Last updated June 29, 2018 at 3:51 pm
Complex organic molecules bubble up from the depths of Saturn’s icy ocean moon.
Large organic molecules have been discovered in the plumes spewing from Enceladus, seen in this photo from Cassini. Credit: NASA/JPL-Caltech/Space Science Institute
Complex organic molecules have been discovered for the first time originating from Saturn’s icy moon Enceladus, according to a new study.
The finding strengthens the idea that this ocean-world hosts conditions suitable for life.
The finding was based on data from the Cassini mission, which studied Saturn and its moons in exquisite detail.
While the presence of the molecules isn’t evidence that life has evolved on Enceladus, future NASA- and ESA-led missions could explore what this finding means for the chances of life on other similar icy moons.
The fractured, cratered surface of Enceladus. Credit: NASA/JPL-Caltech/Space Science Institute
Exploring products from the depths
Enceladus was, until quite recently, a mystery to scientists. However that changed in 2005 when Cassini first flew near the 500km diameter moon, revealing a far more complex world than was ever expected – a world which is still revealing surprises after the mission’s end.
One of the stunning findings was a massive sub-surface ocean hidden underneath a thick icy crust. Through cracks in that crust, mighty geysers spewed water and ice grains into space, ejected by the force of powerful hydrothermal vents on the seabed that mix up material from the moon’s water-filled, porous core with the ocean water.
Now, a team from the University of Heidelberg in Germany, has identified fragments of large organic molecules, some up to 15 carbon atoms in size, in those ejected ice grains.
“It is the first-ever detection of complex organics coming from an extra-terrestrial water-world,” said lead author Frank Postberg.
The researchers studied data from Cassini as it passed through the spray from the geysers, and compared it with data from Cassini’s passes through Saturn’s E ring, which is made up of ice ejected from Enceladus.
In both locations they found evidence of large, complex organic molecules.
The origins of complex molecules
The fragments detected were created as the ice grains hit the Cosmic Dust Analyzer (CDA) on Cassini, which measured impacts of molecules against a sensor to determine their mass, at speeds of about 30,000 kilometres per hour.
Some of the impacts registered mass of 200 atomic units, near the limit of the sensor’s range. However, the researchers believe that, prior to the collision, the grains could have originally contained even larger molecules.
Molecules the size of those discovered can only be created by complex chemical processes, say the researchers. While they could include processes related to the development of life or primordial material similar to that found in some meteorites, the most likely explanation is hydrothermal activity.
The scientists believe the molecules were cooked up deep within Enceladus’ hot, rocky core under high pressure, which recent simulations suggest could have ocean water percolating through it.
The molecules are then released into the ocean through hydrothermal vents on the floor of Enceladus’s ocean similar to the hydrothermal sites found at the bottom of the oceans on Earth.
It is expected that, in a process also similar to Earth, bubbles of gas transport the organic material through tens of kilometres of ocean to the surface, where they are ejected along with the frozen spray of salty ocean water in the gigantic plumes detected by Cassini.
Hydrothermal activity in Enceladus’ core and the rise of organic-rich bubbles. Credit: ESA, F Postberg et al (2018)
Complex molecules are not evidence of life
This paper is the most recent in a long series of discoveries that have been revealing Enceladus as a potential life-bearing world.
However, the Cassini findings are not enough 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.
Nor is it evidence that life has evolved on the planet, merely that chemicals that are useful for life exist there.
As well as the favoured hydrothermal explanation, there is also the possibility the molecules are created off planet in a sunlight-driven process.
However, as the highest concentrations of the organic fragments were found in Saturn’s E-ring closest to Enceladus and in the plumes from the moon itself, with less in the outer E-ring further from the moon, signs do point to a planetary-based process.
“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,” said Postberg.
Cassini’s last view of Enceladus and its plumes. Credit: NASA/JPL-Caltech/Space Science Institute
With Cassini having perished last year when intentionally crashed into Saturn at the end of its life, we’ll have to wait until the next mission to find out the answer to those burning questions.
Related: Goodbye Cassini
While we wait for the next mission to Saturn, the Cassini revelations will also have important implications for upcoming missions to the Jupiter system by NASA’s Europa Clipper and ESA’s JUpiter ICy moons Explorer, Juice, which are planned to launch in 2022.
Similar to Saturn, Jupiter has a complex system of moons, with three of the largest — Europa, Ganymede, and Callisto – all thought to have an underground ocean.
“Thanks to the Cassini experience, we will know what to look for and how to study it in the Jupiter system,” said Nicolas Altobelli, who was a project scientist on Cassini and is now responsible for the development of the ESA’s scientific program aboard Juice.
The research has been published in Nature.
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