Navid Marvi/Carnegie Institute
Formative interactions between magma oceans and early atmospheres may be the key to understanding the origin of water on planets — without it, we wouldn’t be here, and we’re beginning to understand it better.
According to new research published in the journal Nature by Francesca Miozzi and Anat Shahar, from the Carnegie Institute, the most abundant type of planet in our Galaxy could be rich in liquid water due to formative interactions between magma oceans and early atmospheres during its early years.
Of the more than 6000 known exoplanets in the Milky Way, the so-called sub-Neptunes are the most common. They are smaller than Neptune and more massive than Earth and are thought to havem rocky interiors with thick atmospheres dominated by hydrogen.
This makes them good candidates for testing ideas about how rocky planets like ours got an abundance of water – which was critical to the emergence of life on Earth and is considered a fundamental component of planetary habitability.
“Our increasing knowledge about the vast diversity of exoplanets has allowed us to imagine new details about the early stages of the formation and evolution of rocky planets,” explained Miozzi. “This opened the door to consideration of a new source of water supply for the planets. – a mystery long debated among terrestrial and planetary scientists – but experiments designed with this goal in mind were absent.”
The work focuses particularly on trying to link observations of planetary atmospheres to the evolution and dynamics of their rocky bodies.
Previous research into mathematical modeling demonstrated that interactions between atmospheric hydrogen and iron-bearing magma oceans during planet formation can produce significant amounts of water. However, until now, extensive experimental testing of this proposed source of planetary water had not been carried out.
Miozzi and Shahar led an international team of researchers from IPGP (Institut de Physique du Globe de Paris) and UCLA (University of California, Los Angeles) to create the conditions under which these interactions between hydrogen – which represents the early planetary atmosphere – and iron-rich molten silica – which represents the forming magmatic ocean – would occur on a young planet.
They got it compressing samples to around 600,000 times atmospheric pressure (60 gigapascals) and heating them to more than 4000º C.
Its experimental environment mimics a critical phase in the evolutionary process of a rocky planet. Such bodies are formed from the disk of dust and gas that surrounds a young star in the period after its birth.
This material is accreted into bodies that collide with each other and become larger and hotter, eventually merging into a vast ocean of magma. These young planets are often surrounded by a thick envelope of molecular hydrogen, H2, what can act as a “thermal blanket”, maintaining the magma ocean for billions of years before cooling.
“Our work provided the first experimental evidence of two critical processes in early planetary evolution,” said Miozzi. “We showed that a large amount of hydrogen is dissolved in the melt and that significant quantities of water are created by the reduction of iron oxide by molecular hydrogen.”
Taken together, these findings demonstrate that large amounts of hydrogen can be stored in the magmatic ocean while water is formed. This has important implications for physical and chemical properties of the planet’s interior, with potential effects also on the development of the nucleus and atmospheric composition.
“The presence of liquid water is considered critical for planetary habitability,” concluded Shahar. “This work demonstrates that large amounts of water are created as a natural consequence of planet formation. It represents a major step forward in the way we think about the search for distant worlds capable of harboring life.”
