Our primitive solar system was a mixed bag of ice and rock


'Time capsules' in stony meteorites show evidence of low-velocity collisions. Belinda Smith reports.


Magnetite-bearing chondrules – the small, round 'droplets' found in the stony meteorites that are among the oldest solid materials in found in the solar system
Yves Marrocchi

Clues to our early solar system sometimes literally fall out of the sky. Tiny glassy blobs trapped in meteorites show rocky material close to the sun mixed with icy material from further out during its first million years or so.

Yves Marrocchi from the University of Lorraine in France and colleagues examined slices of two meteorites and found signs they'd undergone low-velocity collisions between silicate-rich objects from the inner solar system with frozen, ice-rich objects from the far reaches.

The work was published in Science Advances.

The most common meteorite to fall on Earth, stony "chondrites" formed as dust and pebbles clumped together to create asteroids around 4.5 billion years ago. What interests many scientists, though, are the "chondrules" they carry – round, submillimetre silicate-rich blobs mostly composed of metal beads, silicate crystals and the iron-rich mineral magnetite.

These tiny chondrules are like time capsules, preserving the state of the solar system when they formed.

Marrocchi and colleagues examined chondrules in two chondrite meteorites: Vigarano, a 15-kilogram rock that fell to Italy in 1910, and the smaller Kaba, which dropped onto Hungary in 1857.

A thin slice of the Vigarano meteorite.
Yves Marrocchi

They found minerals never before seen in chondrules – in particular, sulfide-associated magnetites of magmatic origin, or SAMs.

They could only have formed under oxidising conditions produced when tiny primitive rocky planets from the inner solar system crashed into icy bodies from further out.

The collisions were low-velocity, they inferred, because of the presence of magnesium iron silicate minerals called olivines. High-energy impacts would have produced enough heat to vapourise olivines.

Of course, that's not to say high-velocity collisions didn't happen – just that Vigarano and Kaba's parent asteroids were involved in low-velocity collisions at some point.

So how did the inner and outer solar system mix?

There are a couple of theories. Gas giants such as Jupiter, scientists think, formed a rocky core early on in the solar system's existence, which then gravitationally captured vast reservoirs of hydrogen.

This core formation may have stirred the pot, jerking objects around and facilitating collisions.

Gas giants also moved around the solar system, creeping close to the sun then heading back out again.

This herding of planets and other objects into their orbits (or kicking them out) may have produced high-velocity impacts between rocky and icy bodies.

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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