NASA’s Cassini spacecraft has provided planetary researchers the first evidence that Enceladus – the sixth-largest of Saturn’s moons – exhibits signs of hydrothermal activity which may resemble that seen in the Earth’s oceans.
This color view of Enceladus was taken by Cassini spacecraft on 31 January 2011, from a distance of 50,330 miles (NASA / JPL-Caltech / SSI / G. Ugarković)
The Cosmic Dust Analyzer (CDA) instrument on Cassini has repeatedly detected miniscule rock particles, rich in the element silicon, near Saturn since 2004. By process of elimination, the CDA team concluded these particles must be grains of silica.
The consistent size of the grains (up to 6-9 nm in diameter) was the clue that told scientists a specific process likely was responsible.
In a 4-year analysis of Cassini data, computer simulations and lab experiments, Dr Hsiang-Wen Hsu of the University of Colorado, Boulder, and his colleagues found that the tiny grains most likely form when hot water containing dissolved minerals from the Enceladus’ 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 Dr Hsu, the first author of the paper published in the journal Nature.
“We methodically searched for alternate explanations for the nano-silica grains, but every new result pointed to a single, most likely origin,” added co-author Dr Frank Postberg of Heidelberg University in Germany.
Dr Sascha Kempf of the University of Colorado, Boulder, who is a co-author on the study, said: “the puzzle of the Enceladus plumes – first identified not long after the Cassini spacecraft reached the realm of Saturn in 2004 – has been solved, at least to some extent.”
“Ten years ago it was a big mystery why the nano-grains were made of silica rather than water ice,” he said.
“Now we know the observations were correct. We know where the silica particles are coming from, and why we are seeing them. We learned something very unexpected, which is why I really like this study.”
In addition, another study suggests hydrothermal activity as one of two likely sources of methane in the plume of gas and ice particles that erupts from the south polar region of Enceladus.
In the study, Alexis Bouquet of the University of Texas at San Antonio and his colleagues from France and the United States found that, at the high pressures expected in the Enceladus’ ocean, icy materials called clathrates could form that imprison methane molecules within a crystal structure of water ice.
Their findings, published in the journal Geophysical Research Letters, indicate that this process is so efficient at depleting the ocean of methane that the team still needed an explanation for its abundance in the plume.
In one scenario, hydrothermal processes super-saturate the ocean with methane. This could occur if methane is produced faster than it is converted into clathrates.
A second possibility is that methane clathrates from the ocean are dragged along into the erupting plumes and release their methane as they rise, like bubbles forming in a popped bottle of champagne.
“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,” Alexis Bouquet said.
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Hsiang-Wen Hsu et al. 2015. Ongoing hydrothermal activities within Enceladus. Nature 519, 207–210; doi: 10.1038/nature14262
Alexis Bouquet et al. Possible evidence for a methane source in Enceladus’ ocean. Geophysical Research Letters, published online March 11, 2015; doi: 10.1002/2014GL063013
