In 2005, NASA's Cassini spacecraft made a discovery. It found towering geysers of water erupting from fractures called "tiger stripes" near Enceladus's south pole. This water comes from a vast ocean hidden beneath the moon's icy crust, kept liquid by the powerful gravitational forces from Saturn that constantly squeeze and stretch the interior of Enceladus.
When Cassini flew through these plumes, it detected not just water and salts, but also various organic compounds. Since organic molecules dissolved in liquid water, the discovery suggested perhaps this was the precursor chemicals that lead to life. Here was evidence of chemistry happening in an alien ocean, exactly what you'd want to see when hunting for life.
Artist impression of the Cassini spacecraft at Saturn (Credit : NASA/JPL)
At least, that is what we thought. A recent study led by Dr. Grace Richards from Italy's National Institute for Astrophysics has rather thrown a spanner in the works. The results of their study suggest these organic molecules might not be coming from the ocean at all. Instead, they could be forming right on Enceladus's surface through a much more mundane process. The team suggest that Saturn's powerful magnetic field traps high energy particles that constantly bombard the icy surface and in the process, create organic molecules.
To test their hypothesis, they recreated Enceladus’ conditions in the laboratory, mixing up ice containing water, carbon dioxide, methane, and ammonia and cooling it to minus 200 degrees Celsius. Then they bombarded this artificial Enceladus ice with high energy particles, mimicking the radiation environment around Saturn.
Plumes of water vapour erupt from Saturn’s moon Enceladus, captured by the Cassini spacecraft (Credit : NASA/JPL-Caltech/Kevin M Gill)
The results were striking. The radiation created a whole mix of molecules, including carbon monoxide, cyanate, and ammonium. Most importantly, it produced molecular precursors to amino acids, the building blocks of proteins that are essential for life as we know it.
This result, whilst showing the organic compounds don’t come from the ocean, still doesn’t rule out that Enceladus's ocean can't support life. But it does mean we can't simply assume that finding organic molecules in the plumes proves the ocean is habitable. Some of these molecules might just be the result of cosmic rays hitting ice, rather than signs of interesting chemistry happening in the hidden ocean below.
"Although this doesn't rule out the possibility that Enceladus's ocean may be habitable, it does mean we need to be cautious in making that assumption just because of the composition of the plumes" - Dr. Grace Richards from Italy's National Institute for Astrophysics
The challenge now is figuring out how to tell ocean derived organics apart from radiation produced ones. That's going to require more sophisticated missions to Enceladus perhaps including the proposed European Space Agency mission currently under consideration for the 2050s.
Source : Study Questions Ocean Origin of Organics in Enceladus’s Plumes
