Ancient asteroids show early solar system was more chaotic than previously thought

(ORDO NEWS) — There is no doubt that young solar systems are chaotic places. Cascading collisions defined our young solar system, with rocks, boulders and planetesimals colliding repeatedly.

A new study based on pieces of asteroids that crashed into Earth sets a time frame for this chaos.

Astronomers know that asteroids have remained virtually unchanged since their formation in the early solar system billions of years ago.

They are like stone time capsules containing the scientific data of that important era, as the differentiated asteroids had mantles to protect their interiors from cosmic weathering.

But not all asteroids remained intact.

Over time, repeated collisions ripped the insulating mantles from their iron cores and then shattered some of them to pieces.

Some of these pieces fell to Earth. Stones that fell from space were of great interest to people and in some cases were a valuable resource; King Tut was buried with a dagger made from an iron meteorite, and the Inuit in Greenland had been making tools from an iron meteorite for centuries.

Scientists have shown great interest in iron meteorites because of the information they contain.

A new study based on iron meteorites – fragments of the core of large asteroids – studied isotopes of palladium, silver and platinum. By measuring the number of these isotopes, the authors were able to more tightly limit the timing of some events in the early solar system.

The work “Scattering of the solar nebula limited by impacts and cooling of the core in planetesimals” was published in the journal Nature Astronomy. The lead author is Alison Hunt of ETH Zurich and the National Center for Research Competence (NCCR) PlanetS.

“Previous scientific studies have shown that asteroids in the solar system have remained relatively unchanged since their formation, billions of years ago,” Hunt said. “So they are an archive that has preserved the conditions of the early solar system.”

The ancient Egyptians and Inuit knew nothing about elements, isotopes and decay chains, but we do. We understand how different elements break down in chains into other elements, and we know how long it takes.

One of such decay chains underlies this work: the short-lived 107Pd-107Ag decay system. This chain has a half-life of about 6.5 million years and is used to detect the presence of short-lived nuclides from the early solar system.

The researchers collected samples of 18 different iron meteorites that were once part of the iron cores of asteroids.

They then isolated palladium, silver and platinum from them and measured the concentrations of various isotopes of these three elements using a mass spectrometer. The specific isotope of silver is of crucial importance in this study.

During the first few million years of the solar system’s history, decaying radioactive isotopes heated the metal cores in asteroids. As the isotopes cooled and decayed, the silver isotope (107Ag) accumulated in the nuclei. The researchers measured the ratio of 107Ag and other isotopes and determined how quickly and when the asteroid cores cooled.

This is not the first time researchers have studied asteroids and isotopes in this way. But previous studies did not take into account the influence of galactic cosmic rays (GCR) on the isotope ratio.

GCRs can disturb the process of neutron capture during decay and reduce the amount of 107Ag and 109Ag. These new results are corrected for GCR interference by platinum isotope counts.

“Our additional measurements of platinum isotopic abundance allowed us to correct silver isotope measurements for distortions caused by cosmic irradiation of samples in space. Thus, we were able to determine the timing of collisions more accurately than ever before,” Hunt said.

“And to our surprise, all of the asteroid cores we studied were irradiated almost simultaneously, between 7.8 and 11.7 million years after the formation of the solar system,” Hunt said.

In astronomy, 4 million years is a short time span. During this short period, all measured asteroids exposed the core, that is, collisions with other objects deprived them of their mantle. Without an insulating shell, the cores were cooled simultaneously.

Other studies have shown that the cooling was rapid, but they have not been able to pinpoint the time frame as clearly.

For the asteroids to have the isotope ratios the team found, the solar system would have to be a very chaotic place, with a period of frequent collisions that stripped the asteroids of their mantle.

“It was like everything was falling apart at the time,” says Hunt. “And we wanted to know why,” she adds.

Why did a period of such chaotic clashes arise? There are several options, the article says.

The first possibility is related to the giant planets of the solar system. If they migrated or were somehow unstable at the time, they could reorganize the inner solar system, disrupt the movement of small bodies such as asteroids, and cause a period of increased collisions. This scenario is called the Nice model.

Another possibility is gas being drawn into the solar nebula.

When the Sun was a protostar, like other stars, it was surrounded by a cloud of gas and dust called the solar nebula. Asteroids were in this disk, and over time, planets formed in the same place. But in the first few million years of the solar system’s existence, the disk changed.

At first, the gas was dense, which slowed down the movement of objects such as asteroids and planetesimals due to gas drag. But as the Sun flared up, it produced more solar wind and radiation.

The solar nebula was still there, but the solar wind and radiation pressed against it, dispersing it. As it dissipated, it became less dense, and the resistance of objects decreased.

Without the cushioning effect of the dense gas, the asteroids accelerated and collided with each other more often.

According to Hunt and her colleagues, it is the decrease in gas resistance that is the reason for this.

“The theory that best explained this energetic early phase of the solar system was that it was caused primarily by the dissipation of the so-called solar nebula,” said study co-author Maria Schönbechler.

“The solar nebula is the remnants of gas left over from the cosmic cloud from which the Sun was born. For several million years, it orbited the young Sun until it was blown away by solar winds and radiation,” Schoenbechler said.

“Our work illustrates how improvements in laboratory measurement methods allow us to infer key processes that took place in the early solar system, such as the probable time by which the solar nebula disappeared.

At that time, planets like Earth were still in the process of birth. Ultimately, this could help us better understand how our own planets were born, as well as give us insight into other planets outside of our solar system,” Schoenbechler concluded.


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