¹Deborah L.Domingue, ²MarioD’Amore, ²Sabrina Ferrari,²Jörn Helbert,³Noam R.Izenberg
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2018.07.018]
¹Planetary Science Institute, 1700 E. Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
²Institute for Planetary Research, DLR, Rutherfordstrasse 2, Berlin, Germany
³The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
Copyright Elsevier

This study examines the level of structural uniformity within Mercury’s regolith as a function of geomorphological unit. Using two categories of photometric models (Hapke versus Kaasalainen-Shkuratov), the variation between and within similar geomorphological units are examined with the Mercury Atmosphere and Surface Composition Spectrometer (MASCS) photometry sequence data sets. The results show evidence for variations in the spectral and photometric scattering properties both within similar geomorphological units and between different geomorphological units. The ejecta and cratering units show the largest differences between the modeling results, each indicating variations in different properties. The results for the intercrater materials, smooth materials, and dark materials show consistent results between both models. The variations include possible differences in grain structures, regolith compaction, and surface roughness on micron to millimeter scales.

¹Jonathan A.Lewis, ^1,2Rhian H.Jones, ³Serafina C.Garcea
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.08.002]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131
²School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK
³Henry Moseley X-ray Imaging Facility, School of Materials, University of Manchester, Manchester, M13 9PL, UK
Copyright Elsevier

Porosity is an important physical property of meteorites and asteroids that affects density, material strength, and thermal conductivity. Porosity can also promote chemical exchange by facilitating the transport of fluids and dissolved ions. We measured the porosity of individual chondrules from the L4 ordinary chondrite Saratov, using X-ray microtomography (μCT) and scanning electron microscopy, to examine the abundance and distribution of porosity in chondrules, and to understand how porosity relates to chemical exchange during parent body processes. Porosity was 1-2% by volume in the chondrules we measured and maximum pore sizes were ∼300 μm. Porosity distribution and morphology indicate that porosity is a secondary feature and most pores >1 μm were formed from the dissolution of chondrule mesostasis glass. Fe and K are preferentially enriched in phases adjacent to the most porous regions: Fe is enriched in pyroxene, and K is enriched in mesostasis where it is observed as either the silica alteration phase merrihueite, or fine-scale, K-feldspar exsolution in albitic feldspar. Some pores can be described as vugs, as they contain euhedral olivine and chromite, with textures indicating vapor deposition. Knowing the chondrule porosity, we estimate the matrix porosity in Saratov to be very high, 40-60%. We suggest that during prograde metamorphism, an aqueous fluid originating from the matrix dissolved chondrule mesostasis glass, producing the observed porosity, and introducing Fe into the pyroxene phenocrysts. Fluids were less abundant through peak metamorphism, and chondrule mesostasis glass crystallized to fine-grained albite. During retrograde metamorphism, high temperature, short duration bursts of a dry, alkali-bearing fluid from the asteroid interior infiltrated the pore network, formed the vug phases, altered silica to merrihueite, and introduced K to the secondary albite. Fine-scale K-feldspar then exsolved from albite during rapid cooling to the ambient temperature. Overall, development of porosity during metamorphism on the L chondrite parent body contributed to the chemical evolution of ordinary chondrite material, as well as affecting physical properties of the parent asteroid.