Planetary pressure cooker: how gas and rock give birth to oceans express
The most common type of planet in our Galaxy — rocky worlds around other stars, or exoplanets, shrouded in the simplest gas hydrogen. Their innards remained a mystery until scientists recreated the conditions of the interior in the lab. It turns out that inside, such a planet works like a giant pressure cooker: at heat above 4000°C and monstrous pressure, hydrogen penetrates molten rock, stealing oxygen from iron oxides. Water is born, and the freed iron gathers into heavy droplets, like grains in a boiling broth. In one cooking cycle, a mass of water comparable to dozens of Earth's oceans accumulates, and simultaneously heat is released that keeps the planet warm from within for a long time. This underground kitchen rewrites the history of water in the Universe: oceans can appear not only from comet impacts, but also directly from the primordial gas that enveloped the newborn world. And if our young Earth once wore a hydrogen coat, its own global ocean could have been brewed in the same rocky cauldron.
Original abstract
as written by the authors arXiv:2511.01351
The most abundant type of planet discovered in the Galaxy has no analogue in our Solar System and is believed to consist of a rocky interior with an overlying thick H2 dominated envelope. Models have predicted that the reaction between the atmospheric hydrogen and the underlying magma ocean can lead to the production of significant amounts of water. The models suffer however from the current lack of experimental data on the reaction between hydrogen and silicate melt at high pressures and temperatures. Here we present novel experimental results designed to investigate this interaction. Laser heating diamond anvil cell experiments were conducted between 16 and 60 GPa at temperatures above 4000 K. We find that copious amounts of hydrogen dissolve into the silicate melt with a large dependence on temperature rather than pressure. We also find that the reduction of iron oxide leads to the production of significant amounts of water along with the formation of iron-enriched blebs. Altogether, the results predict that the typical processes attending planet formation will result in significant water production with repercussions for the chemistry and structure of the planetary interior as well as the atmosphere.
If the young Earth had managed to hold onto its hydrogen envelope, it would have 'cooked' its own global ocean — no comets needed.