Hydrothermal vents on the seafloor of “ocean worlds” could support life, says a new study

We’ve all seen the surreal images of hydrothermal vents on the frigid ocean floor — bubbling up black fountains of super-hot water — and the life forms that cling to them. Now, a new study by UC Santa Cruz researchers suggests that cooler-temperature vents, common across Earth’s seafloor, could help create life-sustaining conditions on “ocean worlds” in our solar system.

Ocean worlds are planets and moons that have, or have had in the past, a liquid ocean, often beneath a layer of ice or in their rocky interior. In Earth’s solar system, several moons of Jupiter and Saturn are ocean worlds, and their existence has been the subject of everything from peer-reviewed academic studies and satellite space missions to popular films such as the 2013 science fiction thriller The Europa Report.

Many lines of research suggest that some ocean worlds emit enough heat internally to drive hydrothermal circulation beneath their seafloors. This heat is generated by radioactive decay as it occurs deep within the Earth, with additional heat possibly generated by tides.

Rock-thermal fluid systems were discovered on Earth’s seafloor in the 1970s when scientists observed leaking fluids that transported heat, particles and chemicals. Many of the vents were surrounded by novel ecosystems, including specialized bacterial mats, red-and-white tubeworms and heat-sensitive shrimp.

Simulation of extraterrestrial seabeds

In this new study, published today in Journal of Geophysical Research: PlanetsThe researchers used a complex computer model based on hydrothermal circulation as it occurs on Earth. After changing variables such as gravity, heat, rock properties and fluid circulation depth, they found that hydrothermal vents can persist under a wide range of conditions. If these types of currents occur on an ocean world such as Jupiter’s moon Europa, they could increase the likelihood that life exists there, too.

“This study suggests that low-temperature hydrothermal systems (not too hot for life) could have been maintained on ocean worlds beyond Earth for timescales comparable to those required for life to evolve on Earth,” said Andrew Fisher, the study’s lead author and distinguished professor of Earth and Planetary Sciences (EPS) at UC Santa Cruz.

The seawater circulation system on which the team based its computer models was found on a 3.5-million-year-old seafloor in the northwestern Pacific, east of the Juan de Fuca Ridge. There, cool bottom water flows through an extinct volcano (seamount), flows about 30 miles underground, and then flows back into the ocean through another seamount. “The water collects heat as it flows and comes out warmer than when it came in, and with very different chemical properties,” explained Kristin Dickerson, the paper’s second author and a doctoral student in earth and planetary sciences.

The flow from one seamount to another is driven by buoyancy, as the density of water decreases when heated and increases when cooled. Density differences create differences in fluid pressure in the rock, and the system is maintained by the flows themselves – 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 primarily by volcanic activity beneath the seafloor, Fisher explained that at lower temperatures, a much larger volume of fluid flows in and out of Earth’s seafloor, caused primarily by the planet’s “background cooling.” “The flow of water through low-temperature vent systems is equivalent to all of Earth’s rivers and streams in terms of the amount of water released, 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 about every half a million years.”

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

The team was particularly excited by one result of the computer simulations presented in the new study. It shows that at very low gravity – such as that found on the seafloor of Enceladus – circulation at low to moderate temperatures can persist for millions or billions of years. This could help explain how small ocean worlds can have long-lived fluid circulation systems beneath their seafloors, despite limited heating: The low efficiency of heat extraction could lead to considerable longevity – essentially the entire lifetime of the solar system.

Planetary scientists use observations from satellite missions to find out what conditions exist or are possible on ocean worlds. The authors of the new paper plan to be present at the launch of the Europa Clipper spacecraft from Cape Canaveral, Florida, this fall, along with colleagues who work on the Exploring Ocean Worlds project.

The researchers are aware of the uncertainty about when the seafloors of ocean worlds can be directly studied for the presence of active hydrothermal systems. Their distance from Earth and their physical properties pose major technical challenges for space missions. “It is therefore important to make the most of the available data, many of which have been collected remotely, and to leverage the knowledge gained from decades of detailed studies of analog Earth systems,” they conclude in the article.