Could there be life on one of Saturn's moons?
An icy moon quietly circling Saturn in the far reaches of the outer solar system has long fascinated Régis Ferrière, an associate professor in the Department of Ecology and Evolutionary Biology. Named after a giant in Greek mythology, Enceladus spans just over 300 miles from pole to pole. Next to Earth's moon, Enceladus looks like a ping-pong ball dwarfed by a bowling ball. It even has that same eggshell-like hue, thanks to its thick, icy shell crisscrossed by fissures and cracks, some of which are dozens of miles long.
Giant water plumes erupting from Enceladus have inspired much research and speculation about the vast ocean that is believed to be sandwiched between the moon's rocky core and its icy shell. Flying through the plumes and sampling their chemical makeup, NASA's Cassini spacecraft detected a relatively high concentration of certain gas molecules. Some of them, particularly hydrogen and methane, looked familiar to scientists who study microbes eking out a living in some of the most extreme environments our own planet has to offer: hydrothermal vents in lightless depths at the bottom of oceans.
"We wanted to know: Could Earthlike microbes that 'eat' the dihydrogen and produce methane explain the surprisingly large amount of methane detected by Cassini?" said Ferrière, who recently published a paper on possible origins of the excess methane. "Searching for such microbes, known as methanogens, at Enceladus' seafloor would require extremely challenging deep-dive missions that are not in sight for several decades."
The members of Ferrière's team took a different route: They constructed mathematical models to calculate the probability that different processes, including biological methanogenesis, might explain the Cassini data.
Lo Que Pasa spoke with Ferrière about some of the intriguing implications resulting from these studies.
Saturn's moon Enceladus is thought to harbor an ocean underneath its thick ice shell. What is known about that hidden ocean?
The subsurface ocean is liquid salty water. It is believed to be between 12 and 15 miles deep and covered by an ice shell up to 20 miles thick. Initially, it was thought to exist only under the south pole, where the plumes erupt, but the "wobble" of Enceladus measured by the Cassini probe signaled that its shell is completely detached from the rocky core. In other words, it floats on top of an ocean that covers the entire moon.
What did scientists expect to find when they sent the Cassini spacecraft flying through plumes spouting from the surface of the icy moon?
The composition of the plume was not that unexpected, because it is similar to that seen at most comets, and Enceladus' material may be of cometary origin. What was really unexpected was the discovery of molecular hydrogen, because that pointed to hydrothermal systems on Enceladus' seafloor. These hydrothermal systems are the most likely explanation for the hydrogen in the plume, and the chemistry involved is also known to produce methane. That raised three questions. First, how much of the methane in the plume might be explained by this hydrothermal activity? Second, on Earth these hydrothermal systems are home to microbes that use the hydrogen as food and produce methane as a waste product; could it be that Enceladus's deep-sea vents are habitable by these types of microbes? And, finally, if such microbes existed at Enceladus' vents, could they, together with the geological chemistry, explain the levels of hydrogen and methane measured by Cassini?
What do we know about the force driving the plumes spouting through cracks in Enceladus' shell?
This boils down to tidal heating. Enceladus has a porous and permeable rocky core. As a result of Saturn's gravity, rocks in the core get moved around, frictions occur, and that process generates heat. This drives convection of water through the core similar to water circulating through a boiler. Pressure builds up, and water from the ocean is ejected into space through cracks in the ice.
Where do scientists think this methane comes from?
As water moves through the heated rocky core, a chemical reaction known as "serpentinization" occurs. You can actually see this in certain places, with a famous example being Yellowstone National Park. As hot water trickles through rocks rich in iron, it oxidizes the iron and releases hydrogen. That hydrogen can then react with carbon dioxide in the ocean water, and that produces methane. So, serpentinization chemistry is one hypothesis to explain both hydrogen and methane in the plume. As I mentioned earlier, the methane may also be of cometary origin, tracing back to Enceladus formation. Or the methane could result from a process called "pyrolysis," which, in this case would involve the decomposition of organic molecules – of cometary origin – by hot temperature in the core, left over from the formation of Enceladus. But we don't have the data to conclude on the contribution of primordial methane outgassing or pyrolysis to contemporary methane production.
If it turns out the methane is indeed produced by organisms, what would be the most likely scenario?
Our model shows that, given the Cassini data, the hypothesis of "habitability" by Earth-like methanogens is very likely. The scenario where such metabolic activity is actually happening is compatible with the data, and achieves a high score in our statistical analysis, if – and that's a big if – the probability that life emerged on Enceladus in the first place is high. This is a very disputed question, of course. If there were indeed microbes, we think a scenario in which they live in colonies around hydrothermal vents would be likely because that is what we see here on Earth. Or there is a possibility microorganisms may live even deeper, underneath the seafloor.
Enceladus is not the only moon in our solar system believed to have a liquid ocean covered by an icy crust. Europa, one of Jupiter's moons, has a similar makeup. Which of the two do you think is the more likely candidate to potentially harbor microbial life-forms?
Europa is about the size of Earth's moon and similar to Earth in several ways. It has a metallic core, a rocky interior and a massive ocean. In fact, the water in Europa's ocean might amount to twice as much water as in all of the oceans on Earth. One big difference, of course, is Europa's icy surface. Models tell us that the moon is probably as old as the solar system, whereas we don't know with high confidence how old Enceladus is. This may be critical to evaluate potential biosignatures, such as methane. Also, Europa has a diversity of alternate environments that may be favorable to life, or that may contain signs of life more accessible than the seafloor, such as pockets of liquid water within the ice crust. So, all things considered, Europa looks even more interesting as a target to search for life beyond Earth.
Visit NASA's collection of stunning Cassini images.