Ocean worlds are planetary bodies with liquid oceans, often beneath an icy shell or within rocky interiors. In our solar system, several moons of Jupiter and Saturn are ocean worlds. Some ocean worlds are thought to have hydrothermal circulation, where water, rocks, and heat combine to pump and expel fluids to the ocean floor. Hydrothermal circulation influences the chemical composition of the water and rocks of ocean worlds and may help life develop deep beneath the icy surface. In a new study, planetary researchers used computer simulations of hydrothermal circulation based on well-understood systems on Earth to measure the effects of low gravity at values appropriate for ocean worlds smaller than our home planet. Simulations of ocean worlds with (lower) gravity result in fluid circulation that is roughly similar to that which occurs above and below the ocean floor on Earth, but with some key differences. Low gravity reduces buoyancy, so fluids do not become lighter as they heat up, which reduces their flow rate. This increases the temperature of the circulating fluids, which could lead to more extensive chemical reactions, possibly including those necessary to support life.
This diagram shows how Cassini scientists think rocks and water at the bottom of Enceladus’ ocean interact to produce hydrogen gas. Image courtesy of NASA/JPL-Caltech/Southwest Research Institute.
Rock-heat-fluid systems were discovered on the Earth’s ocean floor in the 1970s, where scientists observed releases of fluids carrying heat, particles, and chemicals.
Many of the vents were surrounded by a novel ecosystem, including specialized bacterial mats, red and white tube worms and heat-sensing shrimp.
For the new study, Professor Andrew Fisher from the University of California, Santa Cruz, and his colleagues used a complex computer model based on the hydrothermal cycle that occurs on Earth.
After varying variables such as gravity, heat, rock properties and depth of fluid circulation, the researchers found that hydrothermal vents could persist under a wide range of conditions.
If these flows occurred on an ocean world like Jupiter’s moon Europa, they could increase the chances of life surviving there as well.
“This study suggests that extraterrestrial ocean worlds may have supported low-temperature (but not hot enough for life) hydrothermal systems on timescales similar to those it took for life to become established on Earth,” Prof Fischer said.
The ocean circulation system on which the researchers based their computer model was discovered on the 3.5-million-year-old seafloor of the northwest Pacific Ocean, east of the Juan de Fuca Ridge.
There, cold undersea water flows through an extinct volcano (seamount), travels about 30 miles (48.3 km) underground, and then flows out into the ocean through another seamount.
“As water flows, it picks up heat, it’s warmer than when it entered, and its chemistry changes dramatically,” says Kristin Dickerson, a doctoral student at the University of California, Santa Cruz.
“The flow from seamount to seamount is driven by buoyancy – as water warms it becomes less dense and as it cools it becomes more dense,” Prof Fischer added.
“The difference in density creates a difference in fluid pressure within the rock, and the system is sustained by the flow itself. So as long as there is enough heat supplied and the rock properties allow for sufficient fluid circulation, the system will keep running. We call this a hydrothermal siphon.”
“Hot vent systems are primarily driven by sub-sea volcanism, while the Earth’s ocean floor experiences large amounts of fluid flowing in and out at much cooler conditions, driven primarily by Earth’s background cooling.”
“The flow of water through low-temperature vents is equivalent to all the rivers and streams on Earth in terms of the volume of water released, and accounts for about a quarter of the Earth’s heat loss.”
“About every 500,000 years, the entire volume of ocean water is pumped up and out of the ocean floor.”
Many previous studies of the hydrothermal circulation on Europa and Enceladus have considered hotter fluids.
“Cartoons and other illustrations often depict undersea systems that are similar to Earth’s black smokers, where cooler currents could occur just as much or even more than they do on Earth,” said Dr Donna Blackman from the University of California, Santa Cruz.
The results show that in very low gravity, such as on the ocean floor of Enceladus, the circulation can continue at low to moderate temperatures for millions or billions of years.
This could help explain why small ocean planets can have long-lived fluid circulation systems beneath their seafloors despite limited heating: the inefficiency of heat extraction could extend their lifetimes considerably, potentially for the entire lifetime of the solar system.
Scientists acknowledge that it is uncertain when active hydrothermal systems will be directly observed on the ocean planet’s seafloor.
The distance from Earth and physical characteristics pose significant technical challenges for spacecraft missions.
“It is therefore essential to make the most of the available data, much of which is remotely collected, and to leverage the understanding gained from decades of detailed study of the analog Earth system,” the authors concluded.
their paper Published in Journal of Geophysical Research: Planets.
_____
A.T. Fisher others2024. Gravitational maintenance of hydrothermal circulation in relation to the ocean world. Journal of Geophysical Research: Planets 129(6):e2023JE008202; doi:10.1029/2023JE008202
