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A Contemporary View of the Ordinary Chondrite “Boot” that Accounts for Mineralogical Variation

Presentation #509.03 in the session “Asteroids: NEOs Physical Properties 3 - volatiles and composition”.

Published onOct 26, 2020
A Contemporary View of the Ordinary Chondrite “Boot” that Accounts for Mineralogical Variation

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, nominally denoted as Band I and Band II, respectively. Parameterization of Band I and Band II are often used to determine the relative abundance of olivine to orthopyroxene as well as the mol% Fayalite (Fa) and Ferrosilite (Fs). The useful parameters for these determinations are 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 in or near a region called the “OC-boot” in the literature. The boundaries for the OC-boot were determined empirically by Gaffey et al (1993) using an assemblage of ~45 OCs and are also the boundaries for the S(IV)-type asteroids, the analogs to the OCs. The OC-boot has remained unchanged since that work. The precise location in the BIC vs. BAR- space wherein OCs and their asteroid counterparts plot with respect to the OC-boot is dependent on the band parameter analysis (BPA) techniques implemented. Previous investigations of asteroids, e.g., de Leon et al. (1993), Thomas et al. (2014), and Lindsay et al. (2015), show that asteroids frequently cluster within the OC-boot, though often to the left (lower BAR) of the OC-boot. BPAs of asteroids also demonstrate a larger scatter than their OC counterparts, which often places them just outside the OC- boot. Without a deeper investigation and an update to the OC-boot region, it remains unclear if these asteroids are OC-like or not. For this work, we seek to highlight this issue as well as to provide updated boundaries of the OC-boot in an effort to more completely encompass the mineralogical variation observed for the OC subtypes (H, L, and LL). By applying the Spectral Analysis Routine for Asteroids (SARA) to >150 spectra of OCs from Brown University’s Reflectance Experiment Laboratory (RELAB) database, we vary how Band I and Band II are defined to show the BPA-dependency as well as provide an updated OC-boot that encompasses the mineralogical diversity of OCs. We also employ SARA for spectral data of Near-Earth Asteroids (NEAs) from MITHNEOS to compare how OCs and their asteroid analogs plot in regards to the updated OC-boot. We also explore more robust techniques to assign OC subtype analogs for OC-like asteroids.

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