How chondrules formed

Chondrules are millimetre-sized beads of once-molten rock, and they make up much of most stony meteorites. They are among the oldest objects in the Solar System, yet more than a century after they were first studied under the microscope, scientists still disagree about what melted them. This explainer sets out the clues, then the main ideas, fairly, with the evidence for and against each.

1. Step 1 of 7

   What a chondrule is

   A chondrule is a small sphere, usually between a few tenths of a millimetre and about 2 millimetres across, made mostly of the silicate minerals olivine and pyroxene, often with some glass. Chondrules can make up well over half the volume of an ordinary chondrite, and they give chondrites, the most common meteorites, their name.

   Under the microscope they show textures that could only form from a melt. Some have parallel bars of olivine, grown when a fully molten droplet cooled quickly. Others, called porphyritic, have many crystals set in glass, showing they were only partly melted or held seed crystals that survived. The puzzle is what heated so much dust so strongly, so briefly.

2. Step 2 of 7

   The clues they keep

   Chondrules are a record of their own making. Experiments that try to copy their textures show they were heated to roughly 1,500 to 1,900 °C, often within minutes, and then cooled over hours, at tens to hundreds of degrees per hour. That is far too fast for cooling inside a large body, and far slower than a lone droplet radiating its heat into empty space.

   Other clues narrow it further. Many chondrules contain relict grains, crystals left over from an earlier chondrule, so heating happened repeatedly. Most kept volatile elements such as sodium that should have boiled away, which points to dense clouds of chondrules and vapour rather than isolated drops. And each chondrite group has its own typical chondrule sizes and compositions, so they formed in separate batches, in different places.

3. Step 3 of 7

   The clock: when they formed

   Radioactive clocks show chondrules did not form all at once. Their ages, measured with the short-lived isotope aluminium-26 and with lead–lead dating, spread from close to the time of the oldest solids, the CAIs, to about 4 million years later, with most between roughly 1 and 4 million years.

   That long spread matters. Whatever made chondrules worked again and again in the disc for millions of years. It also overlaps the time when the first planetesimals were growing and some were already melting, which keeps both of the main families of explanation in play.

4. Step 4 of 7

   Idea one: shock waves in the disc

   In the nebular shock idea, chondrules melted in the gas of the disc itself. A shock wave is a sudden jump in gas pressure and density, like the sonic boom of a jet. Dust clumps crossing a strong shock are suddenly hit by fast, dense gas and heated by friction and by the glowing gas around them, then cool as the gas behind the shock relaxes.

   Computer models of this process reproduce the measured heating and cooling rates rather well, and can melt many clumps together, as the clues require. The weak point is the source. Proposed drivers include spiral waves in a massive young disc, and bow shocks around planetesimals or young planets moving on stretched orbits, but none is yet proven to give shocks strong and frequent enough.

5. Step 5 of 7

   Idea two: splashes from colliding planetesimals

   In the impact idea, chondrules are splashes. When two planetesimals collide at a few kilometres per second, rock near the point of contact can be squirted out as a jet of melt that breaks into droplets. If the bodies were already partly molten inside, heated by aluminium-26, collisions would spray out even more melt.

   This fits the crowded, vapour-rich setting the chondrules require, and the fact that they formed while planetesimals were growing. One group, the CB chondrites, is widely accepted to have formed in a giant impact plume about 5 million years after the CAIs. The difficulty for other groups is chemistry: many chondrules look like melted primitive dust, not the processed rock expected from inside a planetesimal, and their isotopes often match the dust around them.

6. Step 6 of 7

   Other ideas

   Several other heat sources have been suggested. Lightning in dusty parts of the disc could release electrical energy in sudden bursts, although whether enough charge could build up is uncertain. In the X-wind model, chondrules melted close to the young Sun and were flung outwards by a magnetically driven wind; critics point out that chondrules in different chondrite groups look as if they formed locally, not in one shared place near the Sun.

   Others propose bursts of energy where magnetic field lines snap and reconnect, or heating by radiation from a newly formed hot planet. Each can heat dust in principle, but none has yet been shown to explain all the measured clues together.

7. Step 7 of 7

   Where the debate stands

   There is still no agreed answer, and it is quite possible that more than one process made chondrules, at different times and places. Nebular shocks and planetesimal collisions remain the two leading ideas, and most researchers would not bet heavily on either.

   What is not in doubt is what happened next. Chondrules were swept together with CAIs, metal grains and fine dust and gathered into asteroids, which is how they reached us inside chondrites. Precise ages, isotopes that show which chondrules shared a birthplace with which dust, and pristine samples brought back from asteroids are all helping to close in on the answer.

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