Seabed hydrothermal vents in ‘ocean worlds’ could support life, new study suggests

We’ve all seen the surreal images in nature documentaries showing hydrothermal vents on the frigid ocean floor – bellowing black plumes of piping hot water – and the life forms clinging to them. Now, a new study by UC Santa Cruz researchers suggests that low-temperature vents, common on Earth’s seafloor, could help create conditions suitable for life on “ocean worlds.” of our solar system.

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

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

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

Simulate extraterrestrial seabeds

In this new study, published today in the Geophysical Research Journal: Planets, the 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 could be sustained under a wide range of conditions. If these types of flows occur on an ocean world, like Jupiter’s moon Europa, they could increase the chances that life exists there as well.

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

The seawater circulation system on which the team based its computer models was discovered on a 3.5 million-year-old seafloor in the northwest Pacific Ocean, east of the Juan de Fuca Ridge. There, cool bottom water flows through an extinct volcano (seamount), travels underground for about 30 miles, then returns to the ocean through another seamount. “Water builds up heat as it flows and comes out hotter than when it went in, and with very different chemistry,” explained Kristin Dickerson, second author of the paper and a Ph.D. . candidate in Earth and planetary sciences.

Flow from one seamount to another is governed by buoyancy, because water becomes less dense as it warms and more dense as it cools. Differences in density create differences in fluid pressure within the rock, and the system is supported by the flows themselves, operating as long as sufficient heat is supplied and the properties of the rock allow fluid circulation sufficient. “We call it a hydrothermal siphon,” Fisher said.

Earth’s cooling system

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

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

The team was particularly excited by a result of the computer simulations presented in the new paper showing that, under very low gravity, such as that found on the sea floor of Enceladus, circulation can continue at low to moderate temperatures for millions or billions of years. This could help explain how small ocean worlds can have long-lived fluid circulation systems beneath their seafloors, even if heating is limited: low efficiency of heat extraction could lead to considerable longevity , essentially for the entire lifetime of the solar system.

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

Researchers acknowledge uncertainty about when the seafloors of ocean worlds will be directly observed for the presence of active hydrothermal systems. Their distance from Earth and their physical characteristics present major technical challenges for spacecraft missions. “It is therefore essential to make the most of available data, much of which is collected remotely, and to take advantage of the knowledge gained from decades of detailed studies of analog terrestrial systems,” they conclude in the article .