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Lunar & Planetary Science Conference

The 29th Lunar & Planetary Science Conference, March 1998, will present
around 200 papers, 3/16-20/98.

See http://cass.jsc.nasa.gov/meetings/LPSC98/pdf/program.pdf

 (requires Adobe Acrobat).

Some of the most interesting include these from a session on 
chondrules:

A model for formation of chondrule rims, from accretion of dust in the
early nebula.  Aerodynamic forces can concentrate particles of certain
sizes into zones in the gas cloud, since tiny particles will tend to move
with the gas and larger ones will move relative to it.  (Cuzzi)


A model for formation of chondrules; as planetesimals move through the
nebula, they form a 'bow shock' that could increase pressure and heat, to
facilitate accretion of chondrules.  This shock layer would be near the
planetesimal's surface, and consist of both vaporized incoming material
and evaporated planetesimal material, all confined by the gas pressure
from 'above'.  If accurate, the sizes and quantity of chondrules should
decrease with distance from the sun.  The percent volume of chondrules
found in volatile rich CI chondrites (probably formed far from sun),
compared to percent volume in volatile poor ordinary chondrites (possibly
formed closer to sun) might reinforce this idea.  (Hood)


Chondrules in some ordinary chondrites were exposed to cosmic rays for
200,000-300,000 years before accretion into a matrix and a parent body. 
Previous studies have shown a 900,000 year exposure in other chondrules. 
Isotopes of noble gases produced by cosmic rays were measured, apparently
a difficult task because experimental error was significant. (Polnau)


Chondrules could have formed from hot liquid drops and dust.  Rock dust
of controlled composition and particle size was 'puffed' into a furnace
at 1600 degrees C.  Textures of common chondrules were produced.  An
alternate model that requires 'cold dust balls' to initiate chondrule
formation might not be necessary.  (Yu)


Very fine grained rock dust was melted in an atmosphere of controlled
oxygen content, and cooled at a specific rate.  Fine grained textures
similar to those found in chondrules were produced, but to produce coarse
grain requires very specific conditions.  Coarse grain chondrules
probably are 'recycled'.  (Fox)


Troilite in chondrules can be of various origins; from the initial melt,
or metamorphism (thermal or shock), or aqueous processes.  In type 3
chondrites, troilite indicates that the chondrules were molten for a
short time.  More than a quarter of the chondrules in ordinary chondrites
show signs of multiple heating events.  (Rubin)



These might or might not rewrite the textbooks, but it is interesting to
see that work proceeds on many fronts: by formulating models on a
computer, melting rock in a lab, examining thin sections, measuring
radioactivity, etc.

______________________________________

Peter Abrahams, telscope@europa.com