A fireball beautified the night sky over India on January 23, 1870. Accompanied by a thunderous detonation, the flaming mass crashed into the village of Nedagolla with enough force to leave passers-by stunned. The impact left just over 4 kilograms of cosmic rock – the Nedagolla meteorite.
The meteorite would be just another of the thousands found on Earth without its unusual composition. Researchers have long tried to understand its origin, and now they may have solved the mystery. In a recent study to appear in Meteorites and planetary science (preprint available here), scientists have discovered that the Nedagolla meteorite is the product of a collision between two asteroids of distinct origin. Its unique story opens a new window into research into the early stages of the formation of the solar system.
Two families of meteorites
Meteorites are time capsules that illuminate the era of the formation of the planets. The solar system formed from a cloud of interstellar gas and dust that collapsed under its own gravity. Particles inside the resulting protoplanetary disc collided and stuck together, forming increasingly large planetesimals, which became the parent bodies of meteorites found on Earth.
Meteorites come in different flavors. Depending on whether iron or silicates dominate, meteorites are traditionally classified as iron, stony or stony. The composition also depends on whether the meteorites come from bodies that have melted or the parent body was not melted and therefore no longer virgin. By these classifiers, Nedagolla is an ungrouped iron meteorite.
But we can also look at isotopes. Isotopes are elements with the same number of protons but different number of neutrons, and they can carry a lot of information, including when a rock formed.
“About 10 years ago, the community realized that there was an isotopic dichotomy in meteorite material,” says Fridolin Spitzer, graduate student (University of Münster, Germany), first author of the new study. Cosmochemists thus use isotopes to classify meteorites of all kinds, whatever their chemical composition, either non-carbonaceous chondrite (NC) or the carbonaceous chondrite (CC). (These groups were originally differentiated by the amount of carbon, but now the terms are used more generally.)
There is only one exception: “Nedagolla is the first who does not always fall into one of the two categories, but seems to fall in between,” explains Spitzer.
Scientists suspect that the two classes of isotopes formed in two different parts of the protoplanetary disc: NCs in the inner part of the disc and CCs in the outer solar system, beyond the orbit of Jupiter. So where does the Nedagolla meteorite put it?
Asteroid migrations and collisions
After performing a new independent analysis of the meteorite’s composition, the team proposes that its unique isotopic fingerprint comes from a collision of NC and CC planetesimals. “The two bodies collided, and it caused a fusion due to the high impact velocities, and it caused the materials of these two bodies to mix together,” says Spitzer.
Here things get interesting. Most meteorites originate from the asteroid belt, a region between the orbits of Mars and Jupiter. So, CC-type meteorites must have migrated to the inner part of the solar system at some point, otherwise the Nedagolla meteorite would not exist.
“The reason we have CC material to analyze on Earth, which in itself is an NC body, is that, during the evolution of the disk, planets like Jupiter migrated inward and outward, diffusing material around the solar system, ”says Katherine Bermingham (Rutgers University).
But the details are still unclear. For example, did the movements of Jupiter create the isotopic division? And why did one region of the disc have a systematically different mixture of materials from the other?
With the Nedagolla meteorite, scientists obtained the first isotopic evidence that the NC and CC bodies mingled. Its composition suggests that at least the CC body had a metallic core. Moreover, the formative collision could not have occurred for about 7 million years after the formation of the disc.
Such information measured for a larger sample of similar meteorites would be invaluable. “I think it’s important for the community to do more of this kind of work to see if we can find better time constraints on the NC-CC mix,” Bermingham said. “There are a lot of ungrouped iron meteorites, and maybe this signature will end up in the ones we haven’t looked at yet.”