Scientists with NASA’s Cassini mission, led by LASP and University of Colorado postdoctoral researcher, Sean Hsu, have found that microscopic grains of rock detected near Saturn imply hydrothermal activity is taking place within the moon Enceladus.
This is the first clear indication of an icy moon having hydrothermal activity—in which seawater infiltrates and reacts with a rocky crust, emerging as a heated, mineral-laden solution. The finding adds to the tantalizing possibility that Enceladus, which displays remarkable geologic activity including geysers, could contain environments suitable for living organisms.
The results were published today in the journal Nature.
The surprising new result follows an extensive, four-year analysis of data from the spacecraft, computer simulations and laboratory experiments. From their close examination, the researchers determined the tiny grains most likely form when hot water containing dissolved minerals from the moon’s rocky interior travels upward, coming into contact with cooler water.
Temperatures required for the interactions that produce the tiny rock grains would be at least 194 degrees Fahrenheit (90 degrees Celsius).
“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 Hsu, who served as lead author on the paper.
Cassini’s cosmic dust analyzer (CDA) instrument repeatedly detected minuscule rock particles, rich in the element silicon, orbiting Saturn at the same distance as Enceladus. By process of elimination, the CDA team concluded these particles must be grains of silica, which is found on Earth in sand and the mineral quartz. The consistent size of the grains observed by Cassini—the largest of which were 6 to 9 nanometers—was the clue that told the researchers a specific process likely was responsible.
On Earth, the most common way to form silica grains of this size is hydrothermal activity involving a specific range of conditions; namely, when slightly alkaline water with modest salinity that is super-saturated with silica undergoes a big 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 Frank Postberg, a Cassini CDA team scientist at Heidelberg University in Germany, and a co-author on the paper.
Hsu and Postberg worked closely with colleagues at the University of Tokyo, who performed the detailed laboratory experiments that validated the hydrothermal activity hypothesis. The Japanese team, led by Yasuhito Sekine, verified the conditions under which silica grains form at the same size Cassini detected. The researchers think these conditions may exist at the seafloor on Enceladus, where hot water from the interior meets the relatively cold water at the ocean bottom.
The extremely small size of the silica particles also suggests that they travel upward relatively quickly from their hydrothermal origin to the near-surface sources of the moon’s now famous geysers. From seafloor to outer space, a distance of about 30 miles (50 kilometers), the grains spend a few months to a few years in transit, otherwise they would grow to much larger sizes.
Cassini first revealed active geology on Enceladus in 2005 with evidence of an icy spray issuing from the moon’s south polar region and higher than expected temperatures in the icy surface there. The mission soon revealed a towering plume of water ice and vapor, salts and organic materials that issues from relatively warm fractures on the wrinkled surface. Gravity science results published in 2014 strongly suggest the presence of a 6-mile- (10-kilometer-) deep ocean beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick.
The authors point out that Cassini’s gravity measurements suggest Enceladus’ rocky core is quite porous, which would allow water from the ocean to percolate into the interior. This would provide a huge surface area where rock and water could interact.
“It’s possible much of this interesting hot-water chemistry occurs deep inside the moon’s core,” Hsu said.
Co-authors of the Nature paper include CU-Boulder Professor Mihaly Horanyi and Assistant Professor Sascha Kempf of LASP. Both also are faculty members in CU-Boulder’s physics department and co-investigators of the Cassini CDA. The online version of the publication can be found here: http://dx.doi.org/10.1038/nature14262.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate in Washington. The Cassini cosmic dust analyzer was provided by the German Aerospace Center; the instrument team is based at the University of Stuttgart in Germany.