Complex organics bubble from the depths of ocean-world Enceladus

Credit: European Space Agency

Data from the international Cassini spacecraft have revealed complex organic molecules originating from Saturn’s icy moon Enceladus, strengthening the idea that this ocean-world hosts conditions suitable for life.

Very little was known about Enceladus prior to 2005 – the year when Cassini first flew close. Since then, it has become a continuous source of surprises, with secrets still being revealed even now, after the mission end.

During the spacecraft’s incredible career, scientists discovered that 500 km–diameter Enceladus has a massive sub-surface  hidden underneath a thick 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 .

They detected mighty geysers releasing a mixture of water vapour and  from the oceans into space through cracks – nicknamed ‘tiger stripes’ – in the moon’s icy shell, providing material for one of Saturn’s rings.

Now, a team led by Frank Postberg and Nozair Khawaja of the University of Heidelberg, Germany, has identified fragments of large organic molecules in these ejected ice grains. The results are published today in Nature.

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

“We found large molecular fragments that show structures typical for very complex organic molecules,” adds Nozair. “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 fragments, of up to 200 units of molecular mass, are created as the ice grains hit the dust-analysing instrument on Cassini at speeds of about 30 000 kilometres per 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.

Scientists calculate molecular mass, or weight, as the sum of weights of individual atoms contained in the molecule. Previously, Cassini had only detected lightweight organic molecules at Enceladus that were much smaller than the most recently found fragments.

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  can arise,” says Frank.

Recent simulations show that enough heat to power  for tens of millions of years could be generated through tidal friction if the moon has a porous core with ocean water percolating through it.

Following up on this scenario, the  is injected into the ocean by hydrothermal vents on the floor of Enceladus’ ocean – something akin to the hydrothermal sites found at the bottom of the oceans on Earth, which are one of the possible environments that scientists investigate for the emergence of life on our own planet.

In Earth’s oceans, organic substances from deeper waters can accumulate efficiently on the walls of rising air bubbles, transporting them to the surface where they are dispersed along with sea spray when the bubble bursts.

Scientists think a similar process could be happening on Enceladus. Bubbles of gas, rising through tens of kilometres of ocean, could bring up organic material from depth where they form a thin film floating on the ocean surface beneath the icy shell.

When the bubbles burst at the surface it helps disperse some of the organics, along with spray of salty ocean water. Tiny droplets of the dispersed organic material become ice-coated when water vapour freezes on their surface, and along with the frozen spray of salty ocean water, are ejected in the plumes and then detected by Cassini.

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