Strange meteorites hint at young Jupiter’s travels

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NASA’s Juno spacecraft captured this view as it closed in on Jupiter’s north pole, about two hours before closest approach 27 August 2016. The movement of giant planets such as Jupiter may have been responsible for the development of a unique class of meteorites.
NASA / JPL-Caltech / SwRI / MSSS

Just a few million years after matter in the solar system started to coalesce into solids, Jupiter went on a little trip.

Its migration towards the sun and out again stirred up the asteroid belt and created rare meteorites, computer simulations suggest – meaning the gas giant pretty much fully formed in the solar system’s early days.

Brandon Johnson from Brown University and colleagues in the US modelled the effects of a young, hefty Jupiter’s relatively quick jaunt near the sun and found it could create the violent conditions needed to produce so-called CB metal-rich carbonaceous chondrites.

The work, published in Science Advances, also explains why Mars isn’t bigger than its current size.

Jupiter – the largest planet in the solar system – currently orbits the sun at around five astronomical units (the distance between Earth and the sun in one astronomical unit).

But most planetary scientists think the gas giant moved around in its earlier years, shoving gas, dust, asteroids, comets and developing planets or flinging them away, thanks to its immense gravitational tug. 

(Space programs exploit this “gravitational slingshot” effect when sending spacecraft on long journeys.)

Nailing down exactly where and when Jupiter travelled, though, is no easy feat. The solar system is around 4.6 billion years old – there’s been plenty of time for it to move around.

This is where CB chondrites come in. Planetary scientists are interested in them because the tiny blobs of once-molten material they contain all date back to a very specific, brief period in the early days of the solar system – around five million years after the first solids materialised.

They also contain metallic grains, thought to have condensed from vaporised iron. But vaporising iron takes super-high-velocity impacts – around 20 kilometres per second – which create shockwaves through the material and form a supercritical fluid from which the grains condense.

And in the solar system’s early days, fledging asteroids and planetary embryos were simply too slow to manage this.

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Combined X-ray elemental maps in magnesium (red), calcium (green) and aluminium (blue) of the chondrite Hammadah al Hamra 237.
Alexander Krot, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa

So how did the metallic grains form? Johnson and his colleagues suspected the hulking mass of Jupiter may have had something to do with it.

They simulated what would happen to objects in the asteroid belt if Jupiter executed the “Grand Tack” – a manoeuvre that saw it push in from the outer reaches of the solar system where it formed, towards the inner solar system, then be dragged back thanks to the still-forming Saturn.

The researchers found Jupiter’s presence near the asteroid belt was plenty enough to accelerate bits of material to iron-vaporising-velocities. The most extreme collision was a 90-kilometre-wide object slamming into one 300 kilometres wide at around 33 kilometres per second.

This would have vaporised around a third to two-thirds of the larger object’s iron core, spraying plenty of CB chondrite building blocks.

Jupiter’s round trip only took around half a million years, the researchers found – a cosmic blink of an eye. They also suggest that Jupiter’s mass stole much of the material that would otherwise have packed onto Mars, which is much smaller than planetary models predict.

The team admits that the study doesn’t necessarily preclude other migration scenarios. “It’s possible that Jupiter formed closer to the sun and then migrated outward, rather than the in-then-out migration of the Grand Tack,” Johnson said.

Still, the work suggests that Jupiter reached its present-day size in the solar system’s early days.

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