John T. Wasson^1,2 and Alan E. Rubin²

¹Institute of Geophysics and Planetary Physics, University of California Los Angeles, Los Angeles, California, USA
²Departments of Earth and Space Sciences and Chemistry and Biochemistry, University of California Los Angeles, Los Angeles, California, USA

In numerous past papers, it was concluded that the fine (<1 μm) matrix immediately adjacent to, and radially symmetric around, chondrules in primitive chondrites consists of compact (low-porosity) rims that were attached in the solar nebula. We present here textural and compositional evidence that no matrix-like (or accretionary) rims around chondrules are present in the well-preserved CR2 chondrite LAP 02342. Fine-grained matrix-rich regions (i.e., candidate “rims”) at the edges of chondrules were studied with an electron-microprobe-based matrix-grid technique; comparison of the “rims” data for matrix regions near these chondrules showed the candidate “rims” to be compositionally heterogeneous, inconsistent with origins as radially symmetric, matrix-like rims formed by gradual accretion. This evidence (together with simulations and laboratory studies indicating that accretionary processes produced highly porous aggregates) strongly suggests that nebular processes did not produce compact matrix-like rims around chondrules in any chondrite group.

Reference
Wasson JT and Rubin AE (in press) Absence of matrix-like chondrule rims in CR2 LAP 02342. Meteoritics & Planetary Science
[doi:10.1111/maps.12237]
Published by arrangement with John Wiley & Sons

Link to Article

Michael S. Kelley^a,1,2, Michael J. Gaffey^b,1, Vishnu Reddy^c,1, Juan A. Sanchez^d,1

^aDepartment of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460
^bDepartment of Space Studies, University of North Dakota, Grand Forks, North Dakota 58202
^cPlanetary Science Institute, Tucson, Arizona 85719
^dMax Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
¹Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under Cooperative Agreement No. NNX08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program.
²Currently at Science Mission Directorate, NASA Headquarters, Washington DC

Near-Earth asteroids (NEAs) are interesting as both a threat to the Earth and as the immediate sources for most meteorites. We observed NEA (4953) 1990 MU using the NASA Infrared Telescope Facility (IRTF) and University of Hawaii (UH) telescopes on Mauna Kea to constrain its surface composition and origin. The surface composition of 1990 MU is similar to ordinary chondrites (H chondrites). The calculated olivine and pyroxene chemistry of 1990 MU (Fa_13.5±1.3 and Fs_12.7±1.4) are consistent with the olivine and pyroxene chemistry ranges for H chondrites (Fa_15-21 and Fs_13-19) (Dunn et al. 2010), although the estimated Fa value is at the lower end of H chondrite range. The olivine abundance ratio of 1990 MU (0.57±0.03) is slightly higher but not inconsistent with H chondrites (0.47-0.55±0.03). The radar circular polarization ratio (same circular polarization state or SC/opposite circular polarization state or OC) (Benner et al. 2008) of 1990 MU is 0.36±0.03, which is higher than the mean SC/OC ratio for S-type NEAs (0.270±0.079). 1990 MU SC/OC value is also higher than those of (25143) Itokawa (0.27±0.04), (4179) Toutatis (0.29±0.01) and (433) Eros (0.28±0.06) suggesting a rougher surface at decimeter scale (Benner et al. 2008). We constrained the diameter of 1990 MU (4.4 km) using the average albedo at 0.55 μm of H chondrites (0.21) and absolute magnitude (H) of 14.1 (Flower and Chillemi, 1992). This diameter is higher than the 2.8 km value from Harris et al. (2011) using an albedo of 0.52 for 1990 MU. This albedo value is unusually high for H chondrites, which have an albedo range of 0.12-0.30. We compared olivine and pyroxene chemistries of 1990 MU with main belt asteroid (6) Hebe, probable parent body of H chondrite meteorites and IIE irons (Gaffey and Gilbert, 1998), and found that 1990 MU has more high-calcium pyroxene than Hebe. Fayalite and ferrosilite values of the two asteroids are consistent with H chondrites but don’t overlap each other. The differences could be due to compositional variations observed on Hebe by Gaffey and Gilbert (1998), although the observed rotational variation in spectral parameters does not match well with those of 1998 MU.

