Linking mineralogy and spectroscopy of highly aqueously altered CM and CI carbonaceous chondrites in preparation for primitive asteroid sample return

H C Bates,S S Russell,N E Bowles,A J King,K L Donaldson Hanna

doi:10.1111/maps.13411

Abstract

AbstractThe highly hydrated, petrologic type 1 CM and CI carbonaceous chondrites likely derived from primitive, water‐rich asteroids, two of which are the targets for JAXA's Hayabusa2 and NASA's OSIRIS‐REx missions. We have collected visible and near‐infrared (VNIR) and mid infrared (MIR) reflectance spectra from well‐characterized CM1/2, CM1, and CI1 chondrites and identified trends related to their mineralogy and degree of secondary processing. The spectral slope between 0.65 and 1.05 μm decreases with increasing total phyllosilicate abundance and increasing magnetite abundance, both of which are associated with more extensive aqueous alteration. Furthermore, features at ~3 μm shift from centers near 2.80 μm in the intermediately altered CM1/2 chondrites to near 2.73 μm in the highly altered CM1 chondrites. The Christiansen features (CF) and the transparency features shift to shorter wavelengths as the phyllosilicate composition of the meteorites becomes more Mg‐rich, which occurs as aqueous alteration proceeds. Spectra also show a feature near 6 μm, which is related to the presence of phyllosilicates, but is not a reliable parameter for estimating the degree of aqueous alteration. The observed trends can be used to estimate the surface mineralogy and the degree of aqueous alteration in remote observations of asteroids. For example, (1) Ceres has a sharp feature near 2.72 μm, which is similar in both position and shape to the same feature in the spectra of the highly altered CM1 MIL 05137, suggesting abundant Mg‐rich phyllosilicates on the surface. Notably, both OSIRIS‐REx and Hayabusa2 have onboard instruments which cover the VNIR and MIR wavelength ranges, so the results presented here will help in corroborating initial results from Bennu and Ryugu.

Highlights

The C-complex asteroids, which include the B, C, Cb, Cg, Cgh, and Ch-types in the Bus-DeMeo classification scheme (DeMeo et al 2009), are of interest because they are thought to be chemically primitive, volatile-rich, and can tell us about the evolution of water and organic compounds in the early solar system (e.g., Alexander et al 2012)
We have examined a number of parameters in the visible and near-infrared (VNIR) and mid infrared (MIR) spectra of CM and CI chondrites and established how they vary with mineralogy
Most spectral studies have focussed on the partially altered petrologic type 2 CM chondrites; here we present VNIR and MIR spectra for the highly altered CM1/2, CM1, and CI1 chondrites and describe identified trends related to their composition and the degree of secondary processing

Summary

Introduction

The C-complex asteroids, which include the B-, C-, Cb-, Cg-, Cgh-, and Ch-types in the Bus-DeMeo classification scheme (DeMeo et al 2009), are of interest because they are thought to be chemically primitive, volatile-rich, and can tell us about the evolution of water and organic compounds in the early solar system (e.g., Alexander et al 2012). JAXA’s Hayabusa is at the Cg-type asteroid (162173) Ryugu, and NASA’s Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx, Lauretta et al 2017) is at the B-type asteroid (101955) Bennu. Both missions aim to collect and return samples from the surfaces of their target asteroids. The CIs are all classified as petrologic type 1 meteorites and record aqueous alteration at high water/rock ratios of 1.1–1.2 and temperatures of ~150 °C (Tomeoka and Buseck 1988; Endress et al 1996; Clayton and Mayeda 1999; Barrat et al 2012; King et al 2015a)

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