecules can only be created by complex chemical processes — including those related to life.

Alternatively, they could come from primordial material as found in some meteorites or, more likely, be generated by hydrothermal activity.

“In my opinion the fragments we found are of hydrothermal origin, having been processed inside the hydrothermally active core of Enceladus: in the high pressures and warm temperatures we expect there, it is possible that complex organic molecules can arise,” Dr. Postberg said.

With Cassini data alone, however, it is not possible to confirm the exact origin of the newly-found organics from which the observed fragments derive, as the size of the fragments is at the maximum limit that could be detected by the instruments.

“If we could visit Enceladus again, we would take instruments that can see the entire molecules, not just these fragments, and that would tell us exactly what they are and how they have been created,” Dr. Postberg said.

“It seems that this mysterious moon will keep this secret for some time, but it is in the reach of a future mission to Enceladus to solve this part of the puzzle,” Dr. Khawaja said.

“A future spacecraft could fly through the plume of Enceladus, and analyze those complex organic molecules using a high-resolution mass spectrometer to help us determine how they were made,” Dr. Glein said.

“We must be cautious, but it is exciting to ponder that this finding indicates that the biological synthesis of organic molecules on Enceladus is possible.”

The findings were published in the June 28, 2018 issue of the journal Nature.

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Frank Postberg et al. 2018. Macromolecular organic compounds from the depths of Enceladus. Nature 558: 564-568; doi: 10.1038/s41586-018-0246-4