¹Robert M. Hazen
American Mineralogist 104, 468-470 Link to Article [https://doi.org/10.2138/am-2019-6745]
¹Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A. Orcid 0000-0003-4163-8644
Copyright: The Mineralogical Society of America

The Earth in Five Reactions Workshop posed two significant challenges: (1) the formulation of a conceptual definition of “reaction” and (2) the identification and ranking of the “most important reactions” in the context of planetary evolution. Attempted answers to those challenges, collated in this collection of articles, reflect both the opportunities and hurdles when scientists deal with questions of meaning and value.

¹Kevin J.Walsh,¹Harold F.Levison
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.03.031]
¹Southwest Research Institute, 1050 Walnut St. Suite 300, Boulder, CO 80302, USA
Copyright Elsevier

We present numerical simulations of terrestrial planet formation that examine the growth continuously from planetesimals to planets in the inner Solar System. Previous studies show that the growth will be inside-out, but it is still common practice to assume that the entire inner disk will eventually reach a bi-modal distribution of embryos and planetesimals. For the combinations of disk mass, initial planetesimal radius and gas disk lifetime explored in this work the entire disk never reaches a simple bi-modal mass distribution.
We find that the inside-out growth is amplified by the combined effects of collisional evolution of solid bodies and interactions with a dissipating gas disk. This leads to oligarchic growth never being achieved in different places of the disk at the same time, where in some cases the disk can simultaneously support chaotic growth and giant impacts inside 1 au and runaway growth beyond 2 au. The planetesimal population is efficiently depleted in the inner disk where embryo growth primarily advances in the presence of a significant gas disk. Further out in the disk growth is slower relative to the gas disk dissipation, resulting in more excited planetesimals at the same stage of growth and less efficient accretion. This same effect drives mass loss due to collisional grinding strongly altering the surface density of the accreted planets relative to the initial mass distribution. This effect decreases the Mars-to-Earth mass ratios compared to previous works with no collisional grinding. Similar to some previous findings utilizing vastly different growth scenarios these simulations produce a first generation of planetary embryos that are stable for 10–20 Myr, or 5–10 e-folding times of the gas dissipation timescale, before having an instability and entering the chaotic growth stage.

^1,2Erin L.Walton,³Nicholas E.Timms,⁴Tyler E.Hauck,²Ebberly A.MacLagan,²Christopher D.K.Herd
Earth and Planetary Science Letters 515, 173-186 Link to Article [https://doi.org/10.1016/j.epsl.2019.03.015]
¹Department of Physical Sciences, MacEwan University, City Centre Campus, 10700 104 Ave, Edmonton, AB, T5J 4S2, Canada
²Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Science Building, Edmonton, AB, T6G 2E3, Canada
³Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, Australia
⁴Alberta Geological Survey – Alberta Energy Regulator, 402 Twin Atria Building, 4999 98 Ave, Edmonton, AB, T6B 2X3, Canada
Copyright Elsevier

Hypervelocity bolide impacts deliver vast amounts of energy to the Earth’s near surface. This crustal process almost universally includes sedimentary target rocks; however, their response to impact is poorly understood, in part because of complexities due to layering, pore space and the presence of volatiles that are difficult to model. The response of carbonates to bolide impact remains contentious, yet whether they melt or decompose and liberate gases by the reaction CaCO3(s) → CaO(s) + CO2(g)↑, has significant implications for post-impact climatic effects. We report on previously unknown carbonate impact melts at the Steen River impact structure, Canada, and the first description of naturally shocked barite, BaSO4. Carbonate melts are preserved as groundmass-supported calcite-rich clasts, sampled from an up to 164 m thick, continuous sequence of crater-fill polymict breccias. Electron microscopy reveals fluidal- and ocellar-textured calcite and barite, intimately associated with silicate melt, consistent with these phases being in the liquid state at the same time. Raman spectroscopy and electron backscatter diffraction (EBSD) mapping confirm the presence of high-pressure phases – reidite and coesite – within some Steen River carbonate melt-bearing breccias. These minerals attest to the strong shock provenance of the breccia and provide constraints on their shock history. Preservation of reidite lamellae in zircon indicates a shock pressure >30 GPa <60 GPa and temperatures <1473 K. In addition to melting, we present compelling evidence for widespread (70–100%) decomposition of carbonate target rocks, mixed as lithic clasts into hot impact breccias. In this context, decomposition occurs strictly post-impact due to thermal equilibration-related heating. We demonstrate that this mechanism for CO2 outgassing is likely more widespread than previously recognized. The presence of andradite-grossular garnet serve as mineralogical markers of decomposition, analogous to limestone-replacing skarn deposits. Ca-rich garnet may therefore prove an important indicator mineral for post-shock decomposition of carbonate-bearing target rocks at other craters. These findings significantly advance our understanding of how sedimentary rocks respond to hypervelocity impact, and have wide-reaching implications for estimating the amount and timing of climatically-active volatile release due to impact events.