Citation: Taylor, G. J. (April, 2017) Chondrules: Important, but Possibly Unfathomable. http://www.psrd.hawaii.edu/April17/chondrules.html (date accessed).PSRD-chondrules.pdf
Chondrules are mysterious little droplets of formerly molten silicate found in chondrite meteorites. Harold C. Connolly Jr. (Rowan University, New Jersey) and Rhian H. Jones (University of Manchester, UK) have written a concise review of the properties of chondrules and ideas for their origin. Over a century of chondrule studies have revealed much about their heating and cooling histories, mineralogy, textures (mineral sizes and how the crystals are intergrown), chemical and isotopic compositions, and ages (old–they formed during the first few million years of Solar System history). Nevertheless, cosmochemists are still uncertain how chondrules formed and whether they inform us of conditions and processes in the protoplanetary disk (also called the solar nebula) or are mere by-products of planet formation.
Chondrules are millimeter-sized, originally molten, rapidly cooled, roundish objects in chondrites, a type of primitive, meteorite that contains chondrules. Considering that chondrites are abundant and the oldest rocks in our museums, chondrules must be important. But as Harold Connolly and Rhian Jones point out, from an astrophysical, astronomical, planetary science, or geological perspective, nobody would predict that planetary materials would be processed into tiny molten droplets before accumulating into planetesimals. Once they were identified, inventive ideas blossomed to explain how they formed, with ideas coming from astrophysics and cosmochemistry.
Photomicrograph of the chondrite Semarkona [ Data Link from the Meteoritical Bulletin Database ], showing that it is mostly a pile of chondrules. In this sample, most chondrules are about 1 millimeter in diameter.
The existence of chondrules forced us to invent processes operating in the solar nebula and to test if they fit into our models of protoplanetary disk formation and evolution. Others skipped that stage and examined processes that operated during and after formation of planetesimals. Connolly and Jones raise the important question of whether chondrules are the keys to understanding previously unknown processes in the protoplanetary disk or are the by-products of planet formation. Whichever it turns out to be, chondrules tell a story of our early Solar System, and even in other protoplanetary disks and planetary systems surrounding other stars.
The first step is to use the properties of chondrules (compositions, mineralogy, sizes) to determine thermal histories (how hot and for how long?), the environments in which they formed, how they acquired their chemical compositions, and when did it happen?
To put chondrules in the right cosmochemical context, Connolly and Jones discuss the critical aspects of chondrites, which we can view as long-term chondrule storage facilities. Chondrites come from asteroids, though we do not know which asteroids. We know that some must come from asteroid 25143 Itokawa (see PSRD article: Samples from Asteroid Itokawa). In many ways, chondrites are like sedimentary rocks on Earth: they contain components (mostly chondrules) that formed at different times and in different places, yet end up mingling in a gravitationally-bound pile. Connolly and Jones outline the importance of chondrites, besides being repositories of chondrules.
The top seven reasons why chondrites are important are:
Nobody has questioned the conclusion that Gustav Tschermak made in 1885 that chondrules are little igneous rocks. They are rapidly cooled droplets of molten silicate. Central factors about this melting are open questions, such as the highest temperatures to which they were heated (called the peak temperature), how long they were heated, and how fast they cooled. All these factors contain information about the environment where the melting and cooling occurred.
Chondrules vary in their appearance, main minerals, and chemical compositions (especially in the ratio of oxidized iron to magnesium). The table below gives an idea of their variability and of how stunning chondrules are when thin slices are viewed in polarized light under the microscope. Not all chondrule types are listed here. (The term porphyritic means that some crystals are much larger than the surrounding areas.)
PHOTOMICROGRAPHS OF CHONDRULES IN POLARIZED LIGHT
Courtesy of Ted E. Bunch (Northern Arizona Meteorite Laboratory) and Harold C.Connolly Jr. and Rhian H. Jones.
Barred olivine chondrules have long parallel arms of olivine in a fine-grained to glassy matrix.
Radial pyroxene chondrules have thin sheets of low-calcium pyroxene radiating out from a single point where crystal growth started. Same credit.
Porphyritic olivine chondrules have large crystals of olivine surrounded by a fine-grained to glassy matrix.
Porphyritic pyroxene chondrules have large crystals of low-calcium pyroxene in a fine-grained to glassy matrix. This one also contains (brightly-colored) olivine crystals.
Backscatter electron image of (bright) metallic iron-nickel blobs in porphyritic olivine chondrule. Scale bar is 200 µm.
Chondrule compositions vary impressively, an important fact that hypotheses for their origin need to take into account. An example of the large range in the compositions of chondrules is shown in the graph below that has data from four different chondrites. Another important (and amazing) feature is that any given chondrite contains essentially the full range of chondrule compositions shown in the graph. Models for chondrule origin need to explain how this compositional diversity arose.
|Variation in the concentrations of magnesium oxide (MgO) and silicon dioxide (SiO2) in chondrules from four different chondrites. Any model for chondrule formation must account for such variations in chemical composition. Data are from the Ph.D. dissertation of Jana Berlin (University of New Mexico) and can be found at http://hdl.handle.net/1928/10275. The research was done with Drs. Adrian Brearley and Rhian Jones at the University of New Mexico. (Jana is now at Bruker, Inc. in Berlin, Germany.)|
To understand the environment in which chondrules formed, scientists have performed numerous melting experiments on concoctions with the same chemical compositions as chondrules, paying close attention to reproducing the crystal shapes and intergrowths seen in natural chondrules. This work was valuable in assessing the rates at which chondrules cooled and in demonstrating that many were not completely melted when they formed. (See PSRD article: Dry Droplets of Fiery Rain.) The experiments indicate that some chondrules, such as barred olivine chondrules, were heated to a temperature at which the entire droplet is molten (1750 to 2200 oC, depending on composition). In contrast, experiments show that porphyritic chondrules were heated to only 1550 oC, which implies that some unmelted debris remained from the heating phase before cooling started. Porphyritic textures are matched by experimental cooling at rates between 0.5 and 100 oC/hour.
