New Group Paper about the Origin of Chondrules

This blog has been written by group member Dr Rhian Jones

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Chondrules are small round stony beads that are the main component of chondrite meteorites. They date to within about 2 million years of the formation of the solar system, making them some of the oldest rock particles that we can study in the laboratory (around 4.6 billion years old). We know that chondrules were formed from separate droplets of melted rock, each one about a millimetre across. But discussions about exactly how they formed have been intriguing meteorite research for over a century.

Rhian_1_Parnallee chondrite

The Parnallee chondrite, which consists of a dense pile of chondrules, each about 1 mm across. Photo from the Institute of Meteoritics, University of New Mexico.

From studying chondrites we know a lot about chondrules, such as their chemical and isotopic compositions, the minerals they are made of, and how different types of chondrules are present in different groups of chondrites. From comparisons of natural chondrules with experiments done in laboratories, we know that after they melted they cooled down quite quickly, at rates of around a hundred degrees per hour. But what we don’t know is a fundamental question: what process or processes heated chondrules to the point that they were melted? Chondrule formation has been the subject of many creative ideas, ranging from heating fluffy lumps of dust when shock waves passed through the protoplanetary disk, to producing splashes of melted rock when two planetesimals collided. It is important to understand this question, because chondrule formation could have been a significant first step towards planet formation, and could have been a process that affected a large amount of the rock that eventually made up the Earth.

Rhian_2_Allende_chodnrule

Image of a “barred olivine” chondrule from the Allende chondrite, taken in a polarizing microscope. The chondrule is about 1 mm across. Photo by Rhian Jones.

Group member Rhian Jones, along with Harold Connolly of Rowan University, USA, has recently published a review paper on chondrule formation, titled “Chondrules: The Canonical and Noncanonical Views”. The title refers to the different types of chondrule formation models. By “canonical”, we mean well-accepted arguments that suggest that chondrules were formed from free-floating dust particles in the protoplanetary disk. “Non-canonical” models, which are gaining a lot of current interest, are that chondrules were formed from collisions, and can therefore be considered as the by-products of planetesimal formation. This distinction is important if we want to understand the relationship of chondrule-formation to the process of building planets.

Any valid model for chondrule formation must be able to account for the properties of chondrules that we observe in chondrites. This means that lively debates about chondrules take place between meteorite researchers who study the chondrites themselves, and astrophysicists who produce the various chondrule formation models.

One of the reasons why it is hard to pin down the mechanism for chondrule formation is that if we did not see chondrules in meteorites, we would never predict that they would form. An exciting possibility for the future is that as astronomers’ observations of young stars with active protoplanetary disks improve, we may be able to identify processes where chondrule formation might be taking place today.

Rhian_3_ALMA

Image of a present-day protoplanetary disk, produced by the international astronomy facility, ALMA. The dark rings in the disk may mark regions where planets are accreating.

An easily accessible overview of the paper by Connolly and Jones has now been published on the Planetary Science Research Discoveries (PSRD) website: this is an educational site sharing the latest research by NASA-sponsored scientists on meteorites, asteroids, planets, moons, and other materials in our Solar System.  “Chondrules: Important, but Possibly Unfathomable” was written by G. Jeffrey Taylor of the Hawai’i Institute of Geophysics and Planetology.

The full paper citation is Connolly, H. C., Jr., and R. H. Jones (2016) Chondrules: The canonical and noncanonical views, J. Geophys. Res. Planets, 121, doi:10.1002/2016JE005113.  It can be downloaded from here 

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About Katherine Joy

Hello! I am Katherine Joy. I am part of the University of Manchester Isotope Geochemistry and Cosmochemistry group. More details about my research interests can be found at http://www.seaes.manchester.ac.uk/people/staff/profile/?ea=katherine.joy
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