05 Oct Floating snow and upside-down buoyancy: the secret life of liquids
The 2022 SCINEMA International Science Film festival entry, The weird physics of upside-down buoyancy, explored boats floating upside down on a liquid suspended on a bed of air. The following article by Cosmos journalist Clare Kenyon appeared on cosmosmagazine.com and looked closely at a related aspect that we thought you would find interesting.
There’s been a lot of buzz around Jupiter’s moon, Europa lately.
But, before Europa Clipper is shot off onto its five-and-a-half-year journey, scientists need to know more about the likely make-up of the moon – in particular the characteristics of its icy crust – so they can make sense of the returning data better.
New research from the University of Texas at Austin suggests that snow floats upwards, through the salty brine to rest on the underside of Europa’s ice crust. If you could somehow go for a swim under Europa’s crust, snow would rise up past you and settle on the solid shelf above.
Europa is Jupiter’s sixth closest moon and is about the size of Earth’s own natural satellite. Underneath an icy crust of 15–25 km thick, lies a briny ocean containing more water than all of Earth’s oceans combined, kept from freezing by the constant gravitational tugging (known as tidal forces) from Jupiter.
But why should this snow float up instead of fall?
It’s just one of the secrets in the playbook of liquids – supercooled liquids, more specifically – and it also occurs here on Earth.
In order to freeze, a liquid needs to nucleate around a seed crystal. Supercooled fluids are liquids cooled to below their freezing point but prevented from forming seed crystals by reducing impurities, adding “antifreeze” components or by physical disruption. Aircraft often fly through supercooled water droplets which rapidly crystallise on the aircraft, occasionally causing aircraft or instrument problems.
Frazil ice forms in supercooled liquids (such as under ice sheets) when a small ice flake forms and then settles on the underside of the ice sheet. In some places on Earth, such as large lakes in the US, the formation of frazil ice can result in restriction or blockage of flow through intake pipes as the crystals latch on to the structures.
Both congelation and frazil ice will drive up the purity of Europa’s ice shelf, as they produce ice much less salty than the seawater from which it is derived. The increased likelihood of frazil ice on Europa is important as it will define many of the moon’s physical and dynamical characteristics – how heat is transported and how the crust moves and responds to forces, for instance – as well as how potentially habitable it could be.
Another trick in the secret life of liquids’ playbook is upside-down buoyancy – and it doesn’t even require the liquids to be supercooled, just a good vertical shake is enough.