By G. Jeffrey Taylor and Linda M. V. Martel, Hawai‘i Institute of Geophysics and Planetology
Interstellar space contains dust.
Most of it is not made of well-ordered silicate crystals like those in
typical planetary rocks. It is made of disordered, amorphous
(non-crystalline) smoke particles. The current view of Solar System
formation depicts comets forming far out in the Solar System, in the
Kuiper belt beyond Neptune. If true, comets ought to be made entirely
of silicate smoke particles (plus ice) like those in interstellar
space. This region is too cold to provide enough heat to convert the
smoke to crystalline minerals. Where did the Wild 2 chondrules come
from then?
A reasonable answer,
supported by Nakamura and colleagues, is that dust processed at high
temperature in the inner part of the solar nebula must have been
transported from the inner nebula to the outer, frigid reaches where
comets formed.
Diagram from chapter by Nuth et al. (2005) Chondrites and the Protoplanetary Disk, pg. 678.
This is a
schematic diagram of the solar nebula as it was still accreting dust to
it. Planets have not yet formed. Materials heated near the protosun
circulate to the outer Solar System to the cold regions
where comets formed, driven by forces originating near the early Sun.
Don Brownlee (University
of Washington, Seattle), Principal Investigator of the Stardust
mission, calls this the "Grand Radial Express." Astrophysicists and
cosmochemists have suggested several mechanisms for transporting
materials within and above the nebular disk surrounding the
highly-active infant Sun. The Stardust results give experimental
evidence that such vigorous transport took place.
http://www.sun-nation.org/merkl-power-of-cross.html
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