Ancient Asteroids Reveal That The Early Solar System Was More Chaotic Than We Thought

Young solar systems are undeniably turbulent environments. As rocks, boulders, and planetesimals smashed repeatedly, our infant Solar System was characterized by cascading collisions.

A new research based on asteroids that collided with Earth has given some of the turmoil a chronological frame.

Astronomers believe asteroids have stayed virtually unaltered since their genesis billions of years ago in the early Solar System.

Because differentiated asteroids had mantles that shielded their innards from space weathering, they're like rocky time capsules that carry scientific information from that crucial period.However, not all asteroids stayed intact.

Repeated collisions gradually robbed the insulating mantles of their iron cores, shattering some of the cores into fragments.

Some of the fragments landed on the ground. Tribes were fascinated by meteorites that fell from space, and in certain circumstances, they were a useful resource; King Tut was buried with a knife forged from an iron meteorite, while Inuit people in Greenland manufactured tools out of an iron meteorite for millennia.

Iron meteorites are of great interest to scientists because of the information they contain.

Palladium, silver, and platinum isotopes were studied in a recent research based on iron meteorites, which are debris from the core of bigger asteroids. The authors were able to tighten the time of several early Solar System events by analyzing the levels of those isotopes.

The study was published in Nature Astronomy with the title "The dissipation of the solar nebula constrained by impacts and core cooling in planetesimals". Alison Hunt of the National Center of Competence in Research (NCCR) PlanetS and ETH Zurich are the principal authors on the paper.

"Previous scientific studies showed that asteroids in the Solar System have remained relatively unchanged since their formation, billions of years ago," Hunt explained. "They, therefore, are an archive in which the conditions of the early Solar System are preserved." 

We know more about elements, isotopes, and decay chains than the ancient Egyptians and the Inuit. We know how elements degrade into other elements in chains and how long it takes.

The short-lived 107Pd–107Ag decay system is one of those decay chains at the center of this research. This chain, which has a half-life of around 6.5 million years, is used to identify the existence of short-lived nuclides from the Solar System's early days.

The scientists collected samples from 18 distinct iron meteorites that used to be part of asteroids' iron cores.

The palladium, silver, and platinum in them were then separated, and the amounts of distinct isotopes of the three elements were measured using a mass spectrometer. In this study, a certain silver isotope is crucial.

Decaying radioactive isotopes heated the metallic cores of asteroids during the first few million years of the Solar System's history. An isotope of silver (107Ag) accumulated in the cores as they cooled and additional isotopes decayed. The researchers estimated how quickly and when the asteroid cores cooled by measuring the ratio of 107Ag to other isotopes.

This isn't the first time asteroids and isotopes have been researched in this way. However, previous research did not take into consideration the impact of galactic cosmic rays (GCRs) on isotope ratios.

GCRs can reduce the quantity of 107Ag and 109Ag by disrupting the neutron capture mechanism during decay. By counting platinum isotopes, these new data are adjusted for GCR influence.

"Our additional measurements of platinum isotope abundances allowed us to correct the silver isotope measurements for distortions caused by cosmic irradiation of the 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 the asteroidal cores we examined had been exposed almost simultaneously, within a timeframe of 7.8 to 11.7 million years after the formation of the Solar System," Hunt added.

In astronomy, a 4-million-year time period is considered short. All of the asteroids surveyed had their cores exposed over that brief period, implying that impacts with other objects took away their mantles. The cores all cooled at the same time without the insulating mantles.

Other research have found that the cooling was quick, but they were unable to pinpoint the period.

The Solar System had to be a highly chaotic environment for the asteroids to have the isotope ratios the researchers discovered, with a period of frequent collisions that removed the mantles off asteroids.

"Everything seems to have been smashing together at that time," Hunt says. "And we wanted to know why," she continues.

Why would there be such a flurry of unpredictable collisions? According to the publication, there are a few options.

The first scenario is that the Solar System's large planets will collide. They could've restructured the inner Solar System, disturbed minor things like asteroids, and caused a period of increasing collisions if they moved or were unstable at the time. The Nice model is the name for this scenario.

Another option is solar nebula gas drag.

Like other stars, the Sun was surrounded by a cloud of gas and dust when it was a protostar. The asteroids were trapped within the disk, and the planets would eventually form there as well. However, over the first few million years of the Solar System, the disk shifted.

The gas was initially thick, which delayed the speed of asteroids and planetesimals due to gas drag. However, when the Sun became more active, it began to create more solar wind and radiation.

The solar nebula was still there, but it had been dissipated by the solar wind and radiation. It grew less thick as it dispersed, resulting in reduced drag on things.

Asteroids accelerated and crashed with each other more often without the damping influence of thick gas.

The decrease in gas drag, according to Hunt and her colleagues, is to blame.

"The theory that best explained this energetic early phase of the solar system indicated that it was caused primarily by the dissipation of the so-called solar nebula," Maria Schönbächler, research co-author, noted.

"This solar nebula is the remainder of gas that was left over from the cosmic cloud out of which the Sun was born. For a few million years, it still orbited the young Sun until it was blown away by solar winds and radiation," Schönbächler added.

"Our work illustrates how improvements in laboratory measurement techniques allow us to infer key processes that took place in the early solar system – like the likely time by which the solar nebula had gone. Planets like the Earth were still in the process of being born at that time. Ultimately, this can help us to better understand how our own planets were born, but also give us insights into others outside our solar system," Schönbächler concluded.