Family of Oddball Meteorites Stumped Researchers for Decades – Now an Answer to the Puzzling Chimera

Rare Layered Meteorite Sample

Samples from a rare meteorite family, including the one shown here, reveal that their parent planetesimal, formed in the earliest stages of the solar system, was a complex, layered object, with a molten core and solid crust similar to Earth. Credit: Carl Agee, Institute of Meteoritics, University of New Mexico. Background edited by MIT News.

Study suggests the rare objects likely came from an early planetesimal with a magnetic core.

Most meteorites that have landed on Earth are fragments of planetesimals, the very earliest protoplanetary bodies in the solar system. Scientists have thought that these primordial bodies either completely melted early in their history or remained as piles of unmelted rubble.

But a family of meteorites has befuddled researchers since its discovery in the 1960s. The diverse fragments, found all over the world, seem to have broken off from the same primordial body, and yet the makeup of these meteorites indicates that their parent must have been a puzzling chimera that was both melted and unmelted.

Now researchers at MIT and elsewhere have determined that the parent body of these rare meteorites was indeed a multilayered, differentiated object that likely had a liquid metallic core. This core was substantial enough to generate a magnetic field that may have been as strong as Earth’s magnetic field is today.

Their results, published on July 24, 2020, in the journal Science Advances, suggest that the diversity of the earliest objects in the solar system may have been more complex than scientists had assumed.

“This is one example of a planetesimal that must have had melted and unmelted layers. It encourages searches for more evidence of composite planetary structures,” says lead author Clara Maurel, a graduate student in MIT’s Department of Earth, Atmospheric, and Planetary Sciences (EAPS). “Understanding the full spectrum of structures, from nonmelted to fully melted, is key to deciphering how planetesimals formed in the early solar system.”

Maurel’s co-authors include EAPS Professor Benjamin Weiss, along with collaborators at Oxford University, Cambridge University, the University of Chicago, Lawrence Berkeley National Laboratory, and the Southwest Research Institute.

Oddball irons

The solar system formed around 4.5 billion years ago as a swirl of super-hot gas and dust. As this disk gradually cooled, bits of matter collided and merged to form progressively larger bodies, such as planetesimals.

The majority of meteorites that have fallen to Earth have compositions that suggest they came from such early planetesimals that were either of two types: melted, and unmelted. Both types of objects, scientists believe, would have formed relatively quickly, in less than a few million years, early in the solar system’s evolution.

If a planetesimal formed in the first 1.5 million years of the solar system, short-lived radiogenic elements could have melted the body entirely due to the heat released by their decay. Unmelted planetesimals could have formed later, when their material had lower quantities of radiogenic elements, insufficient for melting.

There has been little evidence in the meteorite record of intermediate objects with both melted and unmelted compositions, except for a rare family of meteorites called IIE irons.

“These IIE irons are oddball meteorites,” Weiss says. “They show both evidence of being from primordial objects that never melted, and also evidence for coming from a body that’s completely or at least substantially melted. We haven’t known where to put them, and that’s what made us zero in on them.”

Magnetic pockets

Scientists have previously found that both melted and unmelted IIE meteorites originated from the same ancient planetesimal, which likely had a solid crust overlying a liquid mantle, like Earth. Maurel and her colleagues wondered whether the planetesimal also may have harbored a metallic, melted core.

“Did this object melt enough that material sank to the center and formed a metallic core like that of the Earth?” Maurel says. “That was the missing piece to the story of these meteorites.”

The team reasoned that if the planetesimal did host a metallic core, it could very well have generated a magnetic field, similar to the way Earth’s churning liquid core produces a magnetic field. Such an ancient field could have caused minerals in the planetesimal to point in the direction of the field, like a needle in a compass. Certain minerals could have kept this alignment over billions of years.

Maurel and her colleagues wondered whether they might find such minerals in samples of IIE meteorites that had crashed to Earth. They obtained two meteorites, which they analyzed for a type of iron-nickel mineral known for its exceptional magnetism-recording properties.

The team analyzed the samples using the Lawrence Berkeley National Laboratory’s Advanced Light Source, which produces X-rays that interact with mineral grains at the nanometer scale, in a way that can reveal the minerals’ magnetic direction.

