¹Zhongwu Lan,¹Ross N.Mitchell,²Thomas M.Gernon,³Adam R.Nordsvan
Earth and Planetary Science Letters 581, 117407 Link to Article [https://doi.org/10.1016/j.epsl.2022.117407]
¹State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²School of Ocean and Earth Science, University of Southampton, Southampton SO22 4JR, UK
³Department of Earth Sciences, University of Hong Kong, Pokfulam, Hong Kong
Copyright Elsevier

The ca. 717 Ma low-latitude Sturtian “snowball Earth” glaciation lasted ∼56 Myr. However, sedimentological evidence for transient, open ocean conditions during the glaciation appears to contradict the concept of a global deep freeze. We demonstrate multiple lines of geologic evidence from five continents for a temporary, localized sea-ice retreat during the middle of the Sturtian glaciation, which coincides with one, perhaps two, asteroid impacts, and arguably more terrestrial impacts as inferred from the lunar impact record. The well-dated Jänisjärvi impact (ca. 687 Ma) is synchronous with repeated volcanic ash falls whose deposition is most parsimoniously interpreted to indicate a partially ice-free ocean. Temporary greenhouse warming caused by the vaporization of sea ice can explain localized glacial retreat within restricted seaways between these continents, where ice flow would have been constricted and sea ice thinnest before impact.

^1,2Lucas T.McClure,²Sean S.Lindsay
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114907]
¹Department of Physics & Astronomy, The University of Tennessee, Knoxville, TN 37916, United States
²Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011, United States
Copyright Elsevier

The visible and near-infrared spectra (0.5–2.5 μm) of ordinary chondrite (OC) meteorites are characterized by absorptions at 1 and 2 μm, typically denoted as Band I and Band II, respectively. Previous works have connected parameterization of Band I and Band II, a so-called band parameter analysis (BPA) of mineralogical abundances and chemistry of OC meteorites. In particular, parameters for these determinations include the center of the Band I feature (BIC) and band area ratio (BAR), the ratio of Band II’s area to that of Band I. Through treating BIC as a function of BAR, OCs plot within a region called the “OC-boot,” first shown in Gaffey et al. (1993). The boundaries for the OC-Boot have remained unchanged since their foundational work, and numerous investigations using various different methods have employed the same boundaries for the OC-Boot’s original zoning. By applying the Spectral Analysis Routine for Asteroids (SARA) to >150 spectra of OCs from Brown University’s NASA/Keck Reflectance Experiment Laboratory (RELAB) database, we highlight the issue of the OC-Boot’s dependency on BPAs. Namely, we vary how Band I and Band II are defined to highlight the BPA-dependency by producing band edge-specific OC-Boots that encompass the mineralogical diversity of OCs (H, L, and LL subtypes) with corresponding spectral ranges. We conclude that there is no single canonical OC-boot and suggest that researchers create their own OC-Boot using their specific BPA or select an OC-boot in the literature that most closely matches their methods of determining band parameters.

^1,2Lucas T.McClure,¹Sean S.Lindsay
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.114944]
¹Department of Physics & Astronomy, The University of Tennessee, Knoxville, TN 37916, United States
²Northern Arizona University, Department of Astronomy and Planetary Science, Flagstaff, AZ 86011, United States
Copyright Elsevier

The ordinary chondrite boot (OC-Boot) is a diagnostic region generated from spectral analyses of the features caused by electron absorptions in the olivine and orthopyroxene of OCs. In our companion article to this one, McClure & Lindsay (2022a) demonstrated that the boundaries of the OC-Boot are band parameter analysis (BPA) dependent. Here, we highlight how using OC-Boot boundaries that are not derived from a self-consistent BPA analysis can lead to potential misidentification of ordinary chondrite-like asteroid analogs. We compare S-type asteroid spectral band parameters to the OC-Boot defined in McClure & Lindsay (2022a) and the OC-Boot defined in Gaffey et al. (1993). We choose the Gaffey et al. (1993) OC-Boot for this comparison since its use is frequently seen in the literature without updated boundaries. By applying the Spectral Analysis Routine for Asteroids (SARA) to spectra from the MIT-Hawaii Near-Earth Object Spectroscopic (MITHNEOS) Survey, we demonstrate an overlap between the contemporary view of the OC-Boot and OC analogs, showing that a self-consistent OC-Boot framework captures the variation of the Near-Earth asteroids (NEAs) more than the original OC-Boot. In particular, we show the OC-Boots from McClure & Lindsay (2022a) encompass relatively more NEAs. We also apply a set of calibration equations derived using SARA to determine the mineral abundances and compositions for the S-type asteroids. We find that 59.57% of NEAs exhibit LL-like mineralogies and that H-like and L-like mineralogies are exhibited 19.15% and 6.38% of cases, respectively. There are a couple of cases wherein the mineralogies could be in between subtypes and five cases where no subtype designation could be determined. The high-frequency of LL-like mineralogies is in agreement with previous studies on S-type NEAs.