Isochrons reveal Bennu's hybrid origin in solar system's transition zone

New analysis of samples returned by NASA's OSIRIS-REx mission suggests that the asteroid Bennu is not a primitive body from the outer solar system, as previously believed. Instead, scientists propose that Bennu is a hybrid, formed from materials originating from both the inner and outer reaches of our solar system. This revised understanding challenges existing models of asteroid formation.

Photo: Wikimedia Commons by Marina Brozović, https://creativecommons.org/public-domain/pdm/

Asteroid Bennu

Hybrid Origins Revealed by Isotopic Analysis

The capsule carrying Bennu's samples landed in Utah's West Desert in September 2023, containing approximately 120 grams of rock. A portion of this material, about half a gram, was analyzed by a team led by Maria Schönbächler at ETH Zurich. Her team focused on the isotopes of iron, titanium, and chromium, whose relative abundances serve as a unique fingerprint of a celestial body's origin.

The measurements yielded definitive results: titanium and iron are uniformly distributed throughout Bennu. Its isotopic profile aligns with that of the asteroid Ryugu and CI-class meteorites, indicating that all three originated from the same primordial cosmic dust cloud. However, their chemical composition significantly differs from other known asteroids, meteorite groups, and planets.

Water as a Cosmic Binder

Previous theories posited that asteroids like Bennu formed far from the sun. The new data contradicts this, suggesting a birth zone near the frost line, where water vapor freezes. In this region, materials from both the inner and outer solar system could have mixed, with ice acting as a binding agent that coalesced fine dust particles.

Jupiter played a critical role in this process. Having formed within the first million years of the solar system's existence, the gas giant acted as a barrier to larger particles. Fine dust, however, could flow around it. In the transitional zone near the water ice line, dust from different parts of the solar disk intermingled, providing the building blocks for Bennu's precursors.

This scenario also accounts for Bennu's significant water content. Ice near the frost line would have evaporated, with some vapor subsequently re-condensing. Jupiter's gravitational influence ensured that the body accreted predominantly fine dust. This type of dust mixes readily, explaining why Bennu's chemical composition closely mirrors that of the sun itself.

A Window to Planetary Origins

Schönbächler emphasizes that Bennu offers an unparalleled glimpse into the initial chemical makeup from which the terrestrial planets, including Earth, ultimately formed. The asteroid is considered "primitive," its material dating back to the solar system's birth and having undergone minimal alteration since then.

With the analysis of Bennu's samples largely complete, scientific attention is now shifting to Japan's JAXA Hayabusa2 mission successor, aiming to collect samples from Phobos, a moon of Mars. The launch is scheduled for late October 2026, with the sample return expected in 2031. Schönbächler plans to apply for a share of these samples to investigate whether they exhibit the same isotopic signature as Bennu, further probing Jupiter's influence on particle selection and dust aggregation.

Source: This commentary was adapted from an original Russian-language publication by Pravda.Ru.