Hydrothermal vents on seafloors of ‘ocean worlds’ could support life, new study says

We’ve all seen the surreal footage in nature documentaries that shows hydrothermal vents on the cold ocean floor — roaring black plumes of super-hot water — and the life forms clinging to them. A new study by UC Santa Cruz scientists now suggests that low-temperature vents that are common on Earth’s sea floor may help create the conditions for life on “ocean worlds” in our solar system.

Oceanic worlds are planets and moons that have—or had in the past—a liquid ocean, often under an icy shell or in their rocky interior. In Earth’s solar system, several of the moons of Jupiter and Saturn are ocean worlds, and their existence has motivated everything from peer-reviewed academic studies and spacecraft missions with satellites to popular movies such as the 2013 sci-fi thriller The Europa Report.

Many lines of research suggest that some oceanic worlds release enough heat internally to drive hydrothermal circulation beneath their seafloor. This heat is generated by radioactive decay that occurs deep within the Earth, with additional heat that can be generated by tides.

Rock-heat-fluid systems were discovered on the Earth’s seafloor in the 1970s, when scientists observed discharges of fluids that carried heat, particles, and chemicals. Many vent sites were surrounded by new ecosystems, including specialized bacterial mats, red-and-white tubeworms, and heat-sensitive shrimp.

Alien seabed simulation

In this new study, published today in Journal of Geophysical Research: Planets, the researchers used a complex computer model based on hydrothermal circulation as it occurs on Earth. After varying variables such as gravity, heat, rock properties, and the depth of fluid circulation, they found that hydrothermal vents could be sustained under a wide range of conditions. If these kinds of flows occur on an oceanic world like Jupiter’s moon Europa, they could increase the likelihood that life exists there as well.

“This study suggests that low-temperature (not too hot for life) hydrothermal systems could have been sustained on oceanic worlds outside of Earth on a time scale comparable to that required for life to take hold on Earth,” said Andrew Fisher, principal study author and Distinguished Professor of Earth and Planetary Sciences (EPS) at UC Santa Cruz.

The seawater circulation system on which the team based their computer models was found on a 3.5-million-year-old seafloor in the northwest Pacific Ocean, east of the Juan de Fuca Ridge. There, cool groundwater flows over an extinct volcano (seamount), travels underground for about 30 miles, and then flows back into the ocean through another seamount. “Water collects heat as it flows and comes out warmer than when it flowed in and with a very different chemistry,” explained Kristin Dickerson, the paper’s second author and a Ph.D. candidate in Earth and Planetary Sciences.

The flow from one seamount to another is driven by buoyancy as the water warms and becomes denser as it cools. Density differences create fluid pressure differences in the rock, and the system is maintained by the flows themselves—running as long as enough heat is supplied and the rock properties allow sufficient fluid circulation. “We call it a hydrothermal siphon,” Fisher said.

Earth’s cooling system

While high-temperature vent systems are driven mainly by volcanic activity beneath the seafloor, Fisher explained that a much larger volume of fluid flows in and out of Earth’s seafloor at lower temperatures, driven mainly by the planet’s “background” cooling. “The flow of water through low-temperature venting is equivalent in terms of water discharge to all the rivers and streams on Earth and is responsible for about a quarter of Earth’s heat loss,” he said. “The entire volume of the ocean is pumped in and out of the seafloor approximately every half a million years.”

Many previous studies of hydrothermal circulation on Europa and Enceladus, a small moon orbiting Saturn, considered higher-temperature fluids. Cartoons and other drawings often depict systems on their seafloor that look like black smokers on Earth, said Donna Blackman, an EPS researcher and third author of the new paper. “Low-temperature flows are at least as likely, if not more likely,” she said.

The team was particularly excited about one result from computer simulations reported in the new paper, which shows that in very low gravity – like that on Enceladus’ seafloor – circulation can continue with low to moderate temperatures for millions or billions of years. This could help explain how small oceanic worlds beneath the seafloor can have long-lived fluid circulation systems even when heating is limited: low heat extraction efficiency could lead to significant lifetimes – essentially the lifetime of the solar system.

Planetary scientists look to observations from satellite missions to help determine what kinds of conditions are present or possible on ocean worlds. The authors of the new documentary plan to attend the launch of the Europa Clipper spacecraft later this fall at Cape Canaveral, Florida, along with colleagues working on the Exploring Ocean Worlds project.

Scientists acknowledge the uncertainty of when the seafloors of oceanic worlds will be directly observed for the presence of active hydrothermal systems. Their distance from Earth and physical properties pose major technical challenges for spacecraft missions. “Therefore, it is essential to make the most of available data, much of it collected remotely, and to use knowledge from decades of detailed studies of Earth’s analog systems,” they conclude in the paper.