Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
The formation of the solar system is a mysterious event with many unresolved details, which is why it is a fascinating topic. Somehow, gas and dust came together to form the sun, planets, moons, and all the other objects we are familiar with. The building blocks for this process, which we have access to, are contained in the meteorites that find their way to Earth, and in them is a chemical and geometrical mystery that so far has no clear resolution. This is the mystery of the chondrules.
What Are Chondrules and Chondrites?
Chondrules are rounded objects typically from 0.1 to 5 mm in diameter, with an average size of roughly 0.4mm. They are made primarily of olivine and pyroxene with the occasional bits of glass and other trace minerals. Based on radioactive dating of Al-26, which is fairly quick and so leads to low levels, we know chondrules formed around 4.567 billion years ago, sometime between dust gathering up to become the planetesimals upon which the rest of the solar system was made from (O’Callaghan 60-1, Kieffer 333, Hutchison 446, Connolly 1887, Weidenschilling 681).
The only material older than chondrules are calcium-aluminum-rich inclusions, or CAIs, beating chondrules by about 1-3 million years. CAIs have been spotted in chondrules, further cementing their precursor status. When grouped together, chondrules form a chondrite meteorite (which are a majority of the 60,000+ meteorites found on Earth so far) which along with CAIs form a record of the earliest days of the solar system (Ibid).
Many subsets of chondrites exist, with special properties pertaining to each one but in general they are made of the same stuff (lots of chondrules, organics, olivine, pyroxene, and such), just in different amounts and forms (Ibid).
The Intrigue of the Chondrules
It is because of their early inception that chondrules are important for us to study. By examining their structure and composition we can build models to explain how they formed in the first place. And there lies a big problem: as amazing as it may sound, we really don’t know how they formed. We have theories, yes, but each has major and seemingly irreconcilable problems. Some of the theories include leftovers from the planets formation, the possible seedlings of the planets, fused dust from lightning strikes, colliding protoplanets, even shockwaves induced. Some of these we will explore further, noting their strengths and weaknesses (O’Callaghan 60).
What we need to establish first are the basic facts that are generally agreed upon for chondrules to have formed. They are igneous rocks, required over 2000 degrees Celsius to heat up the material, then rapid cooling at different rates over days or even hours. We suspect this high temperature/high pressure formation based on their spheroidal shape, the dendrite growth pattern of the crystals seen, the presence of glass, and other patterns indicative of extreme formation. The melting pattern of chondrules is dependent on the maximum temperature reached, the duration of the heating, and the makeup of the chondrule (O’Callaghan 62, Connolly 1887-8, Kieffer 333).
The timing is critical, based on the amount of sodium, potassium, and sulfur present, for these are volatile elements that would disappear if things stayed hot for too long. Their rather uniform nature points to a formation over the entire solar system, rather than be restricted to one location. Also supporting this is the relative match in mineral content between the olivine/pyroxene grains around the chondrules themselves. This also hints at reconstituted chondrules, gaining material from their surroundings. The dust that is present indicates a long wandering time in the early years of the solar system (Ibid).
Now, let’s talk about the problems. And there are plenty of them. For starters, after analyzing their geochemical as well as compositional makeup, it becomes clear that chondrules have multiple reheating events going on. This is especially prevalent in the Fe-rich/MG-rich chondrules, which show many heating events that lead to parts being crystalized and others not. That means we have to try and determine how such an event can happen. Based on the elements present, we have to be careful how that reheating occurred, otherwise we could lose some of those volatile elements. By testing various ways in which heating/cooling is impacted (such as morphology, stability of the elements, and the presence/absence of different materials) separately, scientists will hope to unveil how such a scenario could happen (Connolly 1889).
The dating of chondrules has also led to some interesting issues. We’ve mentioned the use of Al-26 as a dating tool, but other radioactive elements exist too. U-235 decays into Pb-207 with a half-life of about 700 million years and U-238 decays into Pb-206 with a half-life of about 4 billion years. These too give that age estimate of about 4.567…but that’s about. There is an uncertainty of about 3 million years, possibly indicating the general span of chondrule formation. Frequently, CR chondrules are used for aging purposes, having been shown to be the oldest type, having very little thermal history and generally low levels of lead in them. When one looks at this sample group, most of them are older than the rest, indicating that chondrule formation was more prevalent in the earliest days, then tapered off as time went on (Bollard 1).
This is a big issue, because small millimeter sized objects in this era should have a short lifespan courtesy of aero drag from the surrounding gas and dust, causing them to fall into the early sun within 100,000 years. But here our data shows a range of formation, indicating that somehow something had to push out significant mass to counter this drag or something had to capture chondrules to prevent them from falling in. Neither of these seem to have any promising leads, confounding the mystery (Bollard 3, Weidenschilling 682).
By the way, that reheating issue also plays a part in the aging of the chondrules. If a material is heated to a sufficient temperature, this can cause the lead in the object to be evaporated. This can cause our U to Pb ratios to be thrown off, possibly putting the age of a sample into question. Different Al-26 levels seen amongst similar types of chondrules also points to this potential problem of reconstitution as well as unaltered and reprocessed CAIs being seen in the same chondrites (Bollard 3, Weidenschilling 681).
