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June 26, 2024

Low Gravity on Ocean Worlds Might Help Sustain Warm Water Circulation

Freshly emplaced (since 2018) lava mounds (black with yellow hydrothermal mineral staining) at the crater floor of KSM (Jan 2023). ROV Jason prepares to insert a temperature senor into a newly formed hydrothermal vent (yellow stained area, bottom right) within the recently erupted lavas. (©Woods Hole Oceanographic Institution)

Freshly emplaced (since 2018) lava mounds (black with yellow hydrothermal mineral staining) at the crater floor of KSM (Jan 2023). ROV Jason prepares to insert a temperature senor into a newly formed hydrothermal vent (yellow stained area, bottom right) within the recently erupted lavas. (©Woods Hole Oceanographic Institution)

A new study led by team members at UC Santa Cruz used a complex computer model to investigate how the influence of low gravity, as found on ocean worlds in our outer solar system, could influence flows of water and heat below their seafloors.

The work was conducted as part of a multi-institutional “Exploring Ocean Worlds” NASA program, led by Woods Hole Oceanographic Institution (WHOI) Senior Scientist Chris German. It has shed new light on the potential for seafloor hydrothermal venting —which hosts some of the most primitive life forms on Earth — to occur on other “Ocean World” moons orbiting giant planets in the outer Solar System.

ROV Jason’s manipulator arm using an Isobaric Gas-Tight (IGT) sampler to collect warm (~40°C) vent-fluids at the Kama’ehuakanaloa Seamount (KSM), January 2023. (©Woods Hole Oceanographic Institution)

Our solar system contains many "ocean worlds," planets and moons that currently have, or have had in the past, a liquid ocean. Some of these ocean worlds may release enough heat internally to drive hydrothermal circulation - water that flows into the seafloor, circulates, and is warmed, and flows back out. On Earth, these flows can carry heat and chemicals, some of which are key to supporting lush seafloor ecosystems. These rock-heat-fluid systems were discovered on Earth's seafloor in the 1970s, and many scientists think they may exist elsewhere in our solar system - this is a topic of great interest, especially because there is potential to support life. The research team at UC Santa Cruz, in collaboration with colleagues at the Blue Marble Space Institute of Science, WHOI, and Nantes Université, have published their new study in the Journal of Geophysical Research: Planets, showing how hydrothermal systems like those seen on Earth might differ under lower gravity conditions of other ocean worlds.

Many people have heard about high-temperature vents on Earth's seafloor, sometimes called "black smokers," where fluids heated above 300 °C (much hotter than the boiling point of water at sea level on Earth) jet into the ocean, depositing metal ores and helping to support exotic life. While these high-temperature systems are driven mainly by subseafloor volcanic activity, a much larger volume of fluid flows in and out of Earth's seafloor at lower temperatures, driven mainly by "background" cooling of the planet.

"The flow of water through low-temperature venting is equivalent, in terms of the amount of water being discharged, to all of the rivers and streams on Earth, and is responsible for about a quarter of Earth's heat loss," said Andrew Fisher, study lead author and a distinguished professor of earth and planetary sciences (EPS) at UC Santa Cruz. "The entire volume of the ocean is pumped in and out of the seafloor about every half-million years."

"Many previous studies of hydrothermal circulation on Europa and Enceladus (moons of Jupiter and Saturn) have considered higher temperature fluids, and cartoons and other drawings often illustrate systems on their seafloors that look like black smokers on Earth," explained Donna Blackman, an EPS researcher at UC Santa Cruz and third author on the new paper. "Lower temperature flows are at least as likely to occur, if not more likely."

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Kristin Dickerson, the paper's second author and a Ph.D. candidate in EPS at UC Santa Cruz, explained the basis for the study, "We looked at a seawater circulation system beneath Earth's seafloor that has been studied for years. It was discovered deep in the northwestern Pacific Ocean, where cool bottom water flows in through one seamount (an extinct volcano), travels for 50 km, then flows out through another seamount." This water gathers heat as it flows and comes out warmer than when it flowed in, and with very different chemistry. The researchers used a computer model that was developed for that Earth system, changing the value of gravity, and examining how flows would vary under a wide range of conditions (like different amounts of heating, rock properties, fluid circulation depth).

The flow from one seamount to another is driven by buoyancy, because water gets less dense as it warms, and more dense as it cools. Differences in density create differences in fluid pressure in the rock, and the system is sustained by the flows themselves. "We call it a hydrothermal siphon," said Fisher, "and it can run as long as there is a supply of heat and rock properties continue to allow circulation." Some ocean worlds are heated by large tides, which can generate heat as an ocean world is flexed during an eccentric orbit around a giant planet.

The new paper shows that, when gravity is lower than on Earth, there is a smaller buoyancy force driving flow in and out of the seafloor - this tends to slow circulation of water and remove heat. At the same time, less buoyancy when gravity is lower also results in less secondary mixing below the seafloor, a process that tends to use up energy and so reduce the flow between outcrops.

One exciting result from simulations featured in the new paper is that, under very low gravity (like that found on the seafloor of Enceladus, a small moon of Saturn), circulation can continue with low to moderate temperatures for millions or billions of years - i.e. throughout the life of the Solar System. This could help to explain how small ocean worlds, with gravity much lower than on Earth, can have long-lived fluid circulation systems below their seafloors: the low efficiency of heat extraction could lead to considerable longevity. In addition, some simulations resulted in vent fluid temperatures up to 150 °C, just above the upper limits to life on Earth, despite relatively shallow circulation below the seafloor. Overall, these simulations show that low gravity significantly changes temperatures and flow rates, and therefore likely impacts the chemistry of discharging fluids compared to what would be found on Earth.

Planetary scientists are looking to observations from satellite missions to help determine what kinds of conditions are present or possible on ocean worlds. The author team for the new paper will be attending the launch of the Europa Clipper spacecraft at Cape Canaveral, FL later this fall, along with colleagues collaborating on the Exploring Ocean Worlds project.

According to WHOI’s German, who is also co-author on the paper, “A significant outcome from this study is that it suggests that low temperature (not too hot for life) hydrothermal systems could have been sustained on Ocean Worlds beyond Earth over timescales much longer than it took for similar life to first take hold on Earth. Thus, Ocean Worlds in the outer solar system could also be habitable and, perhaps, host life.”

The February 2024 edition of Marine Technology Reporter is focused on Oceanographic topics and technologies.
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