Panel a: Near-IR spectrum of 2I/2019 Q4 (Borisov) in comparison with NIR spectrum of 1I/’Oumuamua, taken from Fitzsimmons et al. (2018). Both ISOs appear featureless and the spectral slopes of the two NIR spectra are comparable. Panel b: comparison between the spectrum of 2I and those of Solar System comets and asteroids. The spectrum of C/2013 US10 is from Protopapa et al. (2018), the spectrum of 6P/d’Arrest from Yang et al. (2009), and the three asteroid spectral classes from DeMeo et al. (2009).
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