Planetzymals, sometimes described as the embryonic seeds of planets, were historically considered too dry to supply Earth with water. A Nature article discusses what these ancient bodies reveal about water delivery to our planet. Planetzymals formed in the early solar system from the protoplanetary disk. As dust settled and coalesced, bodies reaching a radius of about one kilometer began to move through the disk with minimal gas resistance. This allowed them to attract nearby material more readily, gradually growing in size. When they expanded to roughly one to three kilometers across, their rocks experienced metamorphism, melting, and differentiation, processes akin to the deep interior changes seen in mature planets. One hypothesis proposed that such minor planets, after melting, could break apart and fall to Earth, potentially delivering water as they impacted our world.
A team from the University of Maryland examined seven achondrite meteorites that landed on Earth. These meteorites are believed to be fragments of planetary material, characterized by evidence of intense compression and high temperatures. Some of the seven samples originated from the dry interior regions of the solar system, while others welded together in the outer zones where stable ice could exist. The rocks studied proved to be exceptionally dry, containing no more than two million parts of water by mass. This stark dryness challenges the idea that outer solar system objects carried abundant water into Earth’s early history.
Sune Nielsen, one of the researchers involved in the study, notes that while outer solar system bodies are known to show signs of differentiation, it was long assumed that they contained substantial amounts of water due to the presence of ice in their formation zones. The new findings from the analysis of these meteorites suggest otherwise. The data indicate that once minor planets melt, they can be left devoid of water, a surprising outcome that reshapes our understanding of how Earth acquired its oceans. The implications are significant for models of planetary formation and the origin of Earth’s hydrosphere. The research highlights that water delivery likely depended on different sources or processes than previously imagined, potentially pointing to other mechanisms or stages in the planetary accretion timeline.
Consequently, the conclusion drawn from these results is that neither small planets that became meteorites nor tiny asteroids detached from them could be credible sources of Earth’s water. Instead, if water was delivered to Earth, it would have to originate from unaltered bodies that retained their original composition until impact. The exact chemical nature of these potential water-bearing materials remains a topic for further investigation. The study contributes a crucial data point in the ongoing quest to understand how Earth acquired its oceans and how water interacts with planetary differentiation in the early solar system.