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Iron Meteorite ‘Fingerprints’ Reveal New Details About Planet Formation
Image of the inside of an iron meteorite, showing the characteristic Widmanstätten pattern of long, iron-nickel crystals that crisscross one another. Many iron meteorites are thought to be vestiges of a planet’s core that was blasted apart early in our solar system’s evolution.
Credit: Johns Hopkins APL
Mon, 08/03/2020 - 13:56
Jeremy Rehm
A new study coauthored by planetary scientist Nancy Chabot from the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, reveals fresh details about the solar system’s ancient iron meteorites. The study, published Aug. 3 in Nature Geoscience, helps explain their distinct chemical differences from other meteorites and deepens our understanding of the solar system’s geochemistry during its infancy.
“This work helps us understand the geological processes that shaped planetary bodies when the planets were just forming,” said Peng Ni, a planetary scientist at the Carnegie Institution for Science in Washington, D.C., and the lead author on the study.
Planetary scientists have substantial evidence that most iron meteorites are fragments of long-gone planetary cores, blasted apart through violent collisions early in the solar system’s evolution.
Chabot, who is also the coordination lead of NASA’s Double Asteroid Redirection Test (DART) mission, has studied these meteorites for over two decades, running experiments in APL’s Meteorite Laboratory where she melts metal powders at roughly 1,600–2,700°F (900–1500°C) to simulate when a young planet’s core starts to cool into a partially liquid, partially solid ball of metal. She studies how various metals, such as iron, nickel and iridium, divvy up or “partition” between these liquid and solid states.
Ni and his colleague Anat Shahar at the Carnegie Institution for Science were interested in a new way to use Chabot’s experimental technique: seeing if iron isotopes — iron atoms with a different number of neutrons — also partition between liquid and solid, crystallized phases.
Researchers have noticed that iron meteorites have a heftier dose of heavy iron isotopes than their more common counterpart, chondritic meteorites, whose chemical composition is representative of the early solar system’s bulk chemical composition. Nobody was sure why that difference exists, but the team suspected the isotopes might split up much like the elements in Chabot’s experiments.
If so, scientists could deduce how iron meteorites — and thus planetary cores — ended up enriched with heavy isotopes of iron, as well as infer what part of the planetary core a meteorite likely came from.
“It’s like leaving a chemical fingerprint,” Chabot explained.
APL planetary scientist Nancy Chabot at a furnace in APL’s Meteorite Laboratory. The Meteorite Laboratory hosts a number of geochemistry facilities to better understand chemical processes happening in the solar system. In a recent study, Chabot used the furnace to melt metal powders to simulate conditions that form iron meteorites, and see how elemental isotopes distribute in the partially solid, partially liquid melt.
Credit: Johns Hopkins APL
The team melted powders with mixes of iron, nickel and sulfur in APL’s Meteorite Laboratory and then allowed crystals to form, just as they would in a planetary core. The researchers found heavy iron isotopes preferred the iron crystal portions rather than the bits that were still liquid.
They then used a mathematical model of this crystallization process in a planetary body and confirmed that the sizeable amount of heavy iron isotopes in the core is a natural consequence of iron crystal formation.
With one puzzle solved, the model underscored another riddle: The elemental and isotope composition of known iron meteorites appears to be missing large portions of these early solar system cores.
In the study, the team modeled the most numerous iron meteorite group and found that the meteorites account for only 60% of a planetary body’s core, particularly the portion of the core that cooled and crystallized first. The remaining 40% seems to be missing. Meteorites from this part would be rich in light iron isotopes and sulfur, but among the worldwide collection of more than 1,200 iron meteorites, very few meet those criteria.
Chabot said she wasn’t shocked by this. Previous studies have suggested this lost, sulfur-rich reservoir, but she does find it curious.
“We’ve gotten a lot of samples, but somehow we’re missing nearly this whole other half of the core,” she said. “Why is that?”
Some researchers have proposed sulfur-rich minerals are more fragile, so they would shatter more easily from a collision. Chabot is skeptical of that explanation, citing that many meteorites are weak and brittle. “It’s a bit of an enduring mystery,” she said.
Ni emphasized the finding underscores the need to search for the missing reservoir, whether by looking for new meteorites here on Earth or by hunting down the missing pieces in space.
NASA’s Psyche mission, set to launch in 2022, could spearhead that effort. The spacecraft will travel to the asteroid Psyche, which is thought to be a metal-rich body, perhaps containing metal from the iron-nickel core of an extinct, early-solar-system planet.
Engineers at APL are building a gamma ray and neutron spectrometer (GRNS) for the mission to measure the chemical composition of the asteroid’s surface. The GRNS could search for sulfur.
“If the APL GRNS finds sulfur on Psyche, I’ll be super excited,” Chabot said.
In the meantime, the team is examining other isotopes, specifically copper isotopes, and intends to look at other light elements beyond sulfur that can show up in planetary cores, such as phosphorus and carbon.