Composition and Space Weathering Characteristics of Tianwen-2 Mission's First Target Near-Earth Asteroid (469219) Kamo`oalewa

TL;DR

The paper reanalyzes Kamo'oalewa’s VNIR spectrum to constrain surface mineralogy and space weathering using Bus-DeMeo spectral similarity, 1 μm band center analysis, and a NOMAT-based Is/FeO model with validations from lunar samples and Chang’e-5 data. It finds an olivine-rich surface within the S-type complex and an immature to submature space weathering state, with a spectral trend resembling lunar-style weathering but amplified by olivine content. Through integrating band-center analyses, continuum-removal methods, and a modified Hiroi space weathering framework, the study links composition to weathering behavior and discusses implications for Kamo’oalewa’s origin and Tianwen-2 sample-return planning. The work provides initial, data-limited constraints that guide interpretation of quasi-satellite surfaces and informs mission design and target selection for future exploration.

Abstract

The near-Earth asteroid Kamo`oalewa, a quasi-satellite of the Earth and the target for sample return by China's Tianwen-2 mission, exhibits distinctive spectral characteristics. This study re-analyzes the visible and near-infrared reflectance spectrum of Kamo`oalewa published by B. N. L. Sharkey et al. (2021), obtained using the Large Binocular Telescope, to infer its mineral composition and space weathering characteristics. Spectral similarity analysis is performed by comparing the spectrum of Kamo`oalewa to the mean spectra of various types in the Bus-DeMeo taxonomy to make a preliminary constraint on the combined characteristics of surface mineralogy and space weathering effects. To further characterize the mineral composition, a detailed analysis of the 1 μm band center is conducted based on spectral data below 1.25 μm that have higher signal-to-noise ratios. Empirical models for normalized spectra are developed to estimate the Is/FeO content. The results suggest that asteroid Kamo`oalewa has higher olivine abundance than that of typical S-type asteroids and the Moon, exhibiting an immature to submature degree of space weathering. These findings enhance our understanding of the evolution of similar quasi-satellites and provide important implication for the future exploration of Tianwen-2 mission.

Figures (7)

• Figure 1: Spectral comparison of asteroid Kamo'oalewa with the mean spectra of spectral types in Bus-DeMeo taxonomy 2009Icar..202..160DBDmeanspecdb exhibiting prominent Band I absorption feature. (a) Full-band comparison with continuum retained, all spectra normalized at 0.55 $\mathrm{\mu m}$. (b) Full-band comparison after continuum removal based on normalized spectra. (c) VNIR comparison with continuum retained, all spectra normalized at 0.55 $\mathrm{\mu m}$. (d) VNIR comparison after continuum removal based on normalized spectra. In each panel, Kamo'oalewa's spectrum is shown at the bottom, with other spectra ranked by similarity from highest to lowest (bottom to top) and offset vertically for clarity. Panels (a) and (b) employ similarity analysis