Expanding Band Parameter Analysis Methods for HED Meteorites and V-type Asteroids

Vesta and Vesta-like asteroids have been convincingly linked, through visible and near-infrared (VNIR; 0.7 - 2.5 µm [micron]) spectral analysis, to a clan of basaltic achondritic meteorites – howardites, eucrites, and diogenites (HEDs). VNIR reflectance spectra of V-type asteroids and HED meteorites have two absorption features centered near 1 µm (Band I) and 2 µm (Band II) caused primarily by Fe2+ [iron] and Ca2+ [calcium] cations in pyroxene. Previous studies have shown a correlation between the mol% Fs and Wo with the central wavelengths of Band I and Band II, hereafter called Band I Center (BIC) and Band II Center (BIIC). This dependency on mineral composition allows for the generation of calibration equations that link BIC and BIIC to mol% Fs and Wo, which can be applied to estimate the mineralogy of V-type asteroids from their NIR spectra. The ability to accurately determine BIC and BIIC wavelength positions is dependent on the signal-to-noise ratio (SNR) of the spectrum. The typical SNR of asteroid spectra is SNR ~ 50, noticeably lower than the SNR of meteorite spectra, which is about ten times higher. Sanchez et al. (2020) artificially reduced the SNR of the ordinary chondrite meteorite spectra to explore mineralogical determinations using S-type asteroid spectra of comparable SNR. Sanchez et al. (2020) also extended the applicability of S-type calibration equations to incomplete spectral data sets lacking visible data or having unreliable/incomplete data between 2.4 - 2.5 µm. Here I performed a similar analysis on HEDs and V-type asteroids using a reduction in SNR for HED meteorite spectra. In addition, I used six versions of each spectra, covering wavelength ranges from five different terminal wavelengths (~0.75 and 0.8 µm for Band I; 2.4, 2.45, and 2.5 µm for Band II), mimicking incomplete spectral data sets. Like Sanchez et al. (2020), I found that the decreased SNR calibration equations mostly yielded a lower R2 [R Squared], and an increased rms error, compared with their high SNR calibrations. I also created calibration equations using the minimum of Band I as an alternative to the center of Band I, and explored the efficacy of those equations. When applied to V-type asteroids, I found that the derived mineralogy from Band I is systematically higher than that of Band II, but that averaging the results from both bands gives mineralogical results for the V-type asteroids most consistent with those of the HEDs. I discuss the potential reasons for this discrepancy, including SNR, temperature effects, the overall slope of the reflectance spectra, and space weathering

A new method for deriving composition of S-type asteroids from noisy and incomplete near-infrared spectra

The surface composition of S-type asteroids can be determined using band parameters extracted from their near-infrared (NIR) spectra (0.7-2.50 $\mu$m) along with spectral calibrations derived from laboratory samples. In the past, these empirical equations have been obtained by combining NIR spectra of meteorite samples with information about their composition and mineral abundance. For these equations to give accurate results, the characteristics of the laboratory spectra they are derived from should be similar to those of asteroid spectral data (i.e., similar signal-to-noise ratio (S/N) and wavelength range). Here we present new spectral calibrations that can be used to determine the mineral composition of ordinary chondrite-like S-type asteroids. Contrary to previous work, the S/N of the ordinary chondrite spectra used in this study has been decreased to recreate the S/N typically observed among asteroid spectra, allowing us to obtain more realistic results. In addition, the new equations have been derived for five wavelength ranges encompassed between 0.7 and 2.50 $\mu$m, making it possible to determine the composition of asteroids with incomplete data. The new spectral calibrations were tested using band parameters measured from the NIR spectrum of asteroid (25143) Itokawa, and comparing the results with laboratory measurements of the returned samples. We found that the spectrally derived olivine and pyroxene chemistry, which are given by the molar contents of fayalite (Fa) and ferrosilite (Fs), are in excellent agreement with the mean values measured from the samples (Fa$_{28.6\pm1.1}$ and Fs$_{23.1\pm2.2}$), with a maximum difference of 0.6 mol\% for Fa and 1.4 mol\% for Fs.Comment: 22 pages, 11 figures, 2 tables, accepted for publication in The Astronomical Journa