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A Challenge to Planetary Migration Theories

Just how young is the average meteorite? One way to study the question is through the chondrules that make up stony meteorites. Chondrules are mineral deposits formed by rapid cooling; they give the appearance of tiny, spherical bits of glassy rock. Stony meteorites are generally called chondrites because they contain such chondrules. And it’s generally assumed that chondrites formed in the early Solar System in the condensation of the first solid materials.

But University of Toronto geologist Yuri Amelin and Alexander Krot (University of Hawaii) now have data that call that conclusion into question. Their paper in an August issue of Nature reports on chondrules that are the youngest ever found. The researchers used meteorites named Gujba and Hammadah al Hamra, studying their minerological structure and fixing an approximate isotopic age. “It soon became clear that these particular chondrules were not of a nebular origin,” says Amelin. “And the ages were quite different from what was expected. It was exciting.”

In fact, the chondrules Amelin, Krot and their colleagues found post-date the oldest asteroids. Amelin again: “We think these chondrules were formed by a giant plume of vapour produced when two planetary embryos, somewhere between moon-size and Mars-size, collided.”

Perhaps the formation of the Solar System wasn’t as cut and dried as some theories suggest. Indeed, this finding is the first suggestion that some chondrules and the meteorites that contain them formed at considerably different times than others, which implies that the planets — at least in early form — were already there when some chondrules were made. Another suggestion that our understanding of planetary formation is far from comprehensive.

The paper is Alexander N. Krot, Yuri Amelin et al., “Young chondrules in CB chondrites from a giant impact in the early Solar System,” Nature 436, (18 August 2005), pp. 989-992.

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  • ljk February 28, 2008, 7:17

    Co-Accretion of Chondrules and Dust in the Solar Nebula

    Authors: C.W. Ormel, J.N. Cuzzi, A.G.G.M. Tielens

    (Submitted on 27 Feb 2008)

    Abstract: We present a mechanism for chondrules to stick together by means of compaction of a porous dust rim they sweep up as they move through the dusty nebula gas. It is shown that dust aggregates formed out of micron-sized grains stick to chondrules, forming a porous dust rim. When chondrules collide, this dust can be compacted by means of rolling motions within the porous dust layer. This mechanism dissipates the collisional energy, compacting the rim and allowing chondrules to stick. The structure of the obtained chondrule-dust agglomerates (referred to as compounds) then consists of three phases: chondrules, porous dust, and dust that has been compacted by collisions. Subsequently, these compounds accrete their own dust and collide with other compounds.

    The evolution of the compound size distribution and the relative importance of the phases is calculated by a Monte Carlo code. Growth ends, and a simulation is terminated when all the dust in the compounds has been compacted. Numerous runs are performed, reflecting the uncertainty in the physical conditions at the chondrule formation time.

    It is found that compounds can grow by 1-2 orders of magnitudes in radius, upto dm-sizes when turbulence levels are low. However, relative velocities associated with radial drift form a barrier for further growth. Earlier findings that the dust sweep-up by chondrules is proportional to their sizes are confirmed. We contrast two scenarios regarding how this dust evolved further towards the densely packed rims seen in chondrites.

    Comments: 23 pages, accepted for publication in ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.4048v1 [astro-ph]

    Submission history

    From: Chris Ormel [view email]

    [v1] Wed, 27 Feb 2008 17:15:44 GMT (714kb)