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Scientists find evidence of complex organic molecules from Enceladus


Using mass spectrometry data from NASA's Cassini spacecraft, scientists found that large, carbon-rich organic molecules are ejected from cracks in the icy surface of Saturn's moon Enceladus. Southwest Research Institute scientists think chemical reactions between the moon's rocky core and warm water from its subsurface ocean are linked to these complex molecules.

Scientists find evidence of complex organic molecules from Enceladus
Cassini has discovered complex organic molecules erupting from Enceladus into space [Credit: ESA]
"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 SwRI's Dr. Christopher Glein, a space scientist specializing in extraterrestrial chemical oceanography. He is coauthor of a paper in the journal Nature outlining this discovery. "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."

Prior to its deorbit in September of 2017, Cassini sampled the plume of material emerging from the subsurface of Enceladus. The Cosmic Dust Analyzer (CDA) and the SwRI-led Ion and Neutral Mass Spectrometer (INMS) made measurements both within the plume and Saturn's E-ring, which is formed by plume ice grains escaping Enceladus' gravity.

"Even after its end, the Cassini spacecraft continues to teach us about the potential of Enceladus to advance the field of astrobiology in an ocean world," Glein said. "This paper demonstrates the value of teamwork in planetary science. The INMS and CDA teams collaborated to reach a deeper understanding of the organic chemistry of Enceladus' subsurface ocean than would be possible with only one data set."

Scientists find evidence of complex organic molecules from Enceladus
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. Centre: 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 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. Note that the insets
 at centre and right are rotated 180ยบ with respect to the global view at left
[Credit: ESA; F. Postberg et al. 2018]
During Cassini's close flyby of Enceladus on Oct. 28, 2015, INMS detected molecular hydrogen as the spacecraft flew through the plume. Previous flybys provided evidence for a global subsurface ocean residing above a rocky core. Molecular hydrogen in the plume is thought to form by the geochemical interaction between water and rocks in hydrothermal environments.

"Hydrogen provides a source of chemical energy supporting microbes that live in the Earth's oceans near hydrothermal vents," said SwRI's Dr. Hunter Waite, INMS principal investigator who also was a coauthor of the new paper. "Once you have identified a potential food source for microbes, the next question to ask is 'what is the nature of the complex organics in the ocean?' This paper represents the first step in that understanding -- complexity in the organic chemistry beyond our expectations!"

"The paper's findings also have great significance for the next generation of exploration," Glein 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. 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."

Source: Southwest Research Institute [June 27, 2018]

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