Reference
Kelley MS, Gaffey MJ, Reddy V and Sanchez JA (2014) Surface Composition of Near-Earth Asteroid (4953) 1990 MU: Possible Fragment of (6) Hebe. Icarus
[doi:10.1016/j.icarus.2014.01.015]
Copyright Elsevier

Link to Article

Richard C. Greenwood^a, Jean-Alix Barrat^b, Akira Yamaguchi^c, Ian A. Franchi^a, Edward R.D. Scott^d, William F. Bottke^e, Jenny M. Gibson^a

^aPlanetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
^bCNRS UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Place Nicolar Copernic, 29280 Plouzané Cedex, France
^cNational Institute of Polar Research, Tokyo 190-8518, Japan
^dHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
^eSouthwest Research Institute and NASA Lunar Science Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80302, USA

Oxygen isotope measurements of a suite of 22 diogenites demonstrate that they have a restricted range of Δ¹⁷O values: . These results indicate that the diogenites form a single population consistent with a single parent body source, rather than multiple sources as has recently been suggested. Our previously published analysis of eucrites and cumulate eucrites (n=34) give very similar results to the diogenites, with  and confirm that diogenites and eucrites are from the same parent asteroid. The isotopic homogeneity displayed by diogenites, eucrites and cumulate eucrites, provides strong evidence for an early large-scale melting event on the HED parent body, possibly resulting in the formation of a magma ocean. The paradox, whereby diogenites show isotopic evidence in favor of global melting, but also geochemical features indicative of late stage interaction with eucritic crust, may reflect a rapid transition from global to serial magmatism on their parent body. The fact that all the lithologically varied HED units have an isotopically homogeneous composition supports the proposal that they are derived from a single, large, diverse asteroid, most likely 4 Vesta. The recent suggestion that the HEDs are not from Vesta, but instead represent material from the same asteroidal source as the main-group pallasites and IIIAB irons can be excluded by our oxygen isotope data.

Reference
Greenwood RC, Barrat J-A, Yamaguchi A, Franchi IA, Scott ERD, Bottke WF and Gibson JM (2014) The oxygen isotope composition of diogenites: Evidence for early global melting on a single, compositionally diverse, HED parent body. Earth and Planetary Science Letters 390:165–174.
[doi:10.1016/j.epsl.2013.12.011]
Copyright Elsevier

Link to Article

Pasquale Tricarico

Planetary Science Institute, Tucson, AZ 85719, USA

The hydrostatic equilibrium of multi-layer bodies lacks a satisfactory theoretical treatment despite its wide range of applicability. Here we show that by using the exact analytical potential of homogeneous ellipsoids we can obtain recursive analytical solutions and an exact numerical method for the hydrostatic equilibrium shape problem of multi-layer planets and synchronous moons. The recursive solutions rely on the series expansion of the potential in terms of the polar and equatorial shape eccentricities, while the numerical method uses the exact potential expression. These solutions can be used to infer the interior structure of planets and synchronous moons from their observed shape, rotation, and gravity. When applied to the dwarf planet Ceres, we show that it is most likely a differentiated body with an icy crust of equatorial thickness 30-90 km and a rocky core of density 2.4-3.1 g cm^-3. For synchronous moons, we show that the J₂/C₂₂  10/3 and the (b – c)/(a – c)  1/4 ratios have significant corrections of order Ω²/(πGρ), with important implications for how their gravitational coefficients are determined from fly-by radio science data and for how we assess their hydrostatic equilibrium state.

Reference
Tricarico P (2014) Multi-layer Hydrostatic Equilibrium of Planets and Synchronous Moons: Theory and Application to Ceres and to Solar System Moons. The Astrophysical Journal 782:99.
[doi:10.1088/0004-637X/782/2/99]

Link to Article