Scientists reveal the ancient history of the solar system

Artist’s impression of the early solar system. Credit: Tobias Stierli / Flaeck / PlanetS

The Chaotic Early Stages of the Solar System

An international team of researchers led by ETH Zurich and the National Research Center PlanetS has recreated the ancient history of several asteroids more accurately than ever. Their findings suggest that the early solar system was more chaotic than previously thought.

Before the formation of Earth and other planets, the young sun was surrounded by cosmic gas and dust. Slowly, rock shards of varying sizes formed from the dust over the millennia. Many of them became building blocks for subsequent planets. Others did not become planets and continue to orbit the sun today, such as asteroids in the asteroid belt.

Iron samples from the cores of asteroids that fell to Earth as meteorites were analyzed by researchers from ETH Zurich and the National Research Center (NCCR) PlanetS in collaboration with an international team. In doing so, they revealed some of their early past when the planets formed. Their results have just been published in the journal Natural astronomy.

Witnesses to the early solar system

“Previous scientific studies have shown that asteroids in the solar system have remained relatively unchanged since their formation billions of years ago,” says Alison Hunt, lead author of the study and researcher at ETH Zurich and the NCCR PlanetS. “So they are an archive in which the conditions of the early solar system are preserved,” says Hunt.

Iron meteorite sample

One of the iron meteorite samples analyzed by the team. Credit: Aurelia Meister

But to unlock this archive, researchers had to carefully prepare and examine the extraterrestrial material. The team took samples from 18 different iron meteorites, which were once part of the metallic cores of asteroids. To perform their analysis, they had to dissolve the samples to be able to isolate the elements Palladium, Silver and Platinum for their detailed analysis. Using a mass spectrometer, they measured the abundances of different isotopes of these elements. Isotopes are separate atoms of given elements, in this case palladium, silver and platinum, which all share the same number of protons in their nucleus but vary in number of neutrons.

During the first million years of our solar system, the metallic cores of asteroids were heated by the radioactive decay of isotopes. As they began to cool, a specific silver isotope produced by radioactive decay began to accumulate. By measuring current silver isotope ratios in iron meteorites, the researchers were able to determine both when and how quickly the asteroid cores had cooled.

The results showed that the cooling was rapid and likely due to severe collisions with other bodies, which broke through the asteroids’ insulating rocky mantle and exposed their metallic cores to the cold of space. While the rapid cooling had been indicated by earlier studies based on silver isotope measurements, the timing was unclear.

“Our additional measurements of the abundance of platinum isotopes allowed us to correct the measurements of silver isotopes for distortions caused by cosmic irradiation of the samples in space. We were therefore able to date the timing of the collisions more accurately than ever before,” Hunt reports. “And to our surprise, all of the asteroid nuclei we examined were exposed almost simultaneously, within 7.8 to 11.7 million years. after the formation of the solar system”, explains the researcher.

The near-simultaneous collisions of the different asteroids told the team that this period must have been a very unstable phase of the solar system. “Everything seemed to fall apart at that point,” Hunt says. “And we wanted to know why,” she adds.

From the laboratory to the solar nebula

The team examined different causes by combining their results with those of the latest and most sophisticated computer simulations of the development of the solar system. Together, these sources could narrow the possible explanations.

“The theory that best explained this first energetic phase of the solar system indicated that it was mainly caused by the dissipation of the so-called solar nebula,” co-author of the study, member of the PRN PlanetS and professor of cosmochemistry at ETH Zurich, Maria Schönbächler explains. “This solar nebula is the remnant of gas that was left behind by the cosmic cloud from which the Sun was born. For a few million years it still circled around the young Sun until it was swept away by winds and solar radiation,” explains Schönbächler.

While the nebula was still there, it slowed down objects orbiting the Sun – the same way air resistance slows down a moving car. After the nebula disappeared, the researchers suggest that the lack of gas trail allowed the asteroids to speed up and collide with each other – like bumper cars that were put into turbo mode.

“Our work illustrates how improvements in laboratory measurement techniques allow us to infer key processes that took place in the early solar system – such as the likely time when the solar nebula disappeared. Planets like Earth were still in the process of to be born at this time. Ultimately, this can help us better understand how our own planets were born, but also give us insight into others outside our solar system,” concludes Schönbächler.

Reference: “Solar Nebula Dissipation Constrained by Impacts and Core Cooling in Planetesimals” by Alison C. Hunt, Karen J. Theis, Mark Rehkämper, Gretchen K. Benedix, Rasmus Andreasen and Maria Schönbächler, May 23, 2022 , Natural astronomy.
DOI: 10.1038/s41550-022-01675-2

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