Outer Solar System Plot Twist: Iron Meteorites Are Second-Chance Cores

Research led by Damanveer S. Grewal, Yale University

Scientists have cracked a puzzle that’s perplexed researchers since the Apollo era: why do iron meteorites from the outer solar system appear strangely stripped of sulfur, even though they formed in the coldest, most sulfur-rich regions of space?

The answer reframes our understanding of the early solar system. According to research published in Science Advances, these meteorites don’t actually come from sulfur-poor asteroids at all. Instead, they sample only fragments of larger parent bodies that were violently smashed apart within the first few million years after the solar system formed, then reassembled into new asteroids minus their sulfur-rich cores. The team argues these are second-generation planetesimals formed after early disruption and reassembly, helping reconcile several contradictions that have accumulated in meteorite studies over decades.

Iron Meteorites: A Tale of Two Metals

Iron meteorites are fragments of the metallic cores of ancient asteroids that separated into layers like miniature planets. Scientists divide them into two broad families based on their isotopic signatures: irons from the inner solar system and irons from beyond Jupiter’s original orbit in the outer regions.

Four groups of outer solar system iron meteorites (designated IID, IIF, IIIF, and IVB) showed chemical patterns that didn’t make sense. They were enriched in precious metals like osmium, iridium, and platinum compared to inner solar system irons, although models slightly under- or over-predict certain elements. Their cores appeared to contain little to no sulfur, with the IID and IVB groups predicted to be nearly sulfur-free.

For the IID group particularly, this created an impossible scenario. How could an asteroid forming in the cold outer solar system accrete normal amounts of elements like germanium and gallium, yet be nearly stripped of sulfur, which should have been equally abundant? The IVB group presented a different puzzle: it showed depletion not just in sulfur but also in these other elements, suggesting an additional process like extreme loss during formation. No known primitive meteorite (the building blocks of asteroids) shows the selective sulfur depletion pattern seen in IID.

Melting and Metal Segregation

The research team approached the problem by modeling how chemical elements separate between solid and liquid metal during core formation. Laboratory experiments show that when asteroids heat up from radioactive decay, the first melting occurs at relatively low temperatures, around 980°C, producing sulfur-rich metallic liquids.

These liquids can trickle through partially molten rock to form early cores (protocores) while the rest of the asteroid remains largely solid. Left behind in the mantle is solid metal depleted in sulfur but enriched in other elements.

Led by Damanveer Singh Grewal, an assistant professor of Earth and planetary science at Yale University, researchers used statistical modeling to determine whether the chemical signatures of IID, IIF, IIIF, and IVB irons matched what would be left behind after sulfur-rich liquid extraction. Results were striking. For the supposedly sulfur-free IVB group, the team calculated that about 82% of the original metal had separated as a liquid containing 11% sulfur. For IID, about 81% of metal had been extracted as liquid with 10% sulfur. IIF required extraction of 51% of metal as 13% sulfur liquid, while IIIF needed 80% extraction as 5% sulfur liquid.

While models successfully reproduced overall chemical patterns, the authors note some elements show small mismatches between predictions and observations. Similar processes had already been documented in another class of meteorites called ureilites, which are known to have lost sulfur-rich metal through this mechanism.

Collision and Reassembly

But if sulfur-rich early cores formed first, where did they go? The answer lies in the violent dynamics of the early solar system.

Grewal’s team proposes that the parent asteroids of these meteorites were disrupted by catastrophic impacts or glancing collisions after their sulfur-rich early cores had formed but before their sulfur-poor solid metal had melted. The timing window was narrow: between roughly 1 and 2.5 million years after the first solar system solids formed, with an upper bound of about 3 million years. This constraint comes from the need for daughter bodies to reach temperatures of 1500 to 1530°C to remelt the sulfur-poor metal, requiring substantial radioactive heating still available.

Collisions shattered these partially separated bodies into fragments. Some fragments, dominated by mantle material containing solid metal, reassembled into daughter asteroids. Meanwhile, fragments containing the sulfur-rich cores either formed separate bodies not sampled in our meteorite collections or remained with remnant parent bodies.

These daughter asteroids then experienced further heating from residual radioactive decay, eventually melting the sulfur-poor solid metal to form new cores. These second-generation cores are what we sample as IID, IIF, IIIF, and IVB iron meteorites.

Reconstructing the Missing Cores

Accounting for the “missing” sulfur-rich component dramatically changes estimates of the original parent bodies. When the team performed calculations including both the sampled metal and the unsampled early core, they found that the supposedly sulfur-free IID and IVB parent cores actually contained about 8% and 9% sulfur respectively. These reconstructed values, which incorporate the unsampled early core component, align with other outer solar system groups and fall within the range seen in primitive meteorites.

Core size estimates also changed substantially. Previous work suggested the IVB parent body had an extraordinarily small core, making up just 3% of its mass. Reconstruction reveals a more typical 7% core mass fraction. The IID parent body’s core mass fraction nearly tripled from 5% to 13%.

Oxidation states of these asteroids also require revision. Because reconstructed core mass fractions increased, the amount of oxidized iron remaining in their mantles must have been higher than previously thought. For IVB, the mantle iron oxide content jumps from 8% to 23%. For IID, mantle iron oxide increases from 7% to 19%.

Turbulent Beginnings

Beyond individual meteorites, the fact that multiple outer solar system asteroid parent bodies experienced disruptive collisions within such a narrow time window suggests that this region was far more violent during the gas disk phase than previously recognized.

Standard models assumed that gas drag would dampen collision speeds enough to favor growth through accretion rather than destruction. But Grewal’s team argues that Jupiter’s core, and the cores of other giant planets, may have grown rapidly enough to dynamically stir up nearby planetesimals, increasing impact speeds despite the gas disk’s presence.

Disruptive and glancing collisions were likely common during the gas disk’s lifetime, the researchers note. This more turbulent picture helps explain the diverse and sometimes puzzling characteristics of meteorites in our collections.

Many meteorites may come from second- or later-generation bodies rather than pristine first-generation planetesimals. That means inferring the chemistry of primordial materials or the disk from these samples requires careful consideration of their complex histories.

Earlier Core Formation

The discovery also affects estimates of when core formation occurred in outer solar system bodies. Previous age determinations based on isotopes suggested that asteroids from this region formed their cores later than inner solar system bodies. But these ages only reflect when the sulfur-poor metal melted in the daughter bodies, not when the original sulfur-rich early cores formed in the parent bodies.

Since the parent bodies must have been disrupted while radioactive heating was still active, their initial early core formation occurred more than a million years earlier than the ages recorded by the meteorites we can study. This brings the timing closer to that of inner solar system bodies, suggesting less difference in core formation histories between the two regions than previously thought.

Grewal and his colleagues note that one outer solar system group, South Byron Trio, displays an anomalously old core formation age that overlaps with inner solar system irons. Rather than being an outlier, this may be typical of early outer solar system bodies, with most other groups representing only the later, second-stage core formation in reassembled daughters.

The work doesn’t apply to all outer solar system groups. Two groups, IIC and South Byron Trio, don’t show the enrichment patterns that indicate missing early cores. These may have avoided early disruption or represent different formation pathways.

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