Ocean worlds are planets and moons with liquid oceans, often under ice or within their rocky interiors. Some of Jupiter's and Saturn's moons are considered ocean worlds. These moons have inspired scientific studies, spacecraft missions, and popular media.
Research indicates that some ocean worlds generate enough internal heat for hydrothermal circulation. This heat comes from radioactive decay and tidal forces.
Hydrothermal systems on Earth's seafloor were discovered in the 1970s. These systems, found in volcanic areas, discharge heat, particles, and chemicals, and are surrounded by unique ecosystems.
In the new study, published in the Journal of Geophysical Research: Planets, the researchers used computer models to simulate hydrothermal circulation on ocean worlds. By adjusting variables like gravity, heat, and rock properties, they found that hydrothermal vents could exist under various conditions. If similar flows occur on an ocean world like Europa, it could increase the chances of life there.
"This study suggests that low temperature (not too hot for life) hydrothermal systems could have been sustained on ocean worlds beyond Earth over timescales comparable to that required for life to take hold on Earth," said Andrew Fisher, study lead author and a distinguished professor of earth and planetary sciences at UC Santa Cruz.
The seawater-circulation system used in the models is based on a system found in the northwestern Pacific Ocean. Cool water flows into an extinct volcano, travels underground, and exits through another seamount. "The water gathers heat as it flows and comes out warmer than when it flowed in, and with very different chemistry," explained Kristin Dickerson, the paper's second author and a Ph.D. candidate in earth and planetary sciences.
Buoyancy drives the flow between seamounts, as water becomes less dense when it warms. Differences in density create pressure differences, sustaining the system as long as heat and fluid circulation continue. "We call it a hydrothermal siphon," Fisher said.
High-temperature vents are driven by volcanic activity, but a larger volume of fluid flows through lower-temperature vents, driven by the Earth's cooling. "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," Fisher said. "The entire volume of the ocean is pumped in and out of the seafloor about every half-million years."
Studies of hydrothermal circulation on Europa and Enceladus have often focused on higher temperature fluids. "Lower-temperature flows are at least as likely to occur, if not more likely," said Donna Blackman, an EPS researcher and third author on the new paper.
The study found that under very low gravity, like on Enceladus, circulation can continue at low to moderate temperatures for millions or billions of years. This could explain the long-lived fluid-circulation systems on small ocean worlds despite limited heating.
Planetary scientists rely on satellite data to understand ocean world conditions. The researchers plan to attend the launch of the Europa Clipper spacecraft this fall, as part of the Exploring Ocean Worlds project.
The paper notes the difficulty of directly observing hydrothermal systems on ocean worlds due to their distance and technical challenges. "Thus, it is essential to make the most of available data, much of it collected remotely, and leverage understanding from decades of detailed studies of analog Earth systems," they conclude in the paper.
Research Report:Sustaining Hydrothermal Circulation With Gravity Relevant to Ocean Worlds
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