Underneath Europa’s thick icy surface it is almost certain there is a gigantic, planet-wide ocean where snow falls upwards onto upside-down ice pinnacles and submerged valleys.
This strange underwater snow occurs below ice shelves on Earth, and a new study shows that the same is probably true for Jupiter’s moon, where it may play a role in building its ice crust.
“When we’re exploring Europa, we’re interested in the salinity and composition of the ocean, because that’s one of the things that will govern its potential habitability or even the type of life that might live there,”
said lead author Natalie Wolfenbarger, a graduate student researcher at the University of Texas Jackson School of Geosciences.
Pure Underwater Snow
Underwater snow is a great deal purer than other forms of ice. So Europa’s ice shell could be much less salty than previously thought.
That’s important for mission scientists developing NASA’s Europa Clipper spacecraft. Clipper will use radar to look through the ice shell to find out if Europa’s ocean could be hospitable to life.
Salt trapped in the ice can affect what and how deep the radar will see into the ice shell, so predicting what the ice is made of will help scientists make sense of the data.
Identifying what sort of ice makes up Europa’s shell will also help unravel the salinity and habitability of its ocean.
Congelation Ice vs Frazil Ice
Europa, about the size of the Earth’s moon, is encased in a miles-thick ice shell. Beneath lies a global ocean.
Earlier studies suggest that the temperature, pressure, and salinity of Europa’s ocean closest to the ice is comparable to what is found beneath an ice shelf in Antarctica.
The new study began with that knowledge to investigate the two distinct ways that water freezes under ice shelves, congelation ice and frazil ice.
Congelation ice grows directly from under the ice shelf. Frazil ice forms as ice flakes in supercooled seawater which float upwards through the water, settling on the bottom of the ice shelf.
Both these freezing mechanisms produce ice that is not as salty as seawater. Wolfenbarger found such ice would be even less salty at the scale and age of Europa’s ice shell.
According to her calculations, frazil ice, which keeps only a tiny fraction of the salt in seawater, could be widespread on Europa.
That means its ice shell could be much purer than previous estimates, which would affect everything from its strength to how heat moves through it and forces that might drive a kind of ice tectonics. The research is an affirmation for modelling the habitability of Europa using the Earth.
“We can use Earth to evaluate Europa’s habitability, measure the exchange of impurities between the ice and ocean, and figure out where water is in the ice,”
said coauthor Donald Blankenship, principal investigator for Europa Clipper’s ice-penetrating radar instrument. Europa Clipper is scheduled to be launched in October 2024.
Earlier evidence for the existence of an ocean on Europa comes from data gathered by NASA’s Galileo spacecraft, which orbited Jupiter from 1995 to 2003. Galileo conducted 12 close flybys of Europa.
One of Galileo’s instruments, a magnetometer, detected that a magnetic field was being created within Europa as Jupiter’s powerful magnetic field swept past the moon. Scientists agree the most probable thing that could make this magnetic signature is a global ocean of salty water.
Original study: Natalie S. Wolfenbarger, Jacob J. Buffo, Krista M. Soderlund, and Donald D. Blankenship. Ice Shell Structure and Composition of Ocean Worlds: Insights from Accreted Ice on Earth. Astrobiology, Aug 2022 937-961