Sure enough, the electrons within a number of grains were aligned in a similar direction — evidence that the parent body generated a magnetic field, possibly up to several tens of microtesla, which is about the strength of Earth’s magnetic field. After ruling out less plausible sources, the team concluded that the magnetic field was most likely produced by a liquid metallic core. To generate such a field, they estimate the core must have been at least several tens of kilometers wide.

Such complex planetesimals with mixed composition (both melted, in the form of a liquid core and mantle, and unmelted in the form of a solid crust), Maurel says, would likely have taken over several million years to form — a formation period that is longer than what scientists had assumed until recently.

But where within the parent body did the meteorites come from? If the magnetic field was generated by the parent body’s core, this would mean that the fragments that ultimately fell to Earth could not have come from the core itself. That’s because a liquid core only generates a magnetic field while still churning and hot. Any minerals that would have recorded the ancient field must have done so outside the core, before the core itself completely cooled.

Working with collaborators at the University of Chicago, the team ran high-velocity simulations of various formation scenarios for these meteorites. They showed that it was possible for a body with a liquid core to collide with another object, and for that impact to dislodge material from the core. That material would then migrate to pockets close to the surface where the meteorites originated.

“As the body cools, the meteorites in these pockets will imprint this magnetic field in their minerals. At some point, the magnetic field will decay, but the imprint will remain,” Maurel says. “Later on, this body is going to undergo a lot of other collisions until the ultimate collisions that will place these meteorites on Earth’s trajectory.”  

Was such a complex planetesimal an outlier in the early solar system, or one of many such differentiated objects? The answer, Weiss says, may lie in the asteroid belt, a region populated with primordial remnants.

“Most bodies in the asteroid belt appear unmelted on their surface,” Weiss says. “If we’re eventually able to see inside asteroids, we might test this idea. Maybe some asteroids are melted inside, and bodies like this planetesimal are actually common.”

Reference: “Meteorite evidence for partial differentiation and protracted accretion of planetesimals” by Clara Maurel, James F. J. Bryson, Richard J. Lyons, Matthew R. Ball, Rajesh V. Chopdekar, Andreas Scholl, Fred J. Ciesla, William F. Bottke and Benjamin P. Weiss, 24 July 2020, Science Advances.
DOI: 10.1126/sciadv.aba1303

This research was funded, in part, by NASA.

11 Comments on "Family of Oddball Meteorites Stumped Researchers for Decades – Now an Answer to the Puzzling Chimera"

  1. “puzzling chimera” — puzzling word choice! The usual meanings of chimera are “a grotesque product of the imagination,” “a grotesque monster,” “a fanciful conception.”

    • Torbjörn Larsson | July 26, 2020 at 8:40 am | Reply

      Good catch! Borrowing words goes both ways. C.f. “theory” which was used in science for “well supported description” but borrowed into modern language as “unsupported idea”. Here it seems chimera has been borrowed first into biology for cellular and later genomic descriptions of disparate mixes: “3: an individual, organ, or part consisting of tissues of diverse genetic constitution
      A hybrid created through fusion of a sperm and an egg from different species is a chimera.” [ https://www.merriam-webster.com/dictionary/chimera ]

      Once used terminology tends to leak into other areas of science. C.f. “tree” in biology as organism and as description of branching processes, and now computer science and what not. “2: something in the form of or resembling a tree: such as
      a: a diagram or graph that branches usually from a simple stem or vertex without forming loops or polygons
      a genealogical tree
      phylogenetic trees
      b: a much-branched system of channels especially in an animal body
      the vascular tree” [ https://www.merriam-webster.com/dictionary/tree ]

    • Torbjörn Larsson | July 26, 2020 at 8:42 am | Reply

      Good catch! Borrowing words goes both ways. C.f. “theory” which was used in science for “well supported description” but borrowed into modern language as “unsupported idea”. Here it seems chimera has been borrowed first into biology for cellular and later genomic descriptions of disparate mixes: “3: an individual, organ, or part consisting of tissues of diverse genetic constitution
      A hybrid created through fusion of a sperm and an egg from different species is a chimera.” [More than one link makes the comment go into moderation, but this and the later quote are from Merriam-Webster online dictionary for “chimera” respectively “tree”.]

