A study of Antarctic ice shelves suggests that the ice shell covering Jupiter’s moon Europa could contain a significant amount of underwater “snow”. This could have important implications for NASA’s upcoming Europa Clipper mission, which aims to use ground penetrating radar to study the ice shell and the ocean beneath.
The research was done by a team in the US led by Natalie Wolfenbarger at the University of Texas at Austin and focused on two processes by which Antarctic ice shelves grow from the bottom. The study also has implications for our understanding of whether life emerged in Europa’s ocean, which is encased in an ice shell some 15–25 km thick.
By examining the dynamic features that appear on the surface of Europa’s ice shell, scientists have found compelling evidence that the ocean beneath is constantly interacting with its ice shell. So far, however, the lower layers of this shell have proven more difficult to study.
Two mechanisms
To learn more about the processes which may be unfolding beneath Europa’s surface, Wolfenbarger and colleagues drew parallels with oceanic ice on our own planet. In Earth’s polar regions, ice shelves grow from the bottom through two possible mechanisms. One is congelation, whereby ice freezes at the interface between the ice and the water directly beneath it. The second mechanism involves the creation of frazil ice, which forms as millimetre-sized, randomly shaped crystals within columns of supercooled water. These columns are prevented from freezing more completely by turbulent currents. Under buoyant forces, these crystals travel upwards to rest on the underside of the ice, where they resemble underwater snow.
The researchers compared the contributions of each mechanism to ice formation by examining a variety of ice cores collected from ice shelves in Antarctica. They say that this environment is a close analogue to the temperature, pressure, and salinity of Europa’s ice sheet. Some of these samples were collected from features like rifts and glacier tongues, where the ice is thinner. Others were gathered from ancient ice shelves, which can reach several kilometres in thickness.
Their analysis revealed that congelation dominated the gradual thickening of older ice. On Europa, this process would be driven by the gradual cooling of the moon’s solid interior. In contrast, frazil ice is most likely to accumulate where the ice thins – either in small-scale rifts and fractures, or in warmer regions, often found at lower latitudes. On Europa, large sections of the ice shell are also warmed and thinned out through tidal heating, which is generated by Jupiter’s gravitational pull.
Low salinity
The team also found that these mechanisms create sea ice with very different salinities. While frazil ice retained just around 0.1% of the salinity of water from which it formed, congelation ice had a salt content of around 10% of the local water. Salinity strongly affects a variety of important properties of sea ice: including its strength, heat conduction, and its mechanical responses to ocean currents beneath.
Jupiter’s moon Europa could glow in the dark
Ice composition may also affect the habitability of the ice-ocean interface. Even if life is only present deeper down in Europa’s ocean, the team suggests that biosignatures could be trapped between accumulated frazil ice crystals on the underside of its ice sheet.
These insights could provide important guidance for NASA’s Europa Clipper mission, scheduled for launch in 2024. Using radar, the spacecraft will search beneath the moon’s icy shell – with the goal to determine whether its liquid ocean could harbour conditions suitable for life to emerge.
The study is described in Astrobiology.
