Cassini spacecraft data suggests hydrothermal activity beneath Enceladus’ surface
As seen in this NASA cutaway image of Saturn's moon Enceladus, the geysers – which have been viewed from orbit – hint at hydrothermal activity below the moon's subsurface ocean. The findings were reported in a recent issue of the science journal, "Nature". Image Credit: NASA / JPL
Scientists working with data from NASA’s Cassini-Solstice mission have discovered evidence of present-day hydrothermal activity beneath the icy surface of Saturn’s moon Enceladus. Hydrothermal activity occurs when seawater penetrates and reacts with a rocky crust and emerges as a heated, mineral-laden solution. While this in a natural occurrence in Earth’s oceans, the new findings are the first clear indications that similar processes may be occurring on the icy moon. The results were recently published in two scientific papers.
“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.”
Cassini arrived in orbit above Saturn on July 1, 2004, and has spent more than a decade exploring the planet and its more than 60 moons. Image Credit: NASA / JPL
The first paper, published in the March 12 issue of the journal Nature, dealt with microscopic grains of rock that Cassini detected in the Saturn system.
Following a four-year investigation that included analysis of the spacecraft’s data, computer simulations and laboratory experiments, the scientists concluded that the grains probably formed when hot water containing dissolved minerals from the moon’s rocky interior traveled upward and came into contact with cooler water. Temperatures of at least 194 degrees Fahrenheit (90 degrees Celsius) would be required for the interactions that produce the tiny rock grains.
“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,” said the paper’s lead author Sean Hsu, a postdoctoral researcher at the University of Colorado at Boulder.
Microscopic rock particles rich in silicon were repeatedly detected by Cassini’s cosmic dust analyzer (CDA) even before the spacecraft entered Saturn’s orbit in 2004. The CDA team determined by a process of elimination that the particles must be grains of silica, which is found on Earth in sand and the mineral quartz. The size of the grains, the largest of which were 6 to 9 nanometers in size, lead the researchers to conclude that a specific process was responsible.
The most common way to form grains of this size on Earth is hydrothermal activity when salty water that is slightly alkaline and supersaturated with silica undergoes a large drop in temperature.
“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.
Streams of water, emitted by geysers on the moon, can be clearly seen in this image. Image Credit: NASA / JPL
The second paper, recently published in Geophysical Research Letters, proposes hydrothermal activity as one of two possible sources of methane in the plume of ice and gas that erupts from the south polar region of Enceladus.The plume was detected by Cassini in 2005. A team of French and American scientists conducted extensive modeling in an effort to explain why methane is surprisingly abundant in the plume.
The team found that at the high pressures expected in the moon’s ocean, icy materials called clathrates could form, imprisoning methane molecules within a crystal structure of water ice. The team’s models indicated that this process is so efficient at depleting methane from the ocean that the scientists still needed an explanation for its abundance in the plume.
The first possibility suggested by the researchers is that hydrothermal processes super-saturate the ocean with methane. This could happen if methane is produced faster than it is converted into clathrates. The other possibility is that methane clathrates from the ocean are dragged into the erupting plumes and release their methane as they rise.
The researchers concluded that while both scenarios are likely occurring to some degree, the presence of nanosilica, as reported in the other paper, favors the hydrothermal scenario.
“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,” said lead author Alexis Bouquet, a graduate student at the University of Texas at San Antonio. Boquet co-wrote the paper with Hunter Waite, leader of the Cassini Ion and Neutral Mass Spectrometer (INMS) at Southwest Research Institute in San Antonio, Texas.
This illustration depicts potential origins of methane found in the plume of gas and ice particles that sprays from Saturn’s moon, Enceladus. Image Credit: Southwest Research Institute
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Jim Sharkey
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.
