Iron core, silicate mantle, crust and oceans above it. This is the order of the Earth. For a long time, this model was transferred to the planets of other star systems. But the picture turned out to be much more complicated. Scientists from Switzerland and the USA modeled the distribution of water in the bowels of super-Earths and sub-Neptuns and found that the main volume is not on the surface at all.

Astronomers often discover planets very close to their stars. These are so-called lava worlds that have not completely cooled to form a hard, sweet silicate like Earth. Water dissolves well in magmatic melts, unlike carbon dioxide, which is quickly released and rises into the atmosphere. What happens to it next?

“It takes time for the iron core to form. Most of the metal is initially present as droplets in the magmatic melt. Water joins them and sinks towards the core. The iron droplets act as an elevator for the water,” explains Carolina Dorn from the Institute for Nuclear Physics and Astrophysics at the Swiss Federal Institute of Technology in Zurich.

He published an article in a journal together with colleagues from Princeton University (USA). Nature AstronomyHe modeled the behavior of water in the bowels of planets.

Recently, researchers have shown that most of the water is located in the Earth’s core: according to some data, 37 percent, according to others, up to 73 percent. In the core conditions, water has a large partition coefficient between iron and silicates. formation (pressure – up to 135 gigapascals, temperature – 4200 Kelvin).

Because super-Earths and sub-Neptunians are much larger, the core develops at much higher pressures and temperatures: up to 1,400 gigapascals and 14,000 kelvins, respectively. To estimate the volume of water hidden in the subsoil, the authors used molecular dynamics modeling combined with thermodynamic integration, relying solely on the laws of nature, without relying on experimental data, to calculate the partition coefficient. Calculations were made for four scenarios for the planet’s interior.

It turns out that in planets with a mass of six Earths or less, water is stored mainly in the mantle, and in larger ones (most of which have been discovered) in the core. Although the core is smaller in volume than the mantle, under certain conditions it can absorb 70 times more water than iron silicates. Due to the enormous pressure in the core, water does not form H molecules₂It exists as hydrogen and oxygen. For example, a planet with a mass fraction of nine Earths at an equilibrium temperature of 1,000 Kelvin and a mass fraction of 50 percent would store more than half of its water in the core, 41 percent in the mantle, and only three percent on the surface.

New information on the distribution of water on planets directly affects the interpretation of astronomical data. Observations using telescopes allow us to estimate the mass and size of planets. However, water is often reduced for calculations. Its amount can be underestimated by a factor of ten. According to a new study, planets are much more saturated with water than previously thought.

For example, this approach could explain the extremely low density of planet 55 Cancer e, which has a radius nearly twice that of Earth. Even though water is lost from the surface and mantle, the core is still significantly water-rich, suggesting that the planet consists of water-rich material beyond the snow line.