Jupiter May Be the Reason This Asteroid’s Dust Looks Like The Sun

This Asteroid’s Chemistry Shouldn’t Make Sense, Until Jupiter Enters the Picture

Asteroid Bennu’s rock and dust carry chemical fingerprints from both the inner and outer solar system, and scientists now have a leading explanation for why: the asteroid’s parent body may have formed in a narrow zone just past the water ice line and just inside a still growing Jupiter’s orbit. That scenario, laid out in a new study in Science Advances, helps explain why Bennu, its cousin Ryugu, and a rare group of meteorites called CI chondrites all share elemental abundances that closely mirror the sun itself.

This mixed origin solves a puzzle that has divided researchers for years. Bennu’s parent asteroid seems to have swept up material from far more of the solar system than one small body should reasonably contain, and researchers now think Jupiter is the reason. As the giant planet grew, it began sorting the debris drifting past it, holding some back while letting the rest continue on.

To trace where the dust came from, scientists measured leftover signatures from ancient exploding stars, preserved as slightly different ratios of iron, titanium, and chromium depending on where material ended up in the early solar system. Testing five separate Bennu samples in two independent labs, researchers found iron and titanium signatures that stayed nearly identical across every sample, while chromium showed small variations that only partly line up with signs of past water alteration inside the asteroid.

Five Bennu Samples Reveal Nearly Identical Chemical Fingerprints

Bennu is what scientists call a rubble pile, a loose cluster of rocky fragments held together mostly by gravity rather than forming one solid chunk. NASA’s OSIRIS-REx mission scooped material off Bennu’s surface and parachuted it back to Earth in 2023. For this study, researchers examined five samples of different sizes and textures, including two powdered batches weighing about 20 and nearly 1,300 milligrams.

Two separate labs, one at ETH Zurich and one at Lawrence Livermore National Laboratory, ran the same samples through independent testing methods. Their results matched closely, giving the team confidence the numbers reflected real chemistry rather than lab error.

Iron and titanium signatures came back nearly identical across every sample, regardless of size or texture. Chromium was the exception. It showed small but real variations from sample to sample, and some of those differences lined up with signs that water once moved through the rock and altered it unevenly in certain spots. One small sample, though, fell outside that pattern entirely, and the researchers say another process they do not yet fully understand may also be at work.

That shared chemistry is the real discovery here. Bennu’s iron signature ties it and its relatives more closely to material usually associated with the inner solar system, the same neighborhood that eventually built Earth. That is surprising, since other chemical clues had always placed these objects with material from far out in the solar system instead.

Jupiter’s Growth Filtered Large Rocks from Fine Dust

To explain how dust from opposite ends of the solar system ended up in the same asteroid, researchers turned to models of how a young, growing Jupiter reshaped the disk of gas and dust around it. Those models suggest Jupiter reached a large size within the first few million years after the solar system’s oldest solid material formed. A planet that size would have carved a gap in the surrounding material, and that gap would have worked like a filter.

Big chunks of rock and ice from farther out got stuck outside that gap, unable to drift any closer to the sun. Fine dust, grains smaller than a fraction of a millimeter, stayed light enough to remain mixed across the disk even as Jupiter grew. Larger, solid rock fragments were mostly blocked from crossing the gap, which is why Bennu’s parent body accreted mostly fine dust and shattered mineral fragments rather than whole, coarse rocks.

Recent dating of Ryugu points to formation as early as roughly 2 million years after the solar system’s oldest solid material, and Bennu’s own chromium measurements in this study are consistent with that early timeline. That timing sits earlier than several other meteorite families and is difficult to square with models in which comet forming regions produce bodies only late in the solar system’s history.

Bennu Bridges Two Distinct Regions of the Solar System

Bennu is not simply a leftover piece of the outer solar system, and it is not a stand-in for the inner planets either. Researchers describe it as something in between, built from a pool of fine dust that Jupiter helped assemble by blocking larger rocks while letting small grains keep mixing from both regions. That remains the team’s favored explanation rather than a settled fact, since a similar dust reservoir could, in principle, have formed elsewhere in the outer disk. Even so, this mechanism accounts for why Bennu, Ryugu, and CI meteorites all carry elemental chemistry that echoes the sun so closely, despite rock that clearly experienced heavy water damage over time.

Pinning down where these asteroids formed does more than settle an old classification argument among scientists. It sharpens the picture of how raw material, including water and organic molecules, moved through the early solar system, with Bennu offering one clue to how such material could have reached the inner planets.

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