Solar Nebula Theories
Some scientists who study chondrules subscribe to the solar nebula, from which the solar system originated, as the possible contributor to chondrules. Shock waves from moving objects would create a front passing through the gas and dust, heating up material and giving sufficient pressure to condense material. Maybe lightning, resulting from static charge differentials between dust and gas, condensed material into chondrules (O’Callaghan 63, Connolly 1890-1, Kieffer 333, Weidenschilling 681, Hutchison 447.
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Then again, maybe it was the early Sun, then and still the dominant energy source of the solar system, somehow contributed to chondrules via outflowing material but our knowledge of protoplanetary discs and their general mechanics remains incomplete. But all signs point to any of these as failures, because they should all have occurred prior to chondrule formation, and yet these theories correctly predict the chemical similarities and gradual changes to chondrules as they grow younger (Ibid).
Others instead turn their eye to colliding objects, something else that would have been a frequent event in the early solar system, as the generators of chondrules. Planetesimals would have been frequently colliding with each other, resulting in high speed and high-temperature material being jetted out of the colliding sites. Some work even shows that partially melted planetesimals would be best for this, allowing more jetting to occur, but how the multiple reheating would have occurred here remains unknown.
At the point of impact, you can get the necessary temperatures required and at the necessary cooling rate, so long as it’s a Fe-rich or Mg-rich chondrule. Other classes require more complexity to get their chemical composition, bringing complexity to the impact model that removes its favorability (O’Callaghan 63, Connolly 1891, Kieffer 333, Weidenschilling 682).
The size of the impacting planetesimals is also a big unknown, but many suspect that asteroid-sized objects bombarded by meteorites could be sufficient. You have to have the right ratios of mass for this to work because if the impacting objects are too close them they will destroy each other, but if too small, then they will not have sufficient energy to create chondrules.
The planetesimals of the time would also have to be large enough to survive aero drag from the surrounding material, meaning that we would need an object at least 1 kilometer in size, which should have taken about 1000 orbits to achieve such a size. It’s certainly possible but asks for much coincidence to have many heating events happen (Ibid).
Some have noted different isotope ratios amongst chondrules, possibly indicating that the supposed uniform formation may not be true. Instead, it could be that an inner and outer solar system variation did exist, but then once Jupiter’s orbit migrated outward it created gravitational perturbations that mixed it all up, creating new chondrules in the process. Maybe Jupiter played a different role, instead acting like a shepherd.
Planetesimals had to have CAIs in them, which would have been broken up and reconstituted in planetesimal collisions. This is supported in chondrite composition, both in the patterns of the material and in the makeup. But somehow you have to have the conditions for high-speed impacts but low-speed accretions of that material, something that seems hard to achieve together (O’Callaghan 64, Weidenschilling 682-3).
Jupiter could have solved this, using gravity to grab planetesimals to collide them, then clear out the area to allow the reconversion of the material. But you need special circumstances for this to work, mainly because gravitational interactions with Jupiter end in ejection from the solar system. But if the planetesimals were in orbital resonances with Jupiter (much like the Galilean moons are) then opportune lineups could have occurred within the solar system plane. This could go on for a long time based on planetesimal populations, also matching up with the extended time formation of chondrules (Ibid).
Radiative Heating Theory
One interesting possibility is that dust clouds near the surface of active planetesimals could get heated as they passed by active lava plumes, condensing into chondrules in the process. But would a planetesimal’s surface ever stay hot enough, for long enough, to this occur? How would surrounding gas inhibit or promote this interaction? (Connolly 1892. Kieffer 333)
Of course, attempting to recreate chondrules in the lab has yielded some interesting results. Some try to physically recreate the materials, such as Aimee Smith (University of Manchester) who use powders and subject them to various conditions as underlined by the theory being tested. Others use simulations to try and model the expected results of each scenario (O’Callaghan 66).
So, a lot is going on in this field. It’s very dynamic right now, and I am sure that before this is all settled there will be even more surprises in store for us when it comes to chondrules. Come back every once in a while, and I will try to update this article as new findings are made.
J. Bollard, J. N. Connelly, M. J. Whitehouse, E. A. Pringle, L. Bonal, J. K. Jørgensen, Å. Nordlund, F. Moynier, M. Bizzarro, Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules. Sci. Adv. 3, e1700407 (2017).
Connolly, H. C., Jr., and R. H. Jones (2016), Chondrules: The canonical and noncanonical views, J. Geophys. Res.Planets, 121, 1885–1899, doi:10.1002/2016JE005113.
Hutchison, R. and C.M.O. Alexander, D.J. Barber. “Chondrules: chemical, mineralogical, and isotopic constraints on theories of their origin.” Phil. Trans. R. Soc. Lond. 446-7.
Kieffer, Susan Werner. “Droplet Chondrules.” Science Vol. 189 No. 4200. 333.
O’Callaghan, Jonathan. “The Curious Science of Chondrules.” Scientific American Mar. 2021. Print. 60-4.
Weidenschilling, S.J. and F. Marzari, L.L. Hood. “The Origin of Chondrules at Jovian Resonances.” Science Vol. 279. 30 Jan. 1998. Print. 681-3.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2022 Leonard Kelley