¹Joanna V. Morgan et al. (>10)*
Science 354, 6314, 878-882 Link to Article [DOI: 10.1126/science.aah6561]
¹Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK.
Reprinted with permission from AAAS
*Find the extensive, full author and affiliation list on the publishers website

Large impacts provide a mechanism for resurfacing planets through mixing near-surface rocks with deeper material. Central peaks are formed from the dynamic uplift of rocks during crater formation. As crater size increases, central peaks transition to peak rings. Without samples, debate surrounds the mechanics of peak-ring formation and their depth of origin. Chicxulub is the only known impact structure on Earth with an unequivocal peak ring, but it is buried and only accessible through drilling. Expedition 364 sampled the Chicxulub peak ring, which we found was formed from uplifted, fractured, shocked, felsic basement rocks. The peak-ring rocks are cross-cut by dikes and shear zones and have an unusually low density and seismic velocity. Large impacts therefore generate vertical fluxes and increase porosity in planetary crust.

¹G. K. Bhatia,¹S. Sahijpal
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12789]
¹Department of Physics, Panjab University, Chandigarh, India
Published by arrangement with John Wiley&Sons

Numerical models dealing with the planetary scale differentiation of Mercury are presented with the short-lived nuclide, 26Al, as the major heat source along with the impact-induced heating during the accretion of planets. These two heat sources are considered to have caused differentiation of Mars, a planet with size comparable to Mercury. The chronological records and the thermal modeling of Mars indicate an early differentiation during the initial ~1 million years (Ma) of the formation of the solar system. We theorize that in case Mercury also accreted over an identical time scale, the two heat sources could have differentiated the planets. Although unlike Mars there is no chronological record of Mercury’s differentiation, the proposed mechanism is worth investigation. We demonstrate distinct viable scenarios for a wide range of planetary compositions that could have produced the internal structure of Mercury as deduced by the MESSENGER mission, with a metallic iron (Fe-Ni-FeS) core of radius ~2000 km and a silicate mantle thickness of ~400 km. The initial compositions were derived from the enstatite and CB (Bencubbin) chondrites that were formed in the reducing environments of the early solar system. We have also considered distinct planetary accretion scenarios to understand their influence on thermal processing. The majority of our models would require impact-induced mantle stripping of Mercury by hit and run mechanism with a protoplanet subsequent to its differentiation in order to produce the right size of mantle. However, this can be avoided if we increase the Fe-Ni-FeS contents to ~71% by weight. Finally, the models presented here can be used to understand the differentiation of Mercury-like exoplanets and the planetary embryos of Venus and Earth.

¹M. C. McCanta,²M. D. Dyar
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12793]
¹University of Tennessee, Knoxville, Tennessee, USA
²Mount Holyoke College, South Hadley, Massachusetts, USA
Published by arrangement with John Wiley & Sons

Oxidation is observed in Ca-pyroxene subjected to a range of shock pressures (21–59 GPa). Changes in the pyroxene redox ratio as measured by the changes in %Fe3+ ranged from 2–6 times the starting composition. Mössbauer and reflectance spectroscopy record the changing Fe3+ concentration as a preferential oxidation of Fe2+ in the M2 crystallographic site. The oxidation is also accompanied by mechanical changes in the pyroxene crystals including fracturing, linear defects, and twinning. As oxygen fugacity is often calculated using mineral redox ratios and thought to represent the prevailing fO2 during crystallization, it is imperative to recognize that the fO2 values measured in impact-derived materials may represent that of the impact and not the magma source region.