The ice shelves of the polar regions replenish themselves through two processes, including an extraordinary phenomenon sometimes known as underwater snow. It is thought the icy shell of Europa, and probably other worlds with interior oceans, experience something similar. A better understanding could help the operators of the Europa Clipper – to be launched in 2024 – know what they need to look for in terms of conditions suited to life.

For most of Earth’s history, there has been life in the water, but not necessarily much on land. Consequently, the oceans beneath icy shells on worlds like Europa and Enceladus may well be better prospects to find alien lifeforms than dry Mars. Just as astrobiologists make expeditions to the Atacama Desert and dry parts of Northern Canada to find the closest Martian proxies, University of Texas at Austin graduate student Natalie Wolfenbarger is using oceanic ice shelves as analogs for outer solar system moons.

In Astrobiology, Wolfenbarger and co-authors describe the composition of these shelves – which they argue are more relevant than the better-known but thinner sea ice – and their effects on the water beneath them. As they point out, “Because existing observations [of ice-covered ocean worlds] are mostly confined to the surface, much attention has been directed towards the uppermost layer of the ice shell.” However, the underside is probably more important, and this has barely been considered.

"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," Wolfenbarger said in a statement. 

Current extrapolations from ice to ocean composition may be flawed if we’re making the wrong assumptions about how the ice forms.

The paper notes the shelves are composed of frazil ice (hopefully the cooler counterpart of Fraggle Rock) and congelation ice.

Congelation ice, formed when water in contact with the existing shelf freezes, is known to contain much less salt than the ocean below. But, it turns out frazil ice – more poetically known as underwater snow – is purer still. Where congelation ice has about 90 percent less salt per liter than the ocean, rising to 99 percent under certain conditions, frazil ice keeps out about 99.9 percent, the paper reports.

That matters because Wolfenbarger thinks frazil ice – which forms in flakes like snow in the water column and floats up until it sticks to the underside of the ice shelf – could be common on Europa. If so, Europa’s ice shell would contain much less salt than previously predicted.

Impurities within ice affect its strength, heat transfer and the way it responds to forces that crack it or shape it into dunes. Calculations based on the purity of the ice could affect how we choose to fine-tune missions to these worlds depending on what they need to look for. Once we do measure the salts trapped in Europa’s ice, we need to understand the formation process to translate that to what lies beneath.

Although we have yet to sample Europa directly, we have reason to believe the most common salts in the oceans of Europa and Enceladus are similar to those on Earth, with slightly different orderings of frequency. However, considerable uncertainty remains that needs to be resolved to really understand these worlds.