Carbonaceous Chondrite-like Compositions for Proposed Captured Trans-Neptunian Objects Justitia and Pompeja

Abstract Red spectral slopes (i.e. increasing reflectance with increasing wavelength) and featureless 0.5–2.5 μ m spectra are often associated with the presence of organic material, and there is a sense that the redder the slope, the more organic material is present and the more distant the formation location. This has led to the D-class asteroids, which are the reddest asteroid spectral type, being used as proxies for outer-solar-system material when considering material transport. The asteroids 269 Justitia and 203 Pompeja have been identified as the reddest main-belt asteroids, a distinction that follow-up observations have confirmed for Justitia. This has been used as evidence that Justitia is a transported trans-Neptunian object. We observed Justitia and Pompeja during their 2022 apparitions to determine their compositions from measurements in the 3 μ m region. We found that other than spectral slope, Justitia’s spectrum is a very good match to typical aqueously altered carbonaceous chondritic material and to C-complex asteroids like Bennu and Polana, but very poor matches to spectra of most D-class asteroids, Jovian Trojans and irregular satellites, and Centaurs. We also found that Pompeja’s spectrum is consistent with Justitia’s, though of lower quality. While we do not know what is causing Justitia’s steep spectral slope, its composition appears to be consistent with common carbonaceous chondrites, and there is no need to invoke a separate formation location. It is not evident that Justitia, or any particularly red-sloped asteroid, can have its formation location inferred from spectral slope alone.