Tiny particles of rock from the asteroid Ryugu, delivered to Earth in 2020 by Japan’s Hayabusa2 space mission, have shed new light on an ancient magnetic field that once existed in our early solar system. 

“Magnetic fields have been theorized to play a key role in shaping the evolution of the early solar system by acting as a method through which mass and angular momentum were transported,” explained Elias Mansbach, a researcher in MIT’s Department of Earth, Atmospheric, and Planetary Sciences and lead author of the study, in an email. 

By analyzing these samples, Mansbach and his colleagues confirmed the current theory of solar system evolution, whose predictions for the strength of the magnetic field in the early solar system matched with the value measured by the team. This discovery helps explain how magnetic forces influenced the formation of planets and other celestial bodies billions of years ago.

“Therefore, understanding the strength of the magnetic field and how it varied across the early solar system provides key insights into the architecture of our present system.”

The role of magnetic fields in our solar system’s formation

Around 4.6 billion years ago, our solar system formed from a dense cloud of gas and dust. This cloud collapsed into a rotating disk, with most of the material concentrating at the center to form the Sun. 

The remaining gas and dust from the early solar system continued to swirl within a vast solar nebula, a cloud infused with ionized gas. This ionization, caused by the intense heat and radiation of the newly formed Sun, meant that many of the gas particles were electrically charged. Scientists believe that as these charged particles interacted with the Sun’s powerful energy and rotational movement, they helped generate a magnetic field that stretched across the entire nebula.

This magnetic field, likely formed through a combination of the Sun’s rotation and the movement of ionized gas, influenced the flow and alignment of the swirling material, guiding the accumulation of particles. This process played a crucial role in shaping the early solar system, helping direct the formation of planets, moons, and other celestial bodies. 

“The magnetic field that we were interested in is called the Solar nebular field, which was the magnetic field coupled to the ionized gas of the solar nebula shortly after the creation of the Sun and lasted for the first ~4 million years of the solar system,” said Mansbach.

Over a few million years, as the nebula’s gas and dust dispersed or clumped into larger objects, the nebular magnetic field weakened and eventually faded.

Scientists are interested in learning more about this early magnetic field because it likely influenced the formation of our solar system’s planets, asteroids, and moons. By driving matter inward, the magnetic field influenced how gas and dust coalesced to form these celestial bodies. Understanding the characteristics and effects of this early magnetic field provides valuable insights into the processes that shaped the layout of our solar system.

“But how far this magnetic field extended, and what role it played in more distal regions, is still uncertain because there haven’t been many samples that could tell us about the outer solar system,” Mansbach said in a press release.

Ryugu’s return

As Ryugu is believed to have formed during the solar system’s infancy, it provides a natural record of the magnetic environment at that time. The return of rock samples to Earth provided a rare opportunity to directly study this ancient environment. Before this, scientists had to rely on meteorites that happened to fall to Earth as their only clues to the conditions that existed billions of years ago.

Meteorites, the fragments of asteroids that reach Earth, often contain ferromagnetic minerals, like magnetite, that can record the direction and strength of ancient magnetic fields.

“Meteorites are currently our best source of studying this field,” said Masbach. “Generally, meteorites (and rocks on Earth) possess ferromagnetic minerals such as magnetite that can align themselves in the direction of an external field, similar to how a compass points north on Earth. What is remarkable about these minerals is that not only can they record the ancient field, but they can retain that record over billions of years to the present day.”

Previous studies on meteorites from the inner solar system — up to around seven astronomical units from the Sun (one astronomical unit being the distance from Earth to the Sun) — suggested that this region once had a magnetic field strength of 50 to 200 microteslas (that of the present Earth is around 25–65 microteslas). This aligns with theoretical models. However, the outer Solar system’s magnetic field has been less understood, although scientists assumed it was likely weaker.

Filling this gap in our understanding of the solar system was one of the goals of Japan’s space agency, JAXA, in the Hayabusa2 mission launched in 2014 to Ryugu, a carbon-rich asteroid thought to have originated in the outer solar system. Building on the achievements of its predecessor, Hayabusa, which was the first mission to return samples from an asteroid, Hayabusa2 aimed to investigate Ryugu’s composition in greater detail, focusing on its physical characteristics and the presence of organic compounds and water-bearing minerals. 

When Hayabusa2 arrived, the spacecraft detected a negligible magnetic field on Ryugu, which was expected for a body from this region. But subsequent lab analyses of returned samples indicated a much higher magnetic field — up to 800 microteslas, vastly stronger than expected.

Resolving conflicting data

To clarify this discrepancy, the MIT team examined three millimeter-sized grains from the Ryugu samples using a technique called alternating field demagnetization. In this process, the researchers erased the samples’ magnetic history by applying an alternating laboratory magnetic field, increasing the strength incrementally.

To analyze the magnetic properties of Ryugu samples, the research team employed a technique called alternating field demagnetization. They gradually erased the samples’ magnetic record by applying an increasing laboratory magnetic field, similar to erasing a tape recorder. This process helped identify any retained magnetic evidence. Afterward, they simulated the acquisition of the primary magnetic field by applying a specific laboratory field, repeating the demagnetization process. By comparing the natural magnetic record with the synthetic one, the team could determine the strength of the original field the samples recorded. 

“We found that Ryugu samples do not show clear evidence of formation in the presence of an external field, and place an upper limit on the nebular field to be 15 microtesla in this region,” said Masbach.

Researchers believe that this discrepancy might be due to unintended magnetization of the samples during Hayabusa2’s return journey or due to some Earth’s magnetization sources.

Supporting evidence from other meteorites

The team further validated their findings by combining their results with data gathered from examining other meteorites, believed to have originated in the outer solar system: Tagish Lake, Tarda, and Wisconsin Range 91600.

These meteorites, which fell to Earth in previous decades, revealed magnetic fields ranging from zero to around 5 microteslas, matching what was measured in Ryugu’s grains. These consistent results align with models of the early solar system’s magnetic evolution.

However, Mansbach and his colleagues emphasize that further studies on similar outer solar system meteorites are needed to confirm these results.

“Out of the meteorites in our study that we think formed in the outer solar system, only one shows clear evidence of a magnetic field,” concluded Masbach. “Future studies should focus on measuring the magnetic record of meteorites with similar provenances to build up our statistics and learn more about the outer nebular field.”

Reference: Elias N. Mansbach et al, Evidence for Magnetically-Driven Accretion in the Distal Solar System, Advancing Earth and Space Sciences (2024). DOI: 10.1029/2024AV001396

Feature image credit: Valera268268 on Pixabay