Evidence for potential life-harbouring hydrothermal activity on Enceladus
Sen—By analysing microscopic grains of rock spewed out into space, NASA's Cassini mission has found strong evidence for hydrothermal activity on Saturn's moon Enceladus. It suggests the moon contains pockets of warm salt-water beneath its icy surface, an environment in which life might develop.
Cassini's Cosmic Dust Analyzer (CDA) has been detecting minute, silicon-rich particles of rock for over 10 years. This week, a paper has been published in the journal Nature revealing the results of an in-depth, four-year analysis of data from CDA, combined with simulations and experiments back on Earth.
The team have deduced that the particles are consistently-sized grains of silica, hinting to the team that Enceladus must be harbouring the means for their creation—something that requires a very specific process.
"They [the research team] knew from the characteristics of the orbits of the particles that they come from the E ring [one of Saturn's many rings], which is supplied by the geysers of Enceladus," Professor Carolyn Porco, leader of the Cassini Imaging team, told Sen. "Hydrothermal activity is possible wherever you have water in contact with certain common kinds of rock-forming minerals."
Cassini first detected active geological processes at the south polar region of Enceladus back in 2005 in the form of plumes of water ice and vapour, salts and organic materials. It was not until 2014 that results were published that strongly suggested the presence of a 10 km (six mile) deep ocean beneath a 30 to 40 km (19-25 miles) thick shell of ice.
"We were all delighted in what we had found," says Porco. "I had stated in a pre-orbit paper that Enceladus would be the Europa of the Saturn system—so we weren't completely surprised! But it was the magnitude and coherence of the phenomenon that was stunning: A whole geological province crossed by geysering fractures. It's a glorious discovery that we had stumbled upon. It made a big impact, and personally, I changed the focus of my research because of it!"
This mosaic is one of the highest resolution views acquired by Cassini of the southern hemisphere of Enceladus, showing the plumes in all their glory. Image credit: NASA/JPL-Caltech/SSI
"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on—and beneath—the ocean floor of an icy moon," the paper's lead author Sean Hsu, a postdoctoral researcher at the University of Colorado, said in a statement.
"We know the jets we see are geysers, which is to say erupting water, liquid and vapour," continues Porco. “So at the moment this is the only place, other than Earth, in the Solar System that has geysers. In fact, Enceladus has 10 per cent of all known geysers, and Yellowstone National Park [USA] has about 50 per cent.”
Artist's illustration showing a cross-section of the ice shell underlying Enceladus' south polar terrain, representing current knowledge of the physical and thermal structure and processes ongoing below and at the surface. Image credit: NASA/Ron Miller
On Earth, the most common way to form silica grains of this size is though hydrothermal activity; specifically when salt water, that is slightly alkaline and saturated with silica, experiences a large drop in temperature. On Enceladus, it is believed that mineral-rich hot water, of minimum 90° C in temperature (194° F), has been forced upwards from the moon's interior to come into contact with relatively colder water on the sea floor to create the necessary temperature drop and form the grains.
Data acquired by Cassini to measure the gravity of the moon suggest that the rocky core of Enceladus is fairly porous, which would allow water from the ocean to seep into the interior and provide a huge surface area where the two could interact.
A second paper published in Geophysical Research Letters suggests hydrothermal activity (backed up by the detection of the silica particles) could be one of two explanations for the presence of an abundance of methane in the geysers of Enceladus. Porco believes that the detection of the gas holds significance.
"First it indicates the presence of biologically important elements. Second, and this is completely speculative, it happens to be the a metabolic product of certain kinds of organisms on Earth. So it carries a 'what if?' fascination."
This illustration depicts potential origins of methane found in the plume of gas and ice particles that sprays from Enceladus, based on research by scientists working with the Cassini Ion and Neutral Mass Spectrometer. Image Credit: Southwest Research Institute
The team found that icy materials called "clathrates" could form under the high pressures in the moon's ocean that then trap the methane within a crystal lattice, similar to the structure of water ice. However, models indicate that this process is very efficient at removing methane molecules from the ocean and it therefore cannot not account for the abundance seen in the plume.
One hypothesis to counteract this is that hydrothermal processes saturate the ocean with methane, which could happen if methane is produced faster than it can be converted into clathrates.
A second hypothesis is that the methane-infused clathrates are dragged through the ocean, releasing the molecules as they rise in the erupting plume—in the same way that bubbles of champagne move when the cork from bottle pops.
With two exciting new results published in a short space of time, are we one step closer to discovering life under the surface? "These are very early days," said Porco. "All we can say is that we have on Enceladus the most promising, accessible environment for life that we know of in our Solar System. So, I say 'What are we waiting for? Let's go back!'"