Stony meteorites reveal the timing of Jupiter’s migration

The gas giant caused iron-vaporizing collisions in the asteroid belt 5 billion years ago.
By | Published: December 13, 2016
An artist’s rendering of Jupiter
WikiMedia Commons/ Ukstillalive

The youngest stony meteorites in the solar system may reveal when Jupiter migrated through the asteroid belt. These meteors contain grains of metal that can only be the remnant of high-velocity collisions driven by Jupiter’s gravitational influence.

New evidence comes from a rare group of meteorites called CB chondrites. Formed around 4.8 to 5 billion years ago, they’re the product of objects slamming into each other at very high speeds in the wild early days of the solar system. CB chondrites contain grains of iron and nickel whose structure means they must have condensed directly from a vapor to solid form. Those tiny irregular grains of condensed metal were once part of the iron-nickel cores of rocky objects in the early solar system, and their presence points to a series of high-speed collisions between those objects, with enough shock and heat to vaporize iron.

Our solar system didn’t always look like it does now. The gas giants may have begun their lives much closer to the Sun and then migrated outward, or they may have made only a brief venture inward before retreating again. The evidence of the gas giants’ migration is etched in the solar system they left behind – the composition and dynamics of the rocky objects of the inner solar system.

“Much of the structure of our solar system is driven by the formation and migration of the giant planets,” said Brandon Johnson, a planetary scientist at Brown University who studies impact dynamics.

High-Impact Astronomy

Vaporizing iron, as you might imagine, isn’t an easy task.  To shock vaporize the iron core of a rocky asteroid requires an impact at around 18 km per second, according to lab experiments and computer modelling. At that speed, the shock wave of impact compresses and heats the asteroid’s metal core and silicate mantle, turning it into a supercritical fluid – material under so much heat and pressure that there’s no longer a difference between its liquid and gas phases.

Once the shock wave passes, the pressure lets up and the fluid becomes a mix of liquid and vapor. As it cools, the vapor condenses onto drops of molten metal and silicates, forming the metal grains we see in CB chondrites today.

The trouble is that the standard model of how the solar system formed doesn’t get objects in the asteroid belt moving at speeds great enough to produce those metal-vaporizing impacts. That requires something big to stir things up – like a gravitational boost from a wandering gas giant. With its tremendous gravity, Jupiter would have been able to slingshot objects at speeds more than sufficient to vaporize iron on impact.

Jupiter’s gravitational pull would also have pulled in material from the outer reaches of the solar system, mixing it with what was already in the asteroid belt. That accounts for the diversity of material we see in the modern asteroid belt, not to mention the presence of material from the outer solar system in today’s CB chondrites.

Scientists already generally accept the idea that Jupiter and the other gas giants didn’t form in their current positions, but instead migrated into their present orbits earlier in the solar system’s history. Most of the debate now centers on when they migrated and what course they followed. Realizing that CB chondrites require Jupiter’s interference to form, and knowing when they formed, helps narrow down the timing.

A Smashing Success

One of the most popular models for Jupiter’s migration, called the Grand Tack, has the gas giant forming in an orbit about 3.5 astronomical units, or AU, from the Sun (1 AU is the distance between Earth and the Sun). As it gobbled up surrounding gas to build its thick atmosphere, Jupiter changed the distribution of material in the solar nebula, which eventually drew the planet inward toward the Sun. It made it to the vicinity of the present-day asteroid belt, and when Saturn formed, its gravitational influence nudged both gas giants outward again, so Jupiter ended up in its present-day orbit at 5.2 AU.

Johnson and his colleagues put the Grand Tack in their model of the early solar system. It turned out that for about half a million years, Jupiter’s massive gravitational influence caused a major spike in impact velocities in the area that is now the asteroid belt. The model produced several smashing collisions, including one 33 km per second impact between a huge 300km-wide object and a smaller 90 km one. That impact, according to the researchers, would have vaporized 30% to 60% of the larger object’s metal core, producing the metal grains in CB chondrites. They published their work in the journal Science Advances.

Since scientists know, thanks to isotope testing, when the CB chondrites formed, this model helps them narrow down when Jupiter must have been passing through the inner solar system. Scientists generally agree on about how long it takes for a gas giant to form its planetary core, accrete most of the nearby gas, and start migrating. They also generally agree that migration stops once all the gas in the planetary nebula has been gobbled up, within about 100,000 to one million years after the gas giants start migrating. Putting all those pieces together means that the gas giants’ planetary cores took about 4.5 to 5 billion years to form, and that the solar nebula dispersed shortly afterward.

And that’s all from working backwards from the presence of weird grains of metal in a rare subtype of asteroid. “Our work demonstrates that meteorites may also offer clues about the young giant planets and their wanderlust,” said Johnson.

Plotting Jupiter’s Course

But narrowing down the timing doesn’t answer all of the potential questions about Jupiter’s long-ago wandering. The Grand Tack isn’t the only plausible model for the gas giant’s path; other models have Jupiter starting in the inner solar system, much closer to the Sun, and slowly moving outward.

Johnson’s modelling doesn’t narrow down exactly which path Jupiter took, but it does make it pretty clear that the gas giant had to be in the neighborhood within a pretty narrow window in order to account for the presence of CB chondrites.  The evidence supports any scenario that puts Jupiter in the right place at the right time.

“I think that models that explore a range of potential migration pathways will narrow down what most likely happened in our system. This is something that we are pursuing as future work,” he said.