Did great balls of fire form the planets?
ASTEROID-SIZED balls of magma hurtled through our infant
solar system, and spray from their many collisions provided much of the raw material that formed Earth and its rocky siblings. That's according to a new take on an old theory that challenges the notion that the
solar system started out as a placid sea of dust motes which simply clumped together to form planets.
The early family tree of our solar system's rocky planets features tiny glassy spheres called
chondrules, found today inside ancient meteorites. The origins of chondrules, which are typically about a millimetre across, are shrouded in mystery. They make up much of the material preserved in meteorites that were formed about 2 million years after the solar system began and are thought to have clumped together to form
asteroid-size planetesimals, which in turn agglomerated to make Earth and its peers.
Chondrules' glassy composition and spheroidal shape show that they were once molten. According to the popular view, they formed when dust grains in the nebula surrounding the infant
sun were suddenly heated, perhaps when cosmic lightning or shock waves shot through the nebula.
But calculations published in 2008 on the retention of sodium by the chondrules suggest they formed in dense swarms (
Science, vol 320, p 1617). This is difficult to reconcile with the melting of dust motes in a nebula, which are expected to be widely spaced.
Now, Ian Sanders of Trinity College Dublin in Ireland says another formation scenario, involving collisions between asteroid-sized balls of magma - kept molten through their high radioactivity - offers a better explanation. Sanders renewed the case for the idea, first proposed in the 1980s, at
this week's meeting of the
Meteoritical Society in Nancy, France.
Decay of
radioactive isotopes today helps to keep the cores of relatively large bodies like Earth molten. Sanders argues that the greater abundance of radioactive material in the fledgling solar system means that if objects larger than 30 kilometres across formed, they would have
melted through.
Collisions between such magma balls would breach their thin crusts of solid rock, spraying molten material into space, where the droplets would quickly cool to form chondrules.
Collisions between magma balls would breach their thin crusts and spray material into space
"It puts a completely different slant on what happened in the early solar system in the first 2 million years," Sanders says. That is the period when chondrules formed, based on measurements of key isotopes within them.
Conel Alexander of the Carnegie Institution of Washington DC, who led last year's
Science study, says the high density of droplets possible in plumes ejected from magma balls could explain his team's results, but finds the idea hard to accept on other grounds. A key issue is that material within magma balls should have quickly sorted into chemically distinct layers, with iron sinking to the core and lighter elements near the surface. The chemical signature of such sorting is not present in chondrules, Alexander says.
Sanders notes that some of the chondrules are indeed depleted in iron, as might be expected if they splashed from near the surface of a layered liquid body, though such chondrules are in a minority.