Iron crystals can form at the boundary between the core and mantle of some celestial bodies, and experts simply refer to these crystals as iron ice. Just as snowflakes that form when the cloud level in our atmosphere cools melt when they descend to the lower, warmer levels, iron ice also melts when it falls toward warmer regions within celestial bodies. Iron snowfall causes periodic currents in the core, which are thought to allow a magnetic field to develop on the affected body.
French research group A Geophysical Research Letters I reported in a journal about a theory that could explain Ganymede's magnetic field. Although Ganymede is the largest moon in the solar system, even in the time that has passed since its formation, its core cannot remain warm enough to form a magnetic field through melting. The main actor in this process is Jupiter's gravity, whose tidal heating keeps the moon's core in a molten state and contributes to its magnetism. However, the details of what goes on inside the nucleus that makes this possible are not entirely clear. It is important to know that in the case of such smaller celestial bodies, the solidification of the nucleus occurs from the outside in, and in the case of the Earth's core from the inside out. This is a fundamental difference.
The researchers experimentally investigated whether the formation of iron ice and its role in magnetism is true. Naturally, we cannot create the conditions that prevail in the core of a celestial body, so experts turned to a similar process to model the processes: simple distilled water. They used a container that could be cooled from the bottom. The processes taking place in the core of the celestial body take place in the opposite direction (from bottom to top), but are identical in other respects. To prevent the water from freezing in the tank, a layer of water with a very high salt content was placed at the bottom of the tank, which handles the temperature but cannot freeze because of the salt. (The salt water was able to stay at the bottom because of its high density.)
On the left side of the figure is the process occurring in Ganymede, and on the right side is the process attempted in the experiment—only the direction is different.
Source: Geophysical Research Letters
Thus, ice crystals begin to form at the surface separating the two types of water, and they do not stick to the tank, but rather float and rise into the warm water, where they dissolve. This allowed them to monitor the entire process and its impact on the entire system.
During experiments, it was found that this process occurs in cycles. The rising crystals created a current, which combined with the latent heat of crystal formation, heated the bottom layer of water. As a result, crystal formation stopped and only started again when the bottom layer of water cooled enough again due to cooling from below. These operations were repeated in the tank approximately according to the measurements. Every 23 minutes. (We can also watch a sped-up video recording of the process, but alas Only after downloading Visible.)
Based on model calculations, a similar process occurs in celestial objects with molten iron cores, where the formation of iron ice creates currents in the molten iron core, which are then responsible for creating the magnetic field. These processes are periodic, similar to those that occur in laboratory experiments, so that magnetism also appears at approximately regular intervals on the given celestial body.
The magnetism formed by these processes on the celestial body changes in time and space. The magnetic fields of Ganymede and Mercury could also be generated in this way, but similar processes prevailed in the case of many other celestial bodies, such as our Moon. This doesn't answer all the detailed questions, but we've taken an important step toward understanding the magnetism of small, rocky planets and their moons.