      Once used terminology tends to leak into other areas of science. C.f. “tree” in biology as organism and as description of branching processes, and now computer science and what not. “2: something in the form of or resembling a tree: such as
      a: a diagram or graph that branches usually from a simple stem or vertex without forming loops or polygons
      a genealogical tree
      phylogenetic trees
      b: a much-branched system of channels especially in an animal body
      the vascular tree”.

  2. Torbjörn Larsson | July 26, 2020 at 8:30 am | Reply

    That was a complex pathway. From the paper:

    “Modern meteorite classification schemes assume that no single planetary body could be source of both unmelted (chondritic) and melted (achondritic) meteorites. This dichotomy is a natural outcome of formation models assuming that planetesimal accretion occurred nearly instantaneously. However, it has recently been proposed that the accretion of many planetesimals lasted over ≳1 million years (Ma). This could have resulted in partially differentiated internal structures, with individual bodies containing iron cores, achondritic silicate mantles, and chondritic crusts. This proposal can be tested by searching for a meteorite group containing evidence for these three layers. We combine synchrotron paleomagnetic analyses with thermal, impact, and collisional evolution models to show that the parent body of the enigmatic IIE iron meteorites was such a partially differentiated planetesimal.”

    “Here, we use paleomagnetic analyses to search for the existence of a molten metallic core within the IIE parent body. We are motivated by the fact that if the IIE parent body had a metallic core, it could have powered a dynamo-generated magnetic field that would imprint a natural remanent magnetization (NRM) in the IIE meteorites (21). We then combine these paleomagnetic measurements with impact simulations, thermal modeling, and collisional evolution considerations to establish the likelihood that impacts could form IIE-like iron meteorites.”

    “Therefore, the measured 40Ar/39Ar ages indicate that Techado and Colomera recorded a magnetic field of approximately 78 ± 13 Ma and 97 ± 10 Ma after the formation of CAIs, respectively (31).”

    “The magnetizing field could conceivably have been generated by at least four main sources: the solar nebula, the early solar wind, impacts, or a core dynamo. The relatively young age of the NRM rules out the solar nebula, which dissipated within 5 Ma after CAI formation (33). On the other hand, the slow kinetics of tetrataenite formation exclude time-variable or transient field sources like the early solar wind and impact-generated plasma, which are expected to have varied on time scales shorter than a few days (34, 35). This leaves one remaining source capable of magnetizing Colomera and Techado: a core dynamo. Because core dynamos powered by thermal convection were only sustainable during <20 Ma after planetesimal formation (36, 37), the dynamo would have been powered by the thermochemical convection driven by core crystallization."

    They also discuss formation place, which is most likely between Earth and asteroid belt distance (1-2 au), and parent body similarity to Vesta.

  3. Why is this so difficult to comprehend?
    Liquid/semi-liquid and solid states exist all the time in materials that are heated to their melting points. They must remain at, or above those points to ensure full melt, but during the transition, all three states exist at one time. We are so used to our ‘orderly’ solar system, what about a history of wild, uncontrolled convergences, where laws of pravity and order had not yet taken form?
    Even the improbable is possible, regardless if we witness it or not.
    Iron chondrites exist, metallic elements with a Stony bodd, so why not ‘plastic’ irons?
    The state of not quite molten, not solid. They just might be that piece of uncooked chicken in a pot of boiling stew. Unusual, yes, but certainly not impossible.

    • Torbjörn Larsson | July 27, 2020 at 12:30 pm | Reply

      Have you read the paper? It speaks of a complicated pathway to account for the observations, including the problem to account for processes that makes bodies that are neither fully melted nor fully unmelted – the many bodies expected and seen before.

      No, it’s not impossible, it is just a very fine balance. That was part of the problem.

  4. l always chuckle when they confidently say when and how the universe was formed. These are the same kind of people that confidently stated for many years the earth was flat.

    • Torbjörn Larsson | July 27, 2020 at 12:36 pm | Reply

      Relevance? This was about the origin of our planet system, only 1/3 of the universe age and a very minute part was in question.