Non-porphyritic chondrules probably cooled much faster, but there is a complex interplay between peak temperature and cooling rate: the non-porphyritic chondrules might have been heated hot enough to remove all traces of crystal nuclei (the building blocks for crystal formation), yet cooled slowly. In other words, non-porphyritic chondrules might not have had any leftover solids after the heating phase of their formation was done; but porphyritic chondrules did have dusty grains left over. In fact, studies of natural chondrules show that some porphyritic chondrules contain relict olivine grains the size of the porphyritic olivine crystals. This indicates some recycling of pre-existing chondrules during chondrule manufacture. An additional type of chondrule is characterized by being very fine-grained and depleted in volatile elements such as sodium and sulfur. These appear to have been heated to high temperatures and cooled on the order of a few tenths of a degree C/hour. See PSRD article: Relicts from the Birth of the Solar System.
There is no shortage of hypotheses for chondrule formation. Connolly and Jones summarize the ideas and their variants. I will give a brief overview of the main types of ideas, many of which have been discussed in PSRD articles before.
Early active Sun:
The Sun is hot, so logically if material drifts close to it, it ought to be heated. More important, the young Sun was actively spewing out material that in principle could interact with the dust in the solar nebula, or at least transfer heated products to further out in the solar nebula. These ideas are good, but none has detailed how the mechanism reproduces chondrule thermal histories that include rapid heating and somewhat slow cooling.
Impacts between planetesimals:
You don't build planets without collisions, and some might be fast enough to cause melting. In 2011 Erik Asphaug (then at the University of California, Santa Cruz and now at Arizona State University) and colleagues proposed that chondrules formed by collisions between molten or partially molten planetesimals. Ian Sanders (Trinity College, Dublin) and my colleague Ed Scott (University of Hawaii) also have proposed this idea. None of the models so far appears to have the right thermal history for chondrules or explains the compositional diversity.
An interesting variant of the impact model was proposed in 2015 by Brandon Johnson (then at MIT and currently at Brown University) and coworkers. They propose that molten material jets out from the point of impact between two fast-moving planetesimals. See PSRD article: Ancient Jets of Fiery Rain. The mechanism produces reasonable thermal histories for the droplets produced and for the abundance of chondrules. Johnson and coworkers suggest that chondrules are a "byproduct of [planetary] accretion," an important feature of all impact models in that it makes chondrules secondary, not primary solar nebula products. The model does not explain quantitatively the range in chondrule compositions.
Shock waves in the solar nebula:
Chondrule formation has been proposed by lightning discharges in the solar nebula or shock waves generated by materials accreting to the protoplanetary disk or motions of planetesimals and planet-sized objects. An interesting variant of nebula shocks is compression of the gas-dust cloud in front of a fast-moving planetary embryo. Such bodies are Moon- to Mars-sized objects formed rapidly early in Solar System history. The idea is that they eventually accrete to form the inner planets. Their orbits are not circular, which gives them high relative velocities compared to the nebular gas. As a planetary embryo plows through the gas, it creates a shock in front of it (called a "bow shock"), compressing and heating the dust and gas, possibly leading to the formation of chondrules. Melissa Morris (Arizona State University and The State University of New York, Cortland) and colleagues modeled the bow shock surrounding a Mars-sized object as it plowed through the nascent Solar System (see illustration below). They point out that isotopic analysis by Nicolas Dauphas (University of Chicago) and colleagues strongly suggests that Mars formed in 2–4 million years, while the nebular gas was still present.
|[Left] This photo shows the shock wave in front of a blunt object (moving from top to bottom) in a wind tunnel during an experiment at NASA Ames Research Center. This may resemble the shock wave preceding a planetesimal moving through the solar nebula. [Right] Computer simulation by Melissa Morris and colleagues of a Mars-size embryo moving through the dusty solar nebula.|
Chondrules formed somehow, but how? Which of the ideas will win the origin-of-chondrules challenge? Connolly and Jones give a long list of things we need to know. These include:
These are important questions, and some are way harder than others to answer. Perhaps the overriding issue to settle is whether chondrules formed by a fundamental process in the solar nebula, hence provide information about planet formation, or they formed as by-products of the planet formation. Being a by-product would disappoint many cosmochemists, although even by-products have important information about the manufacturing process that produced them.
Chondrules might have broader cosmic significance beyond helping us understand formation of our Solar System. Astronomers have identified other solar systems that contain Earth-like planetary bodies. Did chondrule formation play a role in the formation of those far-off planets? Or could we observe the record of chondrule formation that might indicate their secondary origin as planets swept through dusty disks? As Connolly and Jones point out, such observations could be impossible, at least for now, but observing chondrule formation in action would provide a look back to the time when the protoplanetary disk around the Sun was evolving and planets were growing. Studies of chondrules and ideas for their origin give astronomers something concrete to search for in those distant places.
Protoplanetary disk image produced by the international astronomy facility known as ALMA — The Atacama Large Millimeter/submillimeter Array. This image was acquired during the ALMA Long Baseline Campaign. The dark rings in the disk may mark regions where planets are accreting. (ALMA, C. Brogan, B. Saxton.) Read more about it in this PSRD CosmoSpark.
Over a century of research on these fascinating little spheroids and yet we still haven't found what we are looking for, yet cosmochemists continue their quixotic search for answers.
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