The deep oceans on Earth are teeming with life, despite the cold and darkness, thanks to hydrothermal vents which provide needed heat and nutrients in an otherwise rather uncomfortable environment. Now, the first evidence has been found for current hydrothermal activity elsewhere in the Solar System: on the ocean bottom of Saturn’s moon Enceladus.
“These findings add to the possibility that Enceladus, which contains a subsurface ocean and displays remarkable geologic activity, could contain environments suitable for living organisms,” said John Grunsfeld, astronaut and associate administrator of NASA’s Science Mission Directorate in Washington. “The locations in our solar system where extreme environments occur in which life might exist may bring us closer to answering the question: are we alone in the universe.”
The Cassini spacecraft had already found evidence that the tiny icy moon has a subsurface ocean or sea beneath the surface ice crust, which feeds the huge geysers of water vapor erupting through cracks in the surface at the south pole. The new findings suggest that this subsurface body of water is similar to Earth’s oceans in other ways as well, with hot hydrothermal vents on the rocky bottom of the ocean.
Cassini has flown directly through the plumes, sampling them as it did so. The discovery of water vapor, salts, and organics all pointed to a salty ocean similar to those on Earth. The data also implied that the water was in direct contact with the rocky bottom, as on Earth, which meant that hydrothermal vents might be possible, making nutrients available to any putative life forms. The new evidence supports that theory; the presence of heat, organics, nutrients, and water increase further the possibility of life of some kind being able to exist in this alien, yet familiar, environment.
On Earth, seawater infiltrates and reacts with the rocky crust, becoming a heated, mineral-laden solution which is released through the vents. They can be localized oases in the otherwise dark and cold ocean bottom, supporting a wide variety of life.
Scientists were able to make the discovery by studying tiny microscopic grains of rock found in the plumes found by Cassini’s Cosmic Dust Analyzer (CDA) instrument. They determined that the grains, composed of silica, most likely formed when hot water in Enceladus’ interior moves upward, coming into contact with cooler water. Temperatures of at least 194 degrees Fahrenheit (90 degrees Celsius) would be required. Their extremely small size, 6 to 9 nanometers, pointed to a hydrothermal origin. On Earth, the most common way for silica grains of this size to form is by hydrothermal activity under a specific range of conditions: slightly alkaline and salty water which is super-saturated with silica and then undergoes a big drop in temperature.
These findings were just published this week in the first of two papers, in the journal Nature.
As Sean Hsu, lead author of the new paper and postdoctoral researcher at the University of Colorado at Boulder, stated, “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.”
“We methodically searched for alternate explanations for the nanosilica grains, but every new result pointed to a single, most likely origin,” said co-author Frank Postberg, a Cassini CDA team scientist at Heidelberg University in Germany.
The tiny size of the grains also means that they travel from the hydrothermal vents deep below up to the near-surface, about 30 miles (50 kilometers), fairly quickly; otherwise, they would be much larger.
The vents may be widespread on Enceladus’ ocean bottom, since other gravity measurements by Cassini indicate that the moon’s rocky core is very porous, which would allow ocean water to percolate into the interior, a relatively huge surface area.
The team’s experiments were also validated by another Japanese team of researchers at the University of Tokyo, led by Yasuhito Sekine, who worked closely with the American researchers.
Hydrothermal vents would also help explain another anomaly: methane detected in the water vapor plumes. These results were recently published in the second paper, in the journal Geophysical Research Letters. Studies done by the team found that icy clathrates should form due to high pressures in the ocean; these clathrates might trap methane molecules. But the studies also showed that this process would be very efficient at depleting the ocean of methane. So why is methane still so abundant in the plumes?
Two scenarios have been suggested. In one, the ocean is super-saturated with methane, which could mean the methane is being produced faster than it is converted into clathrates. In the other scenario, the methane clathrates are actually dragged along into the plumes, releasing methane as they rise to the surface. The effect would be similar to bubbles rising in a bottle of opened champagne.
According to lead author Alexis Bouquet, a graduate student at the University of Texas at San Antonio, “We didn’t expect that our study of clathrates in the Enceladus ocean would lead us to the idea that methane is actively being produced by hydrothermal processes.”
In these models, the methane is assumed to be formed abiotically from olivine and water rather than from life. It might be premature though to say that none of the methane could be produced by organisms living in this alien ocean; we simply don’t know.
Ever since the initial discovery of the water geysers by Cassini and the evidence for a subsurface ocean, Enceladus has become a primary astrobiological target in the search for life elsewhere in the Solar System. Along with Europa, it tops the list of those places where future missions can do just that, which are being planned now.
Carolyn Porco, a planetary scientist with the Cassini mission, and other researchers have just submitted a proposal to NASA to fly an Enceladus Life Finder mission, a spacecraft that would hunt for chemical signatures of life in the Enceladus plumes.
All of the ingredients for life (at least as we know it here on Earth) are present on Enceladus. The question is: Did life ever take hold there as well? Cassini itself can’t answer that, but such a subsequent mission could fly through the plumes again, looking for direct evidence of biological activity. What an exciting possibility …