Scientists aren't comfortable yet saying that organics on Saturn's icy moon arose from life, but they have an idea of what to look for next.
Enceladus, one of Saturn's many icy moons, has become famous for the plumes of water-ice crystals seen jetting into space by NASA's Cassini spacecraft. Results from two mass spectrometers aboard the spacecraft found much more than pure ice in the plumes: They're salty and laced with hydrogen gas and organic compounds like methane (CH4).
Now a closer analysis of the spectrometers' data reveals that the organic brew inside Enceladus must contain organic "parent molecules" much more massive and complex than realized. This discovery, published last week in Nature, bolsters scientists' hopes that life is present there and in other locations of the solar system. But stronger evidence is needed to make the case.
Organic molecules are chains or rings of atoms that include the elemental building blocks of life: carbon, hydrogen, oxygen, and nitrogen. The new Cassini findings show that the organics spewing from Enceladus's interior are more than simple C-H chains. Instead, they're complex arrangements that must contain at least some oxygen and nitrogen.
"This new finding completely changes our perspective on the organic chemistry of Enceladus. We've gone from simple to complex. That's exciting," says co-author Christopher Glein (Southwest Research Institute) in an email interview with Sky & Telescope.
"Previously, we detected organic compounds all with masses less than 100 atomic mass units," Glein explains, "and they generally contained fewer than several carbon atoms per molecule. That's chiefly because our instrument, the Ion and Neutral Mass Spectrometer, only [measured] up to 100 amu." (A carbon atom with six protons and six neutrons has an atomic mass of 12 amu.) For this new study, he continues, results from Cassini's Cosmic Dust Analyzer (CDA) reveal that the organic complexity of Enceladus extends up to hundreds and even thousands of atomic mass units.
The CDA researchers, led by Frank Postberg (Heidelberg University, Germany) detected high-mass assemblages containing a likely ratio of one hydrogen atom for every two carbons. This implies the structure is "understaturated" with hydrogen and rich in multiple-bonded carbon atoms, Glein says.
Meanwhile, the INMS team then found variants of the ring-shaped hydrocarbon benzene (C6H6) — but only during high-speed flybys of Enceladus, when Cassini whizzed above the surface at 14 km/s (roughly 31,000 mph).
"This was initially a puzzle," Glein notes, "but it made perfect sense after the high-mass organic cations were discovered." Apparently, as-yet-unidentified macromolecules break apart when they collide with the INMS. Because those organics contain aromatic (ring-shaped) structures, he explains, benzene is released during the fragmentation process."
The team considered but discarded alternate explanations for the organics found, such as Cassini becoming contaminated from flying close by Titan, another potentially life-friendly moon. However, the CDA recorded organics at Enceladus before Cassini made its first flyby of Titan, and contamination alone can't account for all the organic abundances found in the INMS data.
The organic compounds recently found on Mars by NASA's Curiosity rover appear to be richer in sulfur than those at Enceladus. Glein says this could be because more sulfur is available on the Martian surface, whereas most sulfur on Enceladus would be locked up in soluble sulfide materials such as pyrite (FeS2). The Martian organics are likely more ancient as well, which in turn has implications for the timelines for the evolution of life in our solar system.
There are also similarities and contrasts to Mercury, where researchers identified aromatic carbon (likely in the form of graphite) a few years ago. The organics from Enceladus are also rich in aromatic carbon, but they also contain hydrogen, oxygen, and nitrogen — likely due to the water (H2O) and ammonia (NH3) present in the moon's subsurface ocean.
Comets also contain lots of organic material, but Glein says those small bodies exhibit a greater diversity and abundance of prebiotic compounds. "If Enceladus formed from comets, it's interesting to consider what evolutionary processes might relate them," he says.
One possibility, he suggests, is that hydrothermal processes inside Enceladus drastically altered the primordial inventory, so only the most resilient molecules might remain. Or perhaps life arose in the ocean of Enceladus and simply overprinted any organic signatures of its cometary heritage. A group led by Ruth-Sophie Taubner (University of Vienna) recently identified a strain of bacteria that can survive in Enceladus-like conditions by feasting on methane.
But Glein cautions that the default (null) hypothesis is Enceladus lacks any kind of life. With Cassini gone and no firm plans to send another spacecraft to Saturn's system, planetary scientists might wait decades to find out whether Enceladus harbors its own biology.
One good indicator would be finding organic materials with a low ratio of carbon-13 to carbon-12, compared to the moon-wide average, because organisms tend to incorporate the lighter isotope when making biomass. Another biosignature would be finding organic compounds or chemical groups at Enceladus that are thermodynamically and kinetically unstable — an indication that they're being synthesized by some biological process.
Jennifer Eigenbrode (Goddard Space Flight Center), who led the team that identified preserved organic molecules on Mars, said other life-friendly indicators would be finding hydrocarbon chains such as amino acids and lipids.
If biochemical fingerprints are indeed found at Enceladus, then arguably life had an independent origin there than it did on Earth. "That makes everything kind of stop in awe," Eigenbrode says. But she too cautions that researchers will need to find multiple instances of life-friendly organics to even begin to make this case. After all, even in the most extreme environments of Earth, life occurs in clusters - never in isolation.