Enceladus, an ocean-harboring moon of Saturn, erupts a plume that contains gases and frozen sea spray into space. By understanding the composition of the plume, planetary scientists can learn about what the ocean is like, how it got to be this way, and whether it provides environments where Earth-like life could survive. Now, a research team at the Southwest Research Institute has developed a new geochemical model that reveals that carbon dioxide in the moon’s ocean may be controlled by chemical reactions at its seafloor.
Cassini captured this image of Enceladus as it neared the moon for its closest-ever dive past the moon’s active south polar region. The image was taken in visible light with the spacecraft’s narrow-angle camera on October 28, 2015. The image shows heavily cratered northern latitudes at top, transitioning to fractured, wrinkled terrain in the middle and southern latitudes. The wavy boundary of the moon’s active south polar region is visible at bottom, where it disappears into wintry darkness. This view looks towards the Saturn-facing side of Enceladus. The image was taken at a distance of approximately 60,000 miles (96,000 km) from Enceladus and at a Sun-Enceladus-spacecraft, or phase, angle of 45 degrees. Image credit: NASA / JPL-Caltech / Space Science Institute.
“We came up with a new technique for analyzing the plume composition to estimate the concentration of dissolved carbon dioxide in the ocean,” said lead author Dr. Christopher Glein, a researcher in the Space Science and Engineering Division at the Southwest Research Institute.
“This enabled modeling to probe deeper interior processes.”
The analysis of mass spectrometry data from NASA’s Cassini spacecraft indicates that the abundance of carbon dioxide is best explained by geochemical reactions between the moon’s rocky core and liquid water from its subsurface ocean.
Integrating this information with Cassini/Ion Neutral Mass Spectrometer’s previous discoveries of silica and molecular hydrogen — two chemicals that are considered to be markers for hydrothermal processes — points to a more complex, geochemically diverse core.
“Based on our findings, Enceladus appears to demonstrate a massive carbon sequestration experiment,” Dr. Glein said.
“On Earth, climate scientists are exploring whether a similar process can be utilized to mitigate industrial emissions of carbon dioxide.”
“Using two different data sets, we derived carbon dioxide concentration ranges that are intriguingly similar to what would be expected from the dissolution and formation of certain mixtures of silicon- and carbon-bearing minerals at the seafloor.”
This artist’s rendering showing a cutaway view into the interior of Enceladus. A plume of ice particles, water vapor and organic molecules sprays from fractures in the moon’s south polar region. Image credit: NASA / JPL-Caltech.
Another phenomenon that contributes to this complexity is the likely presence of hydrothermal vents inside Enceladus.
“The dynamic interface of a complex core and seawater could potentially create energy sources that might support life,” said co-author Dr. Hunter Waite, also from the Space Science and Engineering Division at the Southwest Research Institute.
“While we have not found evidence of the presence of microbial life in the ocean of Enceladus, the growing evidence for chemical disequilibrium offers a tantalizing hint that habitable conditions could exist beneath the moon’s icy crust.”
“Distinct sources of observed carbon dioxide, silica and hydrogen imply mineralogically and thermally diverse environments in a heterogeneous rocky core,” Dr. Glein added.
“We suggest that the core is composed of a carbonated upper layer and a serpentinized interior.”
Carbonates commonly occur as sedimentary rocks such as limestone on Earth, while serpentine minerals are formed from igneous seafloor rocks that are rich in magnesium and iron.
It is proposed that hydrothermal oxidation of reduced iron deep in the core creates hydrogen, while hydrothermal activity intersecting quartz-bearing carbonated rocks produces silica-rich fluids.
Such rocks also have potential to influence the carbon dioxide chemistry of the ocean via low-temperature reactions involving silicates and carbonates at the seafloor.
“The implications for possible life enabled by a heterogeneous core structure are intriguing,” Dr. Glein said.
“This model could explain how planetary differentiation and alteration processes create chemical (energy) gradients needed by subsurface life.”
The study was published in the journal Geophysical Research Letters.
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Christopher R. Glein & J. Hunter Waite. The carbonate geochemistry of Enceladus’ ocean. Geophysical Research Letters, published online January 22, 2020; doi: 10.1029/2019GL085885
