Astronomy

How Warm Are the Oceans on the Icy Moons? The Ice Thickness Provides a Clue.

Scientists are discovering that more and more Solar System objects have warm oceans under icy shells. The moons Enceladus and Europa are the two most well-known, and others like Ganymede and Callisto probably have them too. Even the dwarf planet Ceres might have an ocean. But can any of them support life? That partly depends on the water temperature, which strongly influences the chemistry.

We’re likely to visit Europa in the coming years and find out for ourselves how warm its ocean is. Others on the list we may never visit. But we may not have to.

Researchers at Cornell University are figuring out how to determine the temperature of an icy world’s ocean by measuring the thickness of its ice shell and associated properties. They published their results in a research article in the journal JGR Planets. It’s titled “Ice-Ocean Interactions on Ocean Worlds Influence Ice Shell Topography,” and the lead author is Justin Lawrence, a visiting scholar at the Cornell Center for Astrophysics and Planetary Science. Lawrence is also a program manager at Honeybee Robotics, a subsidiary of Blue Origin that builds technologies for space exploration.

Their research is based on what’s called ice-pumping, a phenomenon observed under the ice in Antarctica.

“When ice is submerged, a melting and freezing exchange process termed the “ice pump” can affect ice composition, texture, and thickness,” the researchers write. “We find that ice pumping is likely beneath the ice shells of several ocean worlds in our solar system.”

Ice pumping is more commonly called thermohaline ice pumping, where thermo means heat and haline means basically the same thing as saline: salty. But whereas saline refers to fresh water, haline refers to ocean water.

On Earth, large-scale thermohaline ice pumping supplies heated water to the north and south polar regions. On a smaller scale, affects how much ice forms on the underside of an ice sheet since ice is formed from water containing no salt or very little salt. So, the salt from the ice-forming water is concentrated in the water under the ice. Since that salt-concentrated water is so close to the ice, the water under the ice is both higher in salt and colder because it’s close to the ice. That’s why the term thermohaline is used.

The high-salinity shelf water (HSSW) that forms under the ice is denser than the surrounding water and sinks. As it sinks, it then becomes warmer than the freezing point there since the water pressure lowers the freezing point. So now the HSSW is warmer and triggers melting on the underside of the ice shelf. Then, the HSSW mixes with lower-salinity meltwater to create colder, buoyant ice-shelf water (ISF.) The ISF upwells and forms soft ice called frazil ice on the underside of the ice shelf. The process can create ice layers hundreds of meters thick.

The critical part is where the ocean and the ice interact. The researchers say that if they can determine the ice thickness, they can constrain the water temperature from afar. The press release presenting the results calls this “conducting oceanography from space.”

This schematic from the study shows how thermohaline ice pump circulation works below a generalized ice shelf. (1) High salinity shelf water (HSSW) forms at the surface freezing point (Tf = ?1.9°C) as the brine rejected from sea ice growth mixes into the water column. (2) HSSW is dense relative to the surrounding seawater, so it sinks, and a portion circulates beneath the ice shelf to the grounding zone, where it is now warm compared to the pressure-depressed freezing point (positive thermal driving) and drives melting. (3) Fresh meltwater generated at the colder, in situ freezing point mixes with HSSW, generating fresher, colder, and relatively buoyant Ice Shelf Water (ISW). (4) ISW upwells, the freezing point increases and thermal driving commensurately decreases. With a sufficient pressure decrease, supercooling occurs and frazil ice forms, which can accumulate into hundreds of meters thick layers of marine ice at the ice shelf base. Credit: Journal of Geophysical Research: Planets (2024). DOI: 10.1029/2023JE008036

“Anywhere you have those dynamics, you would expect to have ice pumping,” Lawrence said. “You can predict what’s going on at the ice-ocean interface based on the topography—where the ice is thick or thin, and where it is freezing or melting.”

There’s uncertainty around which Solar System bodies have ice pumping and how close ice pumping on Earth is similar to other bodies. For example, if Europa’s ice shell is thicker than about 35 km (22 miles) and has low salt content, then there may be no ice pumping. “However, the majority of predictions for Europa’s ice shell thickness suggest that the interface falls in the marine regime, such that Earth’s ice shelves can serve as system analogs to inform European ice-ocean interactions,” the authors write in their research.

Ice pumping is probable on Ganymede and Titan, according to the authors, as long as bulk ocean salinity isn’t too low. On the other hand, Enceladus almost certainly has ice pumping. But the ice pumping on Enceladus is expected to be weaker, while at Europa, it’s expected to be much stronger.

Jupiter’s icy moon Europa likely has strong ice pumping very similar to the Ross Ice Shelf in Antarctica. Credits: NASA/JPL-Caltech/SETI Institute

What does it all add up to?

“If we can measure the thickness variation across these ice shells, then we’re able to get temperature constraints on the oceans, which there’s really no other way yet to do without drilling into them,” said Britney Schmidt, associate professor of astronomy and of Earth and atmospheric sciences in the College of Arts and Sciences and Cornell Engineering. “This gives us another tool for trying to figure out how these oceans work. And the big question is, are things living there, or could they?” Schmidt asks.

We can only answer that question incrementally right now. To do that, we need to understand the ice shell, the temperature, and how they’re connected to make progress.

“There’s a connection between the shape of the ice shell and the temperature in the ocean,” Schmidt said. “This is a new way to get more insight from ice shell measurements that we hope to be able to get for Europa and other worlds.”

Right now, estimates for Europa’s ice shell thickness range from 10 to 30 km (6 to 20 mi). For Enceladus, estimates range from 20 to 25 km (12 to 16 miles), though the south pole region’s ice is much thinner, only 1 to 5 km thick (1/2 mile to 3 miles.)

Oddly enough, the icy shells and underlying oceans on the Solar System’s icy worlds may be more similar to Earth than any other planets or moons. The interactions between ice and the ocean on Europa are very similar to what researchers see under Antarctica’s Ross Ice Shelf. In 2019, Schmidt and other researchers observed the underside of the shelf with the Icefin robot and observed ice pumping.

Another factor at play here is gravity. “Ice pumping scales with gravity and so may prove important to dynamics at the ice shell-ocean interfaces of other similarly massive ocean worlds such as Ganymede or Titan,” the authors explain. That’s one of the reasons that Enceladus is expected to have weaker ice pumping: its gravity is ten times weaker than Europa’s.

This study is important because it shows how ice pumping can occur on different ocean worlds in the Solar System, and that has implications for life.

Enceladus likely has ice pumping, but it’s expected to be weaker than on Europa because Enceladus’ gravity is much weaker. Image Credit: NASA/JPL/Space Science Institute

“We show that ice pumping can occur for a range of ocean salinity and ice thicknesses relevant to ocean worlds and that ice pumping is an important process linking ice shell dynamics, ocean circulation, and basal ice shell topography,” the authors write. “We show that the relationship between ice-ocean interactions and ice topography establishes a link between variability in ocean temperature and ice shell thickness that potentially makes constraining ocean temperatures possible in the absence of in situ ocean observations.”

That’s a big step. The more we can learn about these worlds without visiting them, the better. Missions to the Solar Systems icy moons are expensive, though one is already planned: NASA’s Europa Clipper. It’s scheduled for launch later this year and should arrive at Jupiter in 2030. A combination of methods will help the Clipper measure Europa’s ice thickness more accurately.

“The concepts described here will enable the thermal state of Europa’s upper ocean to be constrained from ice shell thickness,” the authors conclude.

Evan Gough

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