Asteroids reveal the chaotic childhood of our solar system

The early solar system was more chaotic than previously thought, according to a new, more accurate reconstruction of the ancient history of several asteroids.

Before the formation of Earth and other planets, the young sun was still surrounded by cosmic gas and dust. Over the millennia, rock fragments of varying sizes have formed from the dust. Many of them became building blocks for later planets. Others did not become part of a planet and still orbit the sun today, for example as asteroids in the asteroid belt.

The researchers analyzed iron samples from the cores of such asteroids that landed on Earth as meteorites. In doing so, they unveiled some of their ancient history from when the planets formed.

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

The chaos of the 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, a researcher at ETH Zurich and the National Center of Competence in Research PlanetS (NCCR PlanetS) and lead author. paper in natural astronomy. “So they are an archive, in which the conditions of the early solar system are preserved.”

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 radioactive decay of isotopes heated the metallic cores of asteroids. 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 platinum isotope abundance allowed us to correct silver isotope measurements for distortions caused by cosmic irradiation of samples in space. So we were able to date the timing of the collisions more precisely than ever before,” says Hunt.

“And to our surprise, all of the asteroid nuclei we examined had been exposed almost simultaneously, within 7.8 to 11.7 million years of the formation of the solar system.”

The almost simultaneous collisions of the various asteroids indicated that this period must have been a very unstable phase of the solar system. “Everything seems to have fallen apart at that point,” Hunt says. “And we wanted to know why.”

Planetary births

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,” says co-author Maria Schönbächler, professor of cosmochemistry at ETH Zurich and fellow of the PRN PlanetS.

“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 revolved around the young sun until it was carried away by winds and solar radiation.

While the nebula was still there, it slowed down objects orbiting the sun, much like air resistance slows down a moving car. After the nebula disappeared, the researchers suggest, the lack of a gas trail allowed the asteroids to speed up and crash into 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 probable time when the solar nebula left,” says Schönbächler.

“Planets like Earth were still being born at that time. Ultimately, this can help us better understand how our own planets came to be, but also give us insight into others outside of our solar system.

Source: ETZ Zürich

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