Evidence for Shock Darkening/Impact Melt on Near-Earth Asteroid (52768) 1998 OR2

We present photometric and spectroscopic characterization of near-Earth asteroid (52768) 1998 OR2 that shows the first evidence of shock darkening processes altering the surfaces in the near-Earth asteroid (NEA) population. Visible spectroscopy (0.45 – 0.93 μm) collected during the asteroid’s April 2020 close approach to the Earth are combined with archival MITHNEOS near infrared (NIR; 0.78 – 2.49 μm) measurements [1]. The combined spectrum shows a very flat reflectance spectrum with two weak absorption bands diagnostic of olivine and pyroxene surface chemistry, but with absorption features which appear suppressed compared to typical S-type asteroid spectra. Measurements of the following diagnostic band parameters were taken: Band I at 0.926 ± 0.003 μm and Band II at 2.07 ± 0.02 μm; Band I depth of 4.5 ± 0.15% and Band II depth of 4.0 ± 0.21%; Band Area Ratio of 1.13 ± 0.05. Further analysis of these band parameters using the equations from [2-4] indicate that this is a composition consistent with H chondrites.

Despite this mineralogical analysis, the asteroid’s spectrum is best characterized as an Xn-type in the Bus-DeMeo taxonomy using the SMASS online tool [5]. This is supported by principal component analysis of the combined visible-NIR spectrum falling on the C/X-complex side of the α-line. In order to understand the suppressed band features of the VNIR spectra, we investigated effects from phase reddening, space weathering, grain size, and shock darkening. We were able to rule out phase reddening because the NIR observations were at a low phase angle and 1998 OR2’s observed spectrum was very flat. Principal component analysis was used to eliminate the hypothesis that space weathering or grain size effects were causing the observed spectral features. Shock darkening is the ideal explanation for the observed spectral features because it causes suppressed absorption band features without reddening the asteroid spectrum. Shock darkening is also supported by principal component analysis placing 1998 OR2 near the end of the shock darkening trend in principal component space, as measured by [1, 5]. We use an areal mixing model with lab measurements of the shock darkened H5 chondrite, Chergach, to constrain the amount of shock darkened material on the asteroid’s surface at ~63% dark lithology and ~37% light lithology.

References: [1] Binzel R. P. et al. (2019) Icarus, 324, 41. [2] Sanchez J. A. et al. (2020), Astronomical Journal, 159. [3] Dunn T. L. et al. (2010) Icarus, 208, 789. [4] Gaffey, M., Bell, J., Brown, H., et al. 1993, Icarus, 106, 573 [5] Slivan, S. 2013, MIT Dept. of Earth, Atmospheric, and Planetary Sciences.