Discussion and news about the modern effort to understand the nature of life on Earth, finding planets around other stars, and the search for life elsewhere in the universe

Monday, March 28, 2011

Carbonaceous Cotton Candy

A typical proto-star is surrounded for a few tens of millions of years by a great disk of nebular material. One percent of the mass of this disk is initially microscopic dust, most likely produced in the atmospheric outflows of earlier generations of elderly stars. The other ninety-nine percent is gas, the same mix of gas we see in the great nebula scattered throughout the galaxy. From this orbiting plate of sauce both the central star grows, and the planets coalesce. While there are many hurdles yet to overcome in our understanding of planet formation, one of the trickiest occurs right at the start of this process.

How exactly the microscopic dust grains and gas-phase matter in a proto-planetary disk go from this state to even a tiny crumb of rocky, icy material is a topic of intense debate. We actually have a bit more confidence in what happens once there are meter-sized chunks of stuff flying around than we do in this earliest stage. Observations of proto-stellar systems and laboratory experiments here on Earth have suggested that the first agglomerations of solid material were probably extremely "fluffy" aggregates of the tiniest particles. Now a recent study of the structures in carbonaceous chondrite meteorites seems to shed further light on this primordial stage in planet building.

A new paper by Bland et al. in Nature Geosciences demonstrates the incredible utility of modern microscopic techniques. In this case the backscatter of electrons reveals previously hidden details about the crystalline texture of the meteorite - otherwise impossible to get at owing to the fragile and complex nature of this class of object. In a nutshell, they examine the alignment of microscopic dust grain particles that are coating what are known as chondrules inside the meteorite. Chondrules are some of the most primitive (i.e. oldest) solids from a young planetary system. Whatever they picked up in their travels, and how they picked it up, provides a unique fossil record of conditions.

The outcome is that the very first solids that formed in our solar system were indeed likely to be extremely "fluffy" or porous, with some 85% of their volume just empty space. The Bland et al. results indicate that the chondrules underwent a large amount of "rolling" and even shocking by pressure waves. In effect a turbulent environment acted to compact the initially fluffy, cotton-candy like materials into denser states. Random rollings and collisions naturally produces closely spherical bodies.

The world beneath our feet may well have begun as sticky cosmic fluff.

[But probably not pink]


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