Thought to lie in abundance across our galaxy, water worlds are compelling candidates for hosting extraterrestrial life. These Neptune-like exoplanets, identified to be around two to four times the size of Earth, are believed to be covered in a thick layer of water ice, hundreds to thousands of kilometers deep, underneath which lies a rocky mantle. Questions still surround the exact composition of these common water worlds, as they differ greatly from terrestrial planets like Earth.
One such mystery is at the boundary between water and rock. In general, these components are thought to exist in separate layers. But the extreme temperature and pressure at this partition led an international team of researchers headed by Arizona State University (ASU) to wonder whether the substances were altered in any way by these conditions.
To test this on Earth, they employed a rather sparkling piece of equipment to help: diamond cells. Two of these gem-quality single-crystal diamonds were shaped into anvils and then positioned opposite each other. In between them, a sample of silica (a component of rock) that had been immersed in water was compressed to a high pressure. The sample was then heated using laser beams to over a few thousand degrees Fahrenheit, simulating the conditions in water world interiors.
X-ray measurements were then taken to probe the sample, the results of which shocked the team. Under these extreme conditions, the sample yielded a completely new solid phase with substantial amounts of silicon and hydrogen in oxide form, suggesting that the rock-water boundary in water-rich exoplanets is not as clear-cut as once thought.
“Originally, it was thought that water and rock layers in water-rich planets were well-separated,” Caroline Nisr, lead author and research scientist at ASU, said in a statement. “But we discovered through our experiments a previously unknown reaction between water and silica and stability of a solid phase roughly in an intermediate composition. The distinction between water and rock appeared to be surprisingly 'fuzzy' at high pressure and high temperature.”
Their study, published in the journal Proceedings of the National Academy of Sciences, is one of the first mineralogy lab studies for water-rich exoplanets and has raised new questions about the chemical composition and geochemical cycles of these water worlds.
“Studying the chemical reactions and processes is an essential step toward developing an understanding of these common planet types,” co-author Dan Shim, of ASU's School of Earth and Space Exploration, remarked.