Giant impacts may explain the origin of chondrules

Chondrules, such as the one shown in figure 1, are millimeter-scale, previously molten droplets that make up more than half the volume of most meteorites that fall to Earth. Isotopic dating suggests they formed during the roughly 5 million years when the solar nebula was coalescing into solids (see the article by Robin Canup, Physics Today, April 2004, page 56 ). Unfortunately, there’s no consensus as to how they formed. The conventional and perhaps still leading view holds that chondrules originated as rapidly melted dust balls—flash heated by shock waves, lightning bolts, solar flares, or some other process—that crystallized in hours to days and sometime afterward became part of accreting planetesimals and planetary embryos. But no one has convincingly explained which of those potential heat sources was actually responsible.

Other theories posit collisions between asteroids as the source of chondrule-forming droplets. The idea goes back to the 1950s and gained traction a decade ago when planetary scientists realized that asteroids could grow as large as the Moon within tens of thousands of years—a much shorter time than the millions of years previously believed. But the drag on bodies from nebular dust and gas should slow their relative speeds to just a few kilometers per second, too slow to melt solid objects in a collision. And even if the bodies were to somehow collide fast enough to melt, such events would produce far more fragments of broken rock than melted rock, which doesn’t match the meteorite record. ¹

MIT postdoc Brandon Johnson, his former Purdue University thesis adviser Jay Melosh, and their colleagues now propose an often overlooked process known as impact jetting that overcomes those objections and revives the plausibility of a solid-collision origin. ² The process describes the high-speed squirting of superheated solid, molten, and gaseous debris from the explosive collision of two objects whose curved or irregular surfaces close together like scissors blades. Long exploited in military applications—impact jetting explains how an armor-piercing bazooka rocket works—the pinching of material generates up to several times the pressures encountered in planar impacts. Simulating the jetting process at high spatial resolution is computationally expensive, though, and the MIT–Purdue researchers are the first to numerically resolve it in a planetary context.

Blobs and droplets

Jetting isn’t the only process that can melt relatively slow-moving bodies. The decay of short-lived radioisotopes, such as aluminum-26, whose daughter product, magnesium-26, is found in the most ancient meteorites, would supply more than enough heat to populate the solar nebula with molten asteroids. And the inevitable splashing of any one of them during even low-speed collisions would spew out magma as great plumes of droplets.

But chemical differentiation, which happens as dense metals sink to the center of a large enough body, particularly one already molten, would alter the splashed melt’s composition. Chondrites, by contrast, contain dense metal and reflect the primitive, undifferentiated composition of the solar nebula. Moreover, while working on another project last year, Johnson and Melosh realized that splashing among molten asteroids would produce liquid blobs roughly 40 m in diameter—hardly droplets.

At the time, the pair were running simulations of droplet-forming, crater-making impacts, ³ such as the dinosaur-killing Chicxulub impact 65 million years ago (see Physics Today, May 1982, page 19 ). In that incident, the asteroid hitting Earth was larger than 10 km and its fireball spewed countless spherules that, apart from their chemical compositions, are indistinguishable from chondrules. When Johnson calculated how quickly a big cloud of debris would cool after an even larger solid impact—between planetary embryos hundreds to thousands of kilometers in diameter—he found cooling rates of 10–1000 K/hr, consistent with what molten chondrules must have experienced.

The researchers were puzzled, however, at what mechanism might give rise to the melt needed to make the chondrules, given the constraint of slow collision speeds in the solar nebula. Melosh wondered if the porous nature of rocky asteroids might account for it, since the rock should heat up during compression just like air in an inflating bicycle tire. Johnson, meanwhile, hit on impact jetting.

High-speed jets

Johnson simulated the effect for a variety of embryos, large and small, at a range of collision speeds. When the edge of a spherical projectile collides with and penetrates either a flat plate, as shown in figure 2’s simulation, or another embryo, he found that the resulting high pressures—exceeding 80 GPa in some cases—and nearly 2000-K temperatures can create a pocket of melt and vapor next to the near vacuum of outer space. ^{2 , 4} The enormous pressure gradient then accelerates a plume of molten rock and gas into the nebula at speeds well above the impact velocity.

The ejecta’s speed explains how the impacts—which spew much more, but slower, broken rock than melt—can end up making chondrule-rich meteorites. The only material that escapes the gravitational pull of the embryo, the researchers argue, is a jetted mixture of melt (nascent chondrules) and lightly shocked solids in which the chondrules eventually become embedded. And although the amount of melt squirted into space turns out to be small—just 1% of the impactor’s mass—Johnson’s simulations reveal that collision speeds as low as 2.5 km/s can create it.

To place the jetting model into a planetary context, Purdue University’s David Minton simulated the orbital dynamics of bodies that would grow to become the planets and asteroids we see today. Starting with a reasonable mass of planetary embryos, whose initial size distribution ranged from 100 km to 1000 km in diameter, he used his simulations, supplemented with a lookup table from Johnson’s jetting results, to estimate the locations, frequency, sizes, and velocities of potential chondrule-forming impacts. ² Those impacts began occurring a few thousand years into the simulation and continued for millions of years, a time span consistent with the life span of the nebula and the spread of ages found among real chondrules dated isotopically.

The simulation’s conservative estimate of 10²³ kg for the accumulated melt made by jetting is several times greater than the mass thought to reside in the asteroid belt, says Johnson. And his separate calculation of likely droplet sizes—determined by the balance of surface tension and inertial forces acting on the accelerated plume of gas and melt— reproduced a millimeter-scale distribution over a wide range of impactor sizes. Together, the results explain the chondrules’ abundance in meteorites.

A reinterpretation

According to the impact-jetting theory, chondrules emerge from much larger bodies and preferentially accrete onto smaller ones, which have a larger collective surface area. That logic led the MIT–Purdue collaboration to a succinct conclusion stated in the final line of their paper: “Chondrules are not the direct building blocks of the planets, but merely a byproduct of their accretion.” ²

“That statement has outraged meteoriticists more than anything we could have said or done,” says Melosh. “It had been believed that with up to 95% of chondrites chock-full of them, chondrules must be those building blocks, and generations of researchers have put the motherhood statement in their grant proposals. We’re denying that and arguing that while chondrules are certainly part of the construction debris, they’re not the main game.”

That interpretation doesn’t change the fact that meteorites reflect conditions in the early solar nebula. Rather, the new model is diagnostic of the process—how and in what environment our solar system was built. “Although more work is needed to investigate and test the new model,” says the University of Chicago’s Fred Ciesla, “it’s likely to open a debate I expect to continue for years.”