      Historicity? You don’t give references, likely because if you had bothered to check no literate person – certainly not science – has claimed earth since the iron age. Your claim is a well know myth, related in the closest encyclopedia (10 s search) [ https://en.wikipedia.org/wiki/Flat_Earth ]. “In the early 4th century BC Plato wrote about a spherical Earth, and by about 330 BC his former student Aristotle provided evidence for the spherical shape of the Earth on empirical grounds. Knowledge of the spherical Earth gradually began to spread beyond the Hellenistic world from then on.[1][2][3][4]”

      “Beginning in the 19th century, a historical myth arose which held that the predominant cosmological doctrine during the Middle Ages was that the Earth was flat. An early proponent of this myth was the American writer Washington Irving, who maintained that Christopher Columbus had to overcome the opposition of churchmen to gain sponsorship for his voyage of exploration. Later significant advocates of this view were John William Draper and Andrew Dickson White, who used it as a major element in their advocacy of the thesis[129] that there was a long-lasting and essential conflict between science and religion.[130] Some studies of the historical connections between science and religion have demonstrated that theories of their mutual antagonism ignore examples of their mutual support.[131][132]

      Subsequent studies of medieval science have shown that most scholars in the Middle Ages, including those read by Christopher Columbus, maintained that the Earth was spherical.[133]”

  5. I just wished we knew what star gave us all this material Earth is made up of with!!! Where is it’s core or nebula???

    • Torbjörn Larsson | July 27, 2020 at 12:54 pm | Reply

      Planetary systems are formed from molecular clouds, the generic model is a 3 stage process. The molecular cloud itself derives from many generation stars, the heavy element concentration is ~ 1,000 times the one after thge first generation stars in the universe.

      The planetary system forming stages was that first gravitational and/or shock wave clumping makes a 1st generation lareg stars, which live a short time and go supernova. Shock waves makes a 2nd generation massive stars that lives longer and blow bubbles in the cloud. The compression shells breaks up into 400 – 600ish stars of Sun or smaller size.

      If the star cluster is open it will disperse. The 4.5 billion year Sun has made ~ 20 of ~200 million year long orbits around the Milky Way, so we have a hard time recognize Sun’s “siblings”. (None found for sure yet, a few possible candidates with the same age estimate and spectroscopic element signature.)

      So some of the element making stars are gone, but isotope observations have long made scientists propose 1 -2 supernova that seeded the cloud just before our system was formed. But here is a new hypothesis: “Merging neutron stars gave solar system heavy elements” [ https://astronomy.com/news/2019/05/merging-neutron-stars-gave-solar-system-heavy-elements ]

      “Stars have to make all the other elements in the universe in their nuclear fusion-powered cores. And even they stop when they reach the element iron, only 26th in order on the periodic table (elements are arranged from lightest, with the smallest number of protons, to heaviest, with the most). After that, everything we have comes from more exotic or extreme processes, like the explosion of a star at the end of its life – or the dramatic collision of one star with another. The former is much more common than the latter, at least when we’re talking about neutron stars, the dense cores of massive, dead stars. They make the most dramatic collisions, only slightly less energetic than two black holes colliding.

      Neutron star mergers occur only a few times per million years in our galaxy (though we sometimes track them from even further away via gravitational waves). By contrast, a new supernova explodes a few times per century somewhere in the Milky Way. Bartos and Márka looked at these rates, and compared them to the ages of materials they measured from our solar system.”

      “The researchers then ran simulations of the Milky Way’s evolution, testing different histories of neutron star mergers and how they would affect the composition of our solar system today. They found that a single neutron star merger could have deposited a substantial amount of the heavy elements we have today by exploding less than 1,000 light-years away from the dust cloud that would one day become our solar system.

      It would have dumped something like a tenth of the Moon’s mass worth of heavy material into the solar system. “If a comparable event happened today at a similar distance from the solar system, the ensuing radiation could outshine the entire night sky,” said Márka in a statement.”

      So if you want to have a remarkable star, or star binary, that was involved, that is a candidate. I don’t think we could identify any more remains than what we see in our system though (but I’m no specialist here).

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