A large part of the ice crust of Jupiter's satellite Europa may be formed from ice crystals formed in the supercooled water column beneath the ice cover.
Jupiter's moon Europa is probably the most interesting celestial body in the solar system for astrobiologists. Europa is slightly smaller than the Earth's Moon, but unlike it, it has a surface of water ice, under which an ocean of liquid water about a hundred kilometres deep is concealed.
The presence of an ocean beneath Europa's ice can be considered fairly established. There are almost no meteoritic craters on the surface but cracks, fractures and regions of 'chaotic landscape' composed of fractured, jumbled and frozen ice blocks are abundant.
The subsoil of Europa receives a powerful tidal warming (like that of neighbouring satellite Io, although to a lesser extent), which means that volcanoes must erupt on the ocean floor, supplying the ocean with nutrients and energy sources - the necessary conditions for habitability.
The surface of Europe is frozen at minus 160 to minus 220 degrees Celsius and hence the ice shell is at least several kilometres thick. The study of the subglacial ocean will be quite a challenge, and at the first stage, scientists are going to send the Europa Clipper apparatus to the Jupiter system, which will study Europa and other satellites of Jupiter through numerous close flyovers.
One of its goals will be to study Europa's icy shell using radar. The capabilities of this method depend quite significantly on the composition of the ice. An admixture of salt would make it difficult for radio waves to penetrate, but if the shell is not very thick, and consists of pure ice, it may be possible to shine through.
In a recent study, scientists at the University of Texas, led by Natalie S. Wolfenbarger, have suggested that the shell may contain less salt than originally thought and is caused by underwater snow that can travel upward in the European ocean.
On Earth, the ice cover over the seas grows mainly due to water freezing from below, at the ice-water interface. In the Antarctic seas, another mechanism has been observed to increase ice thickness - a 'snow' of supercooled water accumulating underneath the ice.
What phenomena might cause such an underwater "snowfall"? The freezing point of the water decreases under pressure - by about one degree for every 130 atmospheres. This corresponds to a depth increase of 1 300 m in the Earth's oceans and about 10 km beneath the ice of Europe. At the bottom of the Mariana Trench and the ocean of Europe, the pressures are almost the same - the depth of the former is ten times less, but the gravity on Earth is seven times greater than on Europe. Therefore, the salt water at the very bottom freezes at a temperature almost ten degrees below zero.
In addition, water is subject to adiabatic heating and cooling - a change in temperature with pressure surges and no heat exchange with the environment. Due to its lower compressibility, its temperature does not change as much as that of air in pumps and compressors, but with large changes in pressure this process becomes noticeable: the coefficient is about one degree per 400 atmospheres (4 kilometres on Earth, 30 kilometres in Europe).
Large volumes of water, when surfacing or sinking, do not have time to mix with the surrounding waters and change their temperature, and water rising from great depths can become supercooled for two reasons at once: by adiabatic cooling during decompression, and the initial temperature, if it was below freezing at the surface. Part of the supercooled water freezes, forming a very clear needle ice called a snow storm. This ice floats up and joins the ice column near the surface.