According to the most widely accepted scientific theory, our Solar System formed from a nebula of dust and gas roughly 4.56 billion years ago (aka. Nebula Theory). It began when the nebula experienced gravitational collapse at the center, fusing material under tremendous pressure to create the Sun. Over time, the remaining material fell into an extended disk around the Sun, gradually accreting to form planetesimals that grew larger with time. These planetesimals eventually experienced hydrostatic equilibrium, collapsing into spherical bodies to create Earth and its companions.
Based on modern observations and simulations, researchers have been trying to understand what conditions were like when these planetesimals formed. In a new study, geologists from the California Institute of Technology (Caltech) combined meteorite data with thermodynamic modeling to better understand what went into these bodies from which Earth and the other inner planets formed. According to their results, the earliest planetesimals have formed in the presence of water, which is inconsistent with current astrophysical models of the early Solar System.
The research was conducted in the laboratory of Paul Asimow, the Eleanor and John R. McMillan Professor of Geology and Geochemistry at Caltech. The team was led by assistant professor Damanveer Grewal, the leader of the CosmoGeo Lab at Arizona State University (ASU) and a former postdoctoral scholar with the Division of Geological and Planetary Sciences at Caltech. Grewal and Asimow were joined by planetary scientists from the Massachusetts Institute of Technology (MIT), the University of California Los Angeles (UCLA), and Rice University.
Sample from a rare meteorite family revealing that its parent planetesimal had a layered structure with a molten core and solid crust (similar to Earth). Credit: Carl Agee, Institute of Meteoritics UNM/MIT
Grewal and his colleagues specialize in studying the chemical signatures of iron meteorites to gather information about the early Solar System. These meteorites are remnants of the metallic cores of the first planetesimals that did not accrete to form a planet and continue to orbit within our Solar System today. Over many eons, some of these objects fell into Earth’s gravity well and ultimately crashed to the surface. The chemical composition of these meteorites is of particular interest since it reveals a great deal about the environments in which they formed.
For one thing, the composition of planetesimals can reveal whether they (and Earth) formed closer to or farther away from the Sun. If the former scenario were the case, cooler conditions would have allowed Earth to retain water ice as a building block. If the latter is correct, Earth would have formed dry and obtained its water by some other means later on, which is what current astrophysical models suggest. According to these models, water was delivered to the inner Solar System via comets and asteroids billions of years ago, a period known as the Late Heavy Bombardment.
While water is no longer present in these meteorites, scientists can infer its existence from the presence of other elements. These include iron oxide (FeO), which occurs when iron is oxidization by exposure to water. A sufficient excess of water will drive the process further, creating ferric oxide (Fe2O3) and ferric oxyhydroxide, or FeO(OH) – the ingredients of rust. While the earliest planetesimals would have lost all traces of iron oxide long ago, Grewal and his team were able to determine how much was present by examining the metallic nickel, cobalt, and iron contents of these meteorites.
These three elements should be present in roughly equal ratios relative to other materials in the meteorite, which means that any “missing” iron would have been depleted through oxidation. As Asimow explained in a Caltech news release:
“Iron meteorites have been somewhat neglected by the planet-formation community, but they constitute rich stores of information about the earliest period of Solar System history, once you work out how to read the signals. The difference between what we measured in the inner solar system meteorites and what we expected implies an oxygen activity about 10,000 times higher.”
Artist concept of Earth during the Late Heavy Bombardment period. Credit: NASA’s Goddard Space Flight Center.
The team’s results indicate that meteorites believed to have originated in the inner Solar System had roughly the same amount of missing iron as meteorites from the outer Solar System. This suggests that both groups formed in a part of the Solar System where conditions were cool enough for water. It further implies that planets accreted water from the beginning, which could have profound implications for theories of how life emerged on Earth. “If water was present in the early building blocks of our planet, other important elements like carbon and nitrogen were likely present as well,” said Grewal. “The ingredients for life may have been present in the seeds of rocky planets right from the start.”
This represents a significant challenge for our current models for how the Solar System formed and evolved, which could indicate that conditions in the early inner Solar System were much cooler than previously thought. The results could also mean that Earth and its fellow rocky planets formed farther from the Sun and gradually migrated to their current orbits. However, as Asimow acknowledged, there is a degree of uncertainty when it comes to the study of ancient planetesimals, which means the results may not contradict current astrophysical models:
“However, the method only detects water that was used up in oxidizing iron. It is not sensitive to excess water that might go on to form the ocean. So, the conclusions of this study are consistent with Earth accretion models that call for late addition of even more water-rich material.”
Their study, titled “Accretion of the earliest inner Solar System planetesimals beyond the water snowline,” recently appeared in Nature Astronomy. Their research was made possible thanks in part to funding provided by NASA and through a Barr Foundation Postdoctoral Fellowship.
Further Reading: Caltech, Nature Astronomy
