Two new studies using data from NASA’s Cassini spacecraft reveal the pH of water spewing from a geyser-like plume on Enceladus, and suggest that much of the eruption activity on its surface could be in the form of diffuse curtains, rather than discrete jets.
This color view of Enceladus was taken by Cassini spacecraft on 31 January 2011, from a distance of 50,330 miles. Image credit: NASA / JPL-Caltech / SSI / G. Ugarković.
According to the first study, published in the journal Nature, features that appear to be individual jets of material erupting along the length of prominent fractures in the south polar region of Enceladus might be phantoms created by an optical illusion.
“We think most of the observed activity represents curtain eruptions from the tiger stripe fractures, rather than intermittent geysers along them. Some prominent jets likely are what they appear to be, but most of the activity seen in the images can be explained without discrete jets,” said Dr Joseph Spitale of the Planetary Science Institute in Tucson, Arizona.
Dr Spitale and co-authors spotted the faint background glow present in most Cassini images of the icy moon. The brightest eruption features, which appear to be discrete jets, look to them to be superimposed intermittently upon this background structure.
They modeled eruptions as uniform curtains along the tiger stripe fractures, and found that phantom brightness enhancements appear in places where the viewer is looking through a ‘fold’ in the curtain.
The folds exist because the fractures in the moon’s surface are more wavy than perfectly straight.
The scientists think this optical illusion is responsible for most of what appear to be individual jets.
“The viewing direction plays an important role in where the phantom jets appear. If you rotated your perspective around Enceladus’ south pole, such jets would seem to appear and disappear,” Dr Spitale explained.
Phantom jets in simulated images produced by the team line up nicely with some of the features in real Cassini images that appear to be discrete columns of spray.
The correspondence between simulation and spacecraft data suggests that much of the discrete-jet structure is an illusion.
In the second study, a team led by Dr Christopher Glein from Carnegie Institution of Washington developed a novel chemical model based on mass spectrometry data gathered by Cassini in order to determine the pH of Enceladus’ ocean.
The model shows that the plume is salty with an alkaline pH of about 11 or 12, which is similar to that of glass-cleaning solutions of ammonia.
It contains the same sodium chloride as our oceans here on Earth. Its additional substantial sodium carbonate makes the ocean more similar to our planet’s soda lakes such as Mono Lake in California or Lake Magadi in Kenya. The scientists refer to it as a ‘soda ocean.’
“Knowledge of the pH improves our understanding of geochemical processes in Enceladus’ soda ocean,” said Dr Glein, lead author on the paper in the journal Geochimica et Cosmochimica Acta.
According to the team, the ocean’s high pH is caused by a metamorphic, underwater geochemical process called serpentinization.
On Earth, serpentinization occurs when certain kinds of so-called ultrabasic or ultramafic rocks are brought up to the ocean floor from the upper mantle and chemically interact with the surrounding water molecules. Through this process, the ultrabasic rocks are converted into new minerals, including the mineral serpentine, after which the process is named, and the fluid becomes alkaline.
On Enceladus, serpentinization would occur when ocean water circulates through a rocky core at the bottom of its ocean.
“Why is serpentinization of such great interest? Because the reaction between the metallic rocks and the ocean water also produces molecular hydrogen, which provides a source of chemical energy that is essential for supporting a deep biosphere in the absence of sunlight inside moons and planets,” Dr Glein explained.
“This process is central to the emerging science of astrobiology, because molecular hydrogen can both drive the formation of organic compounds like amino acids that may lead to the origin of life, and serve as food for microbial life such as methane-producing organisms. As such, serpentinization provides a link between geological processes and biological processes.”
“The discovery of serpentinization makes Enceladus an even more promising candidate for a separate genesis of life.”
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Joseph N. Spitale et al. 2015. Curtain eruptions from Enceladus’ south-polar terrain. Nature 521, 57-60; doi: 10.1038/nature14368
Christopher R. Glein et al. The pH of Enceladus’ ocean. Geochimica et Cosmochimica Acta, published online April 16, 2015; doi: 10.1016/j.gca.2015.04.017
