Planetary scientists from the ETH Zürich and elsewhere have determined palladium-silver (Pd-Ag) and platinum (Pt) isotope compositions of 13 iron meteorites that formed early in the history of our Solar System.
Iron meteorites are thought to represent the once heated interior portions of planetesimals and are some of the earliest-formed bodies in our Solar System.
As such, they are survivors of the many dynamical processes that have shaped Solar System architecture, including dissipation of the protoplanetary disk, and runaway growth, migration and reorganization of the giant planets.
“Previous scientific studies showed that asteroids in the Solar System have remained relatively unchanged since their formation, billions of years ago,” said lead author Dr Alison Hunt, a researcher with the Institute of Geochemistry and Petrology at the ETH Zürich.
“They therefore are an archive, in which the conditions of the early Solar System are preserved.”
“But to unlock this archive, we had to thoroughly prepare and examine the extraterrestrial material.”
The researchers took 18 samples from 13 different iron meteorites, which were once part of the metallic cores of asteroids.
To carry out their analysis, they had to dissolve the samples to be able to isolate Pd, Ag and Pt.
With the help of a mass spectrometer, they measured abundances of different isotopes of these elements.
“In the first few million years of our Solar System, the metallic asteroid cores were heated by radioactive decay of isotopes,” they said.
“As they began to cool down, a specific Ag isotope produced by radioactive decay began to accumulate.”
“By measuring the present-day Ag isotope ratios within the iron meteorites, we could determine both when and how quickly the asteroid cores had cooled.”
Their results show that the cooling was rapid and likely occurred due to severe collisions into other bodies, which broke off the insulating rocky mantle of the asteroids and exposed their metal cores to the cold of space.
While the fast cooling had been indicated by previous studies based on Ag isotope measurements, the timing had remained unclear.
“Our additional measurements of Pt isotope abundances allowed us to correct Ag isotope measurements for distortions caused by cosmic irradiation of the samples in space,” Hunt said.
“So we were able to date the timing of the collisions more precisely than ever before.”
“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.”
The near simultaneous collisions of the different asteroids indicated that this period must have been a very unsettled phase of the Solar System.
“Everything seems to have been smashing together at that time. And we wanted to know why,” Hunt said.
The authors considered different causes by combining their results with those from the latest, most sophisticated computer simulations of the solar system development. Together, these sources could narrow down the possible explanations.
“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,” said senior author Professor Maria Schönbächler, also from the Institute of Geochemistry and Petrology at the ETH Zürich.
“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.”
While the nebula was still around, it slowed down the objects orbiting the Sun in it — similar to how air resistance slows a moving car.
After the nebula had disappeared, the lack of gas drag allowed the asteroids to accelerate and collide into each other — like bumper cars that were turned to 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 — like the likely time by which the Solar Nebula had gone,” Professor Schönbächler said.
“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 planets were born, but also give us insights into others outside our Solar System.”
The study was published May 23, 2022 in the journal Nature Astronomy.
AC Hunt and al. The dissipation of the Solar Nebula constrained by impacts and core cooling in planetesimals. Nat Astronpublished online May 23, 2022 doi: 10.1038/s41550-022-01675-2