Most of the meteorites that have landed on Earth are fragments of planetesimals, the earliest protoplanetary bodies in the solar system. Scientists believe that these primordial bodies either completely melted down early in their history, or remained like heaps of untreated space debris. But one family of meteorites has puzzled researchers since its discovery in the 1960s. Various fragments found around the world seem to have come off the same body. The composition of these meteorites indicates that their parent must have been an amazing “chimera” that was both molten and not molten. Scientists have advanced in solving this scientific conundrum and published the results of the study in the journal Science Advances.
MIT researchers, with the support of international colleagues, determined that the parent body of these rare meteorites was indeed a multi-layered, differentiated object that likely had a liquid-metal core. This core was strong enough to generate a magnetic field that could rival the earth’s magnetic field today.
The results of the study suggest that the diversity of the earliest objects in the solar system could be more complex than scientists previously thought.
This is one example of planetesimals that must have had molten and non-molten layers. Such an object motivates the search for new evidence of composite planetary structures. Understanding the full spectrum of structures, from unmelted to completely molten, is the key to deciphering how planetesimals formed in the early solar system.
Clara Maurel, PhD Student, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology (MIT)
Maurel’s co-authors are EAPS Professor Benjamin Weiss, as well as staff from the University of Oxford, Cambridge University, University of Chicago, National Laboratory. Lawrence Berkeley and Southwest Research Institute.
The solar system formed about 4.5 billion years ago as a vortex of very hot gas and dust. As this disk gradually cooled down, pieces of matter collided and merged, gradually forming large bodies, such as planetesimals.
Most of the meteorites that fell to Earth have compositions that suggest they evolved from early planetesimals, which were one of two types: molten and non-molten. Scientists believe that both types of objects formed relatively quickly, in less than a few million years, at the beginning of the evolution of the solar system.
If a planetesimal formed in the solar system’s first 1.5 million years, the short-lived radiogenic elements could completely melt the body due to the heat generated during their decay. Unmelted planetesimals could have formed later when their material contained fewer radiogenic elements that were insufficient to melt.
There was little evidence in meteorite records for intermediate objects with molten and unmelted composition, except for a rare family of meteorites called IIE irons (group IIE metal meteorites).
These IIE irons are really strange meteorites. They show both evidence that they originated from primordial objects that never melt, and evidence that they were part of a body that completely, or at least substantially melted. We didn’t know where to put them, and that’s what made us focus on them.
Benjamin Weiss, Professor at EAPS
Scientists previously discovered that both molten and unmelted IIE meteorites originated from the same ancient planetesimal, which likely had a hard crust covering a liquid mantle like Earth. Scientists wondered if a planetesimal could conceal a molten metalcore.
The researchers concluded that if a planetesimal contains a metal core, it may well generate a magnetic field, much like the Earth’s liquid core produces a magnetic field.
Scientists wondered if they could find such minerals in IIE meteorite samples that fell to Earth. The team analyzed the samples using an advanced light source from the National Laboratory. Lawrence Berkeley, who generates X-rays that interact with grains of minerals on a nanometer scale so that the magnetic direction of the minerals can be determined.
Of course, the electrons in a number of grains were aligned in the same direction. This is evidence that the parent’s body generated a magnetic field, possibly up to several tens of microtesla, which is approximately equal to the strength of the Earth’s magnetic field. After eliminating less likely sources, the team concluded that the magnetic field was most likely created by a liquid metalcore. According to estimates, to create such a field, the core must have a width of at least several tens of kilometers.
Working with staff at the University of Chicago, scientists have conducted high-speed simulations of various scenarios for the formation of these meteorites. They showed that a body with a liquid core could collide with another object, and this impact could knock material out of the core. This material would then migrate to pockets close to the surface of the body where the meteorites originated.
Was such a complex planetesimal an ejection in the early solar system or one of many such differentiated objects? The answer, scientists say, may lie in the asteroid belt.