A recent analysis of how minerals in planetary interiors change over time suggests that the ocean on Saturn's moon Enceladus emerged fairly recently, which means that there hasn't been much time for hydrothermal processes that might support alien life to develop.
Scientists modeled how the weathering of a mineral called olivine produces heat and hydrogen, and found that the process is probably nearing its end on Enceladus. The team reported the findings on April 5 in the journal Icarus.
The little moon Enceladus is covered by an icy shell, but in 2005 NASA's Cassini spacecraft observed water plumes shooting from the liquid ocean beneath the moon's surface. The spacecraft detected many chemical ingredients important for life, including hydrogen, in the plumes. However, it hasn't been clear how long the hydrothermal activity that produces these geysers or the ocean itself have been around, said Amber Zandanel, a postdoctoral researcher at the University of Nantes in France and first author of the study.
To find out, she and her colleagues examined a process called serpentinization that begins with minerals that formed under high heat and pressure.
"When exposed to low pressures ... and temperatures, things dissolve and then reprecipitate to more stable minerals," Zandanel said.
This process consumes water and releases heat and hydrogen.
"It's a way to study part of what makes a planet habitable," Zandanel said. "As long as there is liquid water in a rocky core, you can get some ... byproducts, or products of alteration, that are of interest for habitability."
She and her team focused on the mineral olivine, which reforms into other minerals such as serpentine, brucite and magnetite, and is very abundant and well-studied on Earth.
"Earth is an interesting case because it is a living planet, so it has a molten core, [and] so we have a mantle that creates olivine constantly," Zandanel said.
When olivine and some other minerals come into contact with seawater, they undergo serpentinization and release heat, creating certain types of hydrothermal vents.
"But on smaller satellites, especially something as small as Enceladus, it doesn't have enough pressure in the interior to create molten rock, so there's not a constant resupply of olivine," Zandanel said. "So it starts, basically, with a set amount."
Enceladus is thought to be partly made of olivine-containing meteorites. Zandanel and her colleagues wanted to figure out how long it would take for all of this olivine to be converted into other minerals.
They created a computer model that drew upon existing data on how quickly olivine weathers on Earth and what byproducts it forms, as well as studies describing the liquid ocean on Enceladus and processes such as the fluid flow within its core.
"Cassini in general gave us a lot of geochemical data from sampling [the] plumes that I used to constrain the water and the hydrogen amounts," Zandanel said.
She and her team determined that the process of olivine weathering should generally produce a lot more hydrogen than scientists have actually found in the moon's vapor plumes.
This indicates that the amount of olivine left to be serpentinized on Enceladus could be nearly exhausted.
Additionally, in scenarios with abundant olivine, the model suggested that so much water would be consumed that very little should have remained in the core.
"Since we know that the core is almost certainly water-saturated, it also implies that this process is nearing its end, or that there wasn't very much olivine to begin with," Zandanel said. "It allows the regime that we see today of slow hydrogen production and constant active water circulation in the rocky core."
She and her team estimated that the maximum amount of time that olivine could undergo serpentinization on Enceladus is about 75 million years, "which is pretty short, geologically speaking," Zandanel said. "This was even in very extreme cases, such as assuming the entire core was made of olivine and no other constituents."
There were only limited observations for the researchers to draw on, which makes it difficult to simulate geophysical processes across the entire moon, Zandanel said. Still, the findings suggest that Enceladus's global ocean probably hasn't been around very long, because weathering would have started as soon as the minerals came into contact with liquid water.
"It implies that if there are hydrothermal vents being created by this process in Enceladus, they don't have an indefinite lifespan; they would be fairly short-lived," Zandanel said. "That's not very exciting from an astrobiological perspective, to have such a short length of time for these things to exist."
However, other research indicates that serpentinization isn't the only source of heat in the moon's ocean.
"There is also tidal heating, among other things, that can create the heat and thermal energy that would be needed for astrobiology," Zandanel said.
Additionally, a recent and short-lived period of hydrothermal activity on Enceladus offers a unique opportunity to study these environments during the early stages of planetary evolution, Zandanel and her colleagues wrote.
The processes underway on Enceladus may be representative of hydrothermal systems on other icy bodies, and may offer insights about how life on Earth could have originated in hydrothermal vents, the team concluded.
The study, "Short lifespans of serpentinization in the rocky core of Enceladus: Implications for hydrogen production," published April 5 in Icarus, was authored by A. Zandanel, G. Choblet and G. Tobie, Université de Nantes; L. Truche and R. Hellmann, Université Grenoble Alpes; and A. Myagkiy, Université Grenoble Alpes and Storengy.