New Insights into Erg Chech 002 Analogues in the Main Belt from Near-Infrared Spectroscopy

Abstract

Erg Chech 002 is an andesitic meteorite that formed early in the Solar System's history, and that is thought to have been formed in the primitive crust of an early accreted and differentiated planetesimal. It shows unique spectral features, and no known asteroid spectral type was initially found to match with its compositional type. In the literature, asteroids (10537) 1991 RY16, (7472) Kumakiri and (14390) 1990 QP10 were found to show peculiar spectra, and were not classified in any known existing spectral class. These objects were hypothesised to be fragments of differentiated planetesimals. In a previous study, the Gaia Data Release 3 dataset of visible reflectance spectra of Solar System Objects was exploited to search for potential analogues of Erg Chech 002 in the main belt. As a result, 142 asteroids were found to potentially match this meteorite. In this work, we present NASA's IRTF near-infrared observations of 20 main belt asteroids found as potential analogues of Erg Chech 002. We classified these asteroids based on their visible-near-infrared spectra, then studied and compared their diagnostic spectral features with those of the laboratory spectra of Erg Chech 002. We classified 16 of the 20 observed asteroids as V-types, one as S-complex and one as A-type, and conclude that none of the observed objects match with Erg Chech 002. In addition, we show that asteroids (10537) 1991 RY16 and (14390) 1990 QP10 are good spectral matches to Erg Chech 002 based on the study of their diagnostic spectral features. Asteroid (7472) Kumakiri could match a more olivine-rich Erg Chech 002-like material, and cannot be completely ruled out as a match of the meteorite. This possible link between the meteorite and these asteroids is consistent with the theories regarding their formation, and these asteroids could be part of a new spectral class of andesitic bodies in the main belt.

Figures (10)

• Figure 1: Second order polynomial fit of the 1 $\mu$m band on the continuum-removed spectrum of (1643) Brown. The polynomial fit parameters are displayed inside the red square, the equation of the fit being $y=ax^2+bx+c$.
• Figure 2: Band areas of the first and second bands of the continuum-removed spectrum of (1643) Brown.
• Figure 3: Literature spectra of asteroids (10537) 1991 RY16 moskovitz2008_10537, (7472) Kumakiri leith2017 and (14390) 1990 QP10 leith2017 (grey dots), and Gaia DR3 spectra considering the range from 462 to 946 nm (blue dots for bands with flag=0, orange diamond for bands with flag=1). The V-type template spectrum of the Bus-DeMeo taxonomic scheme demeo2009 is shown as a purple shaded area, and the powder sample's spectrum of meteorite EC 002 is displayed in black lines for comparison.
• Figure 4: Band I Centre (BIC) as a function of the BII/BI band area ratio (BAR). The BIC and BAR of the meteorite powder and slab spectra are displayed as black and grey stars respectively, and the observed asteroids as black dots with their associated uncertainty. The identification number of S-complex asteroid (18780) Kuncham and A-type asteroid (31060) 1996 TB6 are written next to their corresponding data point. The BIC and BAR of the taxonomic end-members of demeo2009 are highlighted in colours, the letter corresponding to the taxonomic type of the end-member. Finally, asteroid (10537) 1991 RY16 is plotted as a black diamond and (7472) Kumakiri and (14390) 1990 QP10 as black triangles. The mineralogical subdivisions defined by Gaffey2011 for S-type asteroids, ordinary chondrites, basaltic achondrites, and olivine assemblages are overlaid in light grey.
• Figure 5: Spectrum of asteroid (18780) Kuncham (grey dots) superimposed with the S-type template spectrum of the Bus-DeMeo taxonomy demeo2009 (green shaded area). The powder sample and low space-weathered modelled spectra of meteorite EC 002 are displayed for comparison.