[Show abstract][Hide abstract] ABSTRACT: We consider the evidence presented by the LL3.0 chondrite Semarkona,
including its chondrule fraction, chondrule size distribution and matrix
thermal history. We show that no more than a modest fraction of the ambient
matrix material in the Solar Nebula could have been melted into chondrules; and
that much of the unprocessed matrix material must have been filtered out at
some stage of Semarkona's parent body formation process. We conclude that
agglomerations of many chondrules must have formed in the Solar Nebula, which
implies that chondrules and matrix grains had quite different collisional
sticking parameters. Further, we note that the absence of large melted objects
in Semarkona means that chondrules must have exited the melting zone rapidly,
before the chondrule agglomerations could form. The simplest explanation for
this rapid exit is that chondrule melting occurred in surface layers of the
disk. The newly formed, compact, chondrules then settled out of those layers on
short time scales.
[Show abstract][Hide abstract] ABSTRACT: The examination of the physical properties of chondrules has generally
received less emphasis than other properties of meteorites such as their
mineralogy, petrology, and chemical and isotopic compositions. Among the
various physical properties of chondrules, chondrule size is especially
important for the classification of chondrites into chemical groups, since each
chemical group possesses a distinct size-frequency distribution of chondrules.
Knowledge of the physical properties of chondrules is also vital for the
development of astrophysical models for chondrule formation, and for
understanding how to utilize asteroidal resources in space exploration. To
examine our current knowledge of chondrule sizes, we have compiled and provide
commentary on available chondrule dimension literature data. We include all
chondrite chemical groups as well as the acapulcoite primitive achondrites,
some of which contain relict chondrules. We also compile and review current
literature data for other astrophysically-relevant physical properties
(chondrule mass and density). Finally, we briefly examine some additional
physical aspects of chondrules such as the frequencies of compound and
'cratered' chondrules. A purpose of this compilation is to provide a useful
resource for meteoriticists and astrophysicists alike.
[Show abstract][Hide abstract] ABSTRACT: The Earth is known to be depleted in volatile lithophile elements in a
fashion that defies easy explanation. We resolve this anomaly with a model that
combines the porosity of collisionally grown dust grains in protoplanetary
disks with heating from FU Orionis events that dramatically raise
protoplanetary disk temperatures. The heating from an FU Orionis event alters
the aerodynamical properties of the dust while evaporating the volatiles. This
causes the dust to settle, abandoning those volatiles. The success of this
model in explaining the elemental composition of the Earth is a strong argument
in favor of highly porous collisionally grown dust grains in protoplanetary
disks outside our Solar System. Further, it demonstrates how thermal (or
condensation based) alterations of dust porosity, and hence aerodynamics, can
be a strong factor in planet formation, leading to the onset of rapid
gravitational instabilities in the dust disk and the subsequent collapse that
[Show abstract][Hide abstract] ABSTRACT: Mercury's regolith, derived from the crustal bedrock, has been altered by a set of space weathering processes. Before we can interpret crustal composition, it is necessary to understand the nature of these surface alterations. The processes that space weather the surface are the same as those that form Mercury's exosphere (micrometeoroid flux and solar wind interactions) and are moderated by the local space environment and the presence of a global magnetic field. To comprehend how space weathering acts on Mercury's regolith, an understanding is needed of how contributing processes act as an interactive system. As no direct information (e.g., from returned samples) is available about how the system of space weathering affects Mercury's regolith, we use as a basis for comparison the current understanding of these same processes on lunar and asteroidal regoliths as well as laboratory simulations. These comparisons suggest that Mercury's regolith is overturned more frequently (though the characteristic surface time for a grain is unknown even relative to the lunar case), more than an order of magnitude more melt and vapor per unit time and unit area is produced by impact processes than on the Moon (creating a higher glass content via grain coatings and agglutinates), the degree of surface irradiation is comparable to or greater than that on the Moon, and photon irradiation is up to an order of magnitude greater (creating amorphous grain rims, chemically reducing the upper layers of grains to produce nanometer-scale particles of metallic iron, and depleting surface grains in volatile elements and alkali metals). The processes that chemically reduce the surface and produce nanometer-scale particles on Mercury are suggested to be more effective than similar processes on the Moon. Estimated abundances of nanometer-scale particles can account for Mercury's dark surface relative to that of the Moon without requiring macroscopic grains of opaque minerals. The presence of nanometer-scale particles may also account for Mercury's relatively featureless visible-near-infrared reflectance spectra. Characteristics of material returned from asteroid 25143 Itokawa demonstrate that this nanometer-scale material need not be pure iron, raising the possibility that the nanometer-scale material on Mercury may have a composition different from iron metal [such as (Fe,Mg)S]. The expected depletion of volatiles and particularly alkali metals from solar-wind interaction processes are inconsistent with the detection of sodium, potassium, and sulfur within the regolith. One plausible explanation invokes a larger fine fraction (grain size <45 μm) and more radiation-damaged grains than in the lunar surface material to create a regolith that is a more efficient reservoir for these volatiles. By this view the volatile elements detected are present not only within the grain structures, but also as adsorbates within the regolith and deposits on the surfaces of the regolith grains. The comparisons with findings from the Moon and asteroids provide a basis for predicting how compositional modifications induced by space weathering have affected Mercury's surface composition.
[Show abstract][Hide abstract] ABSTRACT: Cl-free and Cl-bearing heating/degassing experiments ( at <1 bar for 10 min, 4h, 6 h) on Al3509 chondrule melt reveals the systematic role of Cl on alkali volatility.
[Show abstract][Hide abstract] ABSTRACT: Clustering analyses on MASCS/MESSENGER results in a polar and equatorial distinct spectral unit. Comparison with an X-ray spectrometer shows elemental correlation.
[Show abstract][Hide abstract] ABSTRACT: Fully segmented maps (1567124 pixels over ~157 mm^2) reveal 2D sizes, 73% chondrules, and 27% matrix, and complementary Mg-Si and Ti-Al in chondrules and matrix.
[Show abstract][Hide abstract] ABSTRACT: Combined analysis of 2-D area from EMP maps and 3-D CT-derived volumes shows promise for developing a conversion factor for both chondrules and CAIs independently.
[Show abstract][Hide abstract] ABSTRACT: Some chondrule mesostasis preserves an ultrarefractory HREE-enriched precursor that complements Group II HREE-depleted CAIs producing flat bulk CO REE patterns.