^1,2Jon M. Friedrich,^2,3,4Michael K. Weisberg,^2,4,5Denton S. Ebel,⁶Alison E. Biltz,⁶Bernadette M. Corbett,⁶Ivan V. Iotzov,⁶Wajiha S. Khan,⁶Matthew D. Wolman
¹Department of Chemistry, Fordham University, Bronx, NY 10458, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024, USA
³Department of Physical Sciences, Kingsborough College of the City University of New York, Brooklyn, NY 11235, USA
⁴Graduate Center of the City University of New York, 365 5th Ave, New York, NY 10016, USA
⁵Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
⁶Fordham College at Rose Hill, Fordham University, Bronx, NY 10458, USA

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.

Reference
Friedrich JM, Weisberg MK,Ebel DS, Biltz AE, Corbett BM, Iotzov IV, Khan WS, Wolman MD (2014) Chondrule size and related physical properties: A compilation and evaluation of current data across all meteorite Groups. Chemie der Erde (in Press)
Link to Article [DOI: 10.1016/j.chemer.2014.08.003]

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¹Alexander Hubbard,² Denton S. Ebel
¹Department of Astrophysics, American Museum of Natural History, New York, NY 10024-5192, USA
²Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024-5192, USA

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.

Reference
Hubbard A, Ebel DS (2014) Semarkona: Lessons for chondrule and chondrite Formation. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.09.025]

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¹David Parry Rubincam
¹Code 698, Planetary Geodynamics Laboratory, Solar System Exploration Division, NASA Goddard Space Flight Center, Building 34, Room S280 Greenbelt, MD 20771

Space erosion from dust impacts may set upper limits on the cosmic ray exposure (CRE) ages of stony meteorites. A meteoroid orbiting within the asteroid belt is bombarded by both cosmic rays and interplanetary dust particles. Galactic cosmic rays penetrate only the first few meters of the meteoroid; deeper regions are shielded. The dust particle impacts create tiny craters on the meteoroid’s surface, eroding it away by abrasion at a particular rate. Hence a particular point inside a meteoroid accumulates cosmic ray products only until that point wears away, limiting CRE ages. The results would apply to other regolith-free surfaces in the solar system as well, so that abrasion may set upper CRE age limits which depend on the dusty environment. Calculations based on N. Divine’s dust populations and on micrometeoroid cratering indicate that large stony meteoroids in circular ecliptic orbits at 2 AU will record 21Ne CRE ages of ∼176 × 106 years if dust masses are in the range 10-21 – 10-3 kg. This is in broad agreement with the maximum observed CRE ages of ∼100 × 106 years for stones. High erosion rates in the inner solar system may limit the CRE ages of Near-Earth Asteroids (NEAs) to ∼120 × 106 years. A characteristic of erosion is that the neon concentrations tend to rise as the surface of the meteorite is approached, rather than drop off as for meteorites with fixed radii. Pristine samples recovered from space may show the rise. If the abrasion rate for stones were a factor of ∼6 larger than found here, then the ages would drop into the 30 × 106 y range, so that abrasion alone might be able to explain many CRE ages. However, there is no strong evidence for higher abrasion rates, and in any case would probably not be fast enough to explain the youngest ages of 0.1 – 1 × 106 y. Further, space erosion is much too slow to explain the ∼600 × 106 y ages of iron meteorites.

Reference
Rubincam DP (2014) Space erosion and cosmic ray exposure ages of stony meteorites. Icarus (in Press)
Link to Article [DOI: 10.1016/j.icarus.2014.09.005]

Copyright Elsevier

¹Michael K. Shepard et al. (>10)*
¹Bloomsburg University, 400 E. Second St., Bloomsburg, PA 17815, USA
*Find the extensive, full author and affiliation list on the publishers website

Using the S-band radar at Arecibo Observatory, we observed thirteen X/M-class asteroids; nine were previously undetected and four were re-observed, bringing the total number of Tholen X/M-class asteroids observed with radar to 29. Of these 29 M-class asteroids, 13 are also W-class, defined as M-class objects that also display a 3-micron absorption feature which is often interpreted as the signature of hydrated minerals (Jones et al. Icarus 88, 172-192, 1990; Rivkin et al. Icarus 117, 90-100, 1995; Icarus 145, 351-368, 2000).
Consistent with our previous work (Shepard et al., 2008 and Shepard et al., 2011), we find that 38% of our sample (11 of 29) have radar albedos consistent with metal-dominated compositions. With the exception of 83 Beatrix and 572 Rebekka, the remaining objects have radar albedos significantly higher than the mean S- or C-class asteroid (Magri et al. Icarus 186, 126-151, 2007).
Seven of the eleven high-radar-albedo asteroids, or 64%, also display a 3-micron absorption feature (W-class) which is thought to be inconsistent with the formation of a metal dominated asteroid. We suggest that the hydration absorption could be a secondary feature caused by low-velocity collisions with hydrated asteroids, such as CI or CM analogs, and subsequent implantation of the hydrated minerals into the upper regolith. There is recent evidence for this process on Vesta (Reddy et al. Icarus 221, 544-559, 2012; McCord et al. Nature 491, 83-86, 2012; Prettyman et al. Science 338, 242-246, 2012; Denevi et al. Science 338, 246-249, 2012).
Eleven members of our sample show bifurcated radar echoes at some rotation phases; eight of these are high radar targets. One interpretation of a bifurcated echo is a contact binary, like 216 Kleopatra, and several of our sample are contact binary candidates. However, evidence for other targets indicates they are not contact binaries. Instead, we hypothesize that these asteroids may have large-scale variations in surface bulk density, i.e. isolated patches of metal-rich and silicate-rich regions at the near-surface, possibly the result of collisions between metal and silicate-rich asteroids.

Reference
Shepard MK et al. (2014) A radar survey of M- and X-class asteroids. III. Insights into their composition, hydration state, & structure. Icarus (in Press)
Link to Article: [DOI: 10.1016/j.icarus.2014.09.016]

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