Compositional indication of E- and M-type asteroids by VIS-NIR reflectance spectra of meteorites

Abstract

Context. E-type asteroids have been linked to aubrites, while M-type asteroids have been linked to enstatite chondrites (ECs) and iron meteorites (IMs). However, as ECs and IMs generally lack absorption characteristics, distinguishing their parent bodies by spectroscopy generally poses a challenge. Aims. We aim to develop a method to distinguish two kinds of M-type asteroids, the parent bodies of ECs and IMs, and to infer their composition. Methods. We measured the visible to near-infrared (VIS-NIR) reflectance spectra of aubrite, ECs, and IMs. Then we analyzed and compared their spectral parameters, such as the reflectance at 0.55 µm (R0.55), absorption bands, and spectral slopes. We also compared the geometric albedo and spectral slopes of a total of 13 E-type and 14 M-type asteroids. Furthermore, combining the collected radar albedo and density data of M-type asteroids, we discuss their potential composition at different depths. Results. We find that for most meteorites, with the exception of very weak absorption in an aubrite and an EH7 chondrite, ECs and IMs do not show any absorption characteristics. Aubrite shows extremely high reflectance and a negative near-infrared slope (NIRS) and ECs show relatively low reflectance and moderately positive NIRS, while IMs show relatively moderate reflectance and the steepest positive NIRS. Two diagrams plotting with R0.55 and NIRS calculated in the 1.1–1.2 µm and 1.1–1.4 µm bands were subsequently shown to perform optimally at distinguishing aubrite, ECs, and IMs. In addition, M-type asteroids have a wide range of NIRS and diverse radar albedo and densities, whereby 16 Psyche shows high NIRS, radar albedo, and density, while 21 Lutetia is dominated by low values for these parameters. Conclusions. We demonstrate that NIRS is correlated with metal content and increases with metal content. In particular, the NIRS calculated in the 1.1–1.4 µm band is a potentially useful parameter for inferring the surface metal content of E- and M-type asteroids. Based on our results, we suggest that the featureless M-type asteroids ought to be divided into two subtypes: Mm- (e.g., 16 Psyche) and Me-type (e.g., 21 Lutetia) in the aim of characterizing the sources of IMs and ECs, respectively.