[meteorite-list] Goodness gracious

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Tue, 21 Jul 2009 12:49:55 -0500
Message-ID: <rovb655rp8ffut65r8v1kim68mka5pn7s5_at_4ax.com>

http://www.newscientist.com/article/mg20327174.800-did-great-balls-of-fire-form-the-planets.html

Did great balls of fire form the planets?

20 July 2009 by David Shiga

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.
Received on Tue 21 Jul 2009 01:49:55 PM PDT