The early solar system was total chaos

There is no doubt that young solar systems are chaotic places. Cascading collisions defined our young solar system when rocks, boulders, and planetesimals repeatedly collided. A new study based on pieces of asteroids that crashed into Earth puts a timeline to some of this chaos.

Astronomers know that asteroids have remained essentially unchanged since their formation at the start of the solar system billions of years ago. They’re like rocky time capsules that hold scientific clues to that important epoch, because differentiated asteroids had mantles that protected their interiors from the weather of space.

But not all asteroids remained whole. Over time, repeated collisions ripped the insulating mantles from their iron cores, then shattered some of those cores into pieces. Some of these pieces fell to Earth. Rocks that fell from space were of great interest to people and were a valuable resource in some cases; King Tut was buried with a dagger made from an iron meteorite, and the Inuit of Greenland made tools from it. an iron meteorite for centuries.

Scientists are keenly interested in iron meteorites because of the information they contain. A new study based on iron meteorites – which are fragments of the core of larger asteroids – looked at the isotopes of palladium, silver and platinum. By measuring the amounts of these isotopes, the authors could more tightly restrict the timing of certain events to the early solar system.

The paper “Solar nebula dissipation constrained by impacts and core cooling in planetesimalsis published in Nature Astronomy. The lead author is Alison Hunt from ETH Zurich and the National Center for Research (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. “They therefore constitute an archive in which the conditions of the early solar system are preserved.”

The ancient Egyptians and Inuit knew nothing about elements, isotopes and decay chains, but we did. We understand how different elements decay in chains into other elements, and we know how long it takes. One such decay chain is central to this work: the short-lived 107Pd–107Ag decay system. This chain has a half-life of approximately 6.5 million years and is used to detect the presence of nuclides of the early solar system.

The researchers collected samples from 18 different iron meteorites that were once part of the iron cores of asteroids. Then they isolated palladium, silver and platinum there and used a mass spectrometer to measure the concentrations of different isotopes of the three elements. A particular isotope of silver is essential in this research.

One of the iron meteorites that the researchers analyzed in their study.  Image credit: Aurélia Meister
One of the iron meteorites that the researchers analyzed in their study. Image credit: Aurélia Meister

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

This is not the first time that researchers have studied asteroids and isotopes in this way. But previous studies did not take into account the effects of galactic cosmic rays (GCR) on isotopic ratios. GCRs can interfere with the neutron capture process during decay and can decrease the amount of 107 Ag and 109 Ag. These new results are corrected for GCR interference by also counting platinum isotopes.

“Our additional measurements of platinum isotope abundances 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,” Hunt reported. “And to our surprise, all of the asteroid nuclei that we looked at were exposed almost simultaneously, within 7.8 to 11.7 million years after the solar system formed,” Hunt said.

Artist's impression of an asteroid collision in the belt between Mars and Jupiter.  Credit: NASA/JPL-Caltech
Artist’s impression of an asteroid collision in the belt between Mars and Jupiter. Credit: NASA/JPL-Caltech

A period of four million years is short in astronomy. During this brief period, all asteroids measured had their cores exposed, meaning collisions with other objects stripped their mantle. Without the insulating mantles, the cores all cooled simultaneously. Other studies have shown the cooling to be rapid, but they could not limit the time so clearly.

For the asteroids to have the isotopic ratios found by the team, the solar system had to be a very chaotic place, with a period of frequent collisions that stripped the asteroids’ mantles.

“Everything seems to have fallen apart at that point,” Hunt says. “And we wanted to know why,” she adds.

Why would there be such a chaotic collision period? There are several possibilities, according to the newspaper.

The first possibility concerns the giant planets of the solar system. If they migrated or were unstable in some way at that time, they could have rearranged the inner solar system, disrupted small bodies like asteroids, and triggered a period of increased collisions. This scenario is called the nice model.

The other possibility is the gas trail in the solar nebula.

Artist’s impression of the solar nebula. Image credit: NASA

When the Sun was a protostar, it was surrounded by a cloud of gas and dust called a solar nebula, just like other stars. The disk contained the asteroids, and the planets would eventually form there too. But the disk changed during the first million years of the solar system.

At first, the gas was dense, which slowed the movement of things like asteroids and planetesimals with a trail of gas. But as the Sun got going, it produced more solar wind and radiation. The solar nebula was still there, but the solar wind and radiation pushed on it, dissipating it. As it dissipated, it became less dense and there was less drag on objects. Without the dampening effect of the dense gas, the asteroids accelerated and collided more frequently.

This figure from the study shows the evolution of differentiated iron meteorite bodies in the early solar system.  At the top, parent bodies accumulate and differentiate within the first ~3 Myr after CAI formation.  Then there are two competing scenarios for the period of increased asteroid collisions.  Scenario A is the gas dissipation scenario, and the one that the research team believes best fits the data.  Scenario B is the Nice model, where a giant planet creates instability and causes the period of increased collisions.  Image credit: Hunt et al.  2022.
This figure from the study shows the evolution of differentiated iron meteorite bodies in the early solar system. At the top, parent bodies accumulate and differentiate within the first ~3 Myr after CAI formation. Then there are two competing scenarios for the period of increased asteroid collisions. Scenario A is the gas dissipation scenario and the one that the research team believes best fits the data. Scenario B is the Nice model, where a giant planet creates instability and causes a period of increased collisions. Image credit: Hunt et al. 2022.

According to Hunt and his colleagues, the reduction in gas drag is responsible.

“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,” explained study co-author Maria Schönbächler. “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,” Schönbächler said.

“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 of our solar system,” Schönbächler concluded.

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