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Astrochemistry

Ocean geysers on Enceladus harbor large organic molecules

Moon is a good candidate to host extraterrestrial life, but this find isn’t proof that it does

by Sam Lemonick
June 27, 2018 | APPEARED IN VOLUME 96, ISSUE 27

 

09627-scicon4-jets.jpg
Credit: NASA
Enceladus's geysers contain large organic molecules theorized to sit atop the moon's subsurface ocean.

Saturn’s moon Enceladus hides a water ocean beneath its icy crust, and it’s one of the likeliest candidates to harbor life beyond our own planet. Scientists have now identified large, complex organic molecules in geysers originating from that ocean, though they’ve stopped short of calling them signs of life (Nature 2018, DOI: 10.1038/s41586-018-0246-4).

When the Cassini spacecraft passed within 50 km of Enceladus’s surface in 2008, it flew through the plumes of those geysers. Early Cassini data and observations from ground-based telescopes identified simple organic molecules including methane and formaldehyde. Now, Frank Postberg and Nozair Khawaja of Heidelberg University in Germany and colleagues analyzed data collected by Cassini’s two mass spectrometers during that pass and others and its encounters in one of Saturn’s rings with ice grains known to originate from Enceladus.

The spacecraft’s Cosmic Dust Analyzer recorded spectra of cations up to 8,000 atomic mass units but its resolution at those masses is too low to characterize the molecules, says Khawaja. Similar-size molecules have been spotted on Saturn’s largest moon, Titan.

By adding in data from a second spectrometer, the group identified molecular fragments with masses below 200 atomic mass units. They report evidence of aliphatic unsaturated carbon chains; hydroxy and ethoxy functional groups; carbonyls; nitrogen-containing moieties; and aromatic rings adjacent to unsaturated carbons. Khawaja says the identity of the parent molecules of the fragments remains an open question. Some of the peaks related to aromatic rings are not consistent with polycyclic aromatic hydrocarbons, which are common in interstellar space. Other possibilities include polymers of aromatic rings and aliphatic sections, or aromatic rings with cross-linking aliphatic chains.

Based on the salt content of the ice analyzed alongside the organics, the researchers say the molecules would not be dissolved in the ocean. They instead propose that these molecules float in films on top of the ocean. Khawaja explains that bubbles from undersea vents could aerosolize organic compounds as they burst at the water’s surface. Ice grains can then nucleate around these molecules and eject into space. Similar processes happen in Earth’s ocean.

The tantalizing possibility is that these molecules are one more clue that Enceladus supports life. “Scientists would like to see an extraterrestrial object where we have an energy source, where we have liquid water, where we have organic molecules,” says Khawaja. “Enceladus fulfills all these conditions.” He says researchers don’t have evidence of life there, but they can’t rule out a biotic origin for these organic molecules.

Others agree. This is not a sign of life, says Dave Clements of Imperial College London, who has identified organic molecules in Enceladus’s jets from Earth-based telescopes. “But it is a sign that life, or some biochemical process that could lead to life, might be there,” he says.

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