We’ve all seen the surreal images in nature documentaries showing hydrothermal vents on the frigid ocean floor – roaring black plumes of superheated water – and the life forms that cling to them. Now, a new study by researchers at UC Santa Cruz suggests that the lower-temperature vents, which are common on 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 used to have) a liquid ocean, often under an icy shell or in their rocky interior. In Earth’s solar system, several moons of Jupiter and Saturn are ocean worlds, and their existence has motivated everything from peer-reviewed academic studies and satellite spacecraft missions to popular films such as the 2013 science fiction thriller. The Europe Report.
Many lines of research suggest that some ocean worlds release enough heat internally to drive hydrothermal circulation beneath their seafloor. This heat is generated by radioactive decay, such 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 the release of 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 heat-sensitive shrimp.
Simulating alien seabeds
In this new study, published today in the Journal of Geophysical Research: Planetsthe researchers used a complex computer model based on the 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 maintained 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 chance that life exists there too.
“This study shows that low-temperature hydrothermal systems (not too hot for life) may have been maintained on oceanic planets beyond Earth, for timescales comparable to the time required for life to emerge on Earth ” said Andrew Fisher, lead author of the study and 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 bottom water flows in through an extinct volcano (seamount), travels 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 flowed in, and with a very different chemistry,” explained Kristin Dickerson, the paper’s second author and a doctoral candidate in earth and planetary sciences.
The flow from one seamount to another is driven by buoyancy, because water becomes less dense as it warms, and denser as it cools. Differences in density cause differences in the fluid pressure in the rock, and the system is maintained by the currents themselves, as long as sufficient heat is supplied and the properties of the rock allow sufficient fluid circulation. “We call it a hydrothermal siphon,” Fisher said.
Earth’s cooling system
While ventilation systems at high temperatures are driven primarily by volcanic activity beneath the seafloor, Fisher explained that at lower temperatures a much greater volume of fluid flows in and out of Earth’s seafloor, driven primarily by “background” cooling of the planet. “Water flow through low-temperature venting is, in terms of the amount of water discharged, equal to all the rivers and streams on Earth, and is responsible for about a quarter of the Earth’s heat loss,” he said. “The entire volume of the ocean is pumped in and out of the seabed about every half a million years.”
Many previous studies of the hydrothermal circulation on Europa and Enceladus, a small moon orbiting Saturn, have considered higher-temperature fluids. Cartoons and other drawings often show 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 currents are at least as likely, if not more likely,” she said.
The team was particularly excited by one result of the computer simulations in the new paper, which shows that under very low gravity – such as that found on the seafloor of Enceladus – circulation can continue for millions or billions of years with low to moderate temperatures. This could help explain how small ocean worlds can have long-lived fluid circulation systems beneath their seafloors, even though heating is limited: the low efficiency of heat extraction could lead to significant longevity – essentially for the entire lifetime of the solar system.
Planetary scientists look to observations from satellite missions to help determine what types of conditions are present or possible on ocean worlds. The authors of the new paper plan plan to attend the launch of the Europa Clipper spacecraft at Cape Canaveral, Florida, later this fall, along with colleagues working on the Exploring ocean worlds project.
The researchers acknowledge the 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 pose major technical challenges for spacecraft missions. “It is therefore essential to make the most of the available data, much of which has been collected remotely, and to leverage the insight from decades of detailed studies of analog Earth systems,” they conclude in the article .
In addition to the UC Santa Cruz team, the paper included co-authors from the Blue Marble Space Institute of Science, Woods Hole Oceanographic Institution, and Laboratoire de Planétologie et Géodynamique de Nantes. The study, “Sustaining Hydrothermal Circulation with Gravity Relevant to Ocean Worlds,” was funded by the NASA Astrobiology Program (award #80NSSC19K1427) and the National Science Foundation (OCE-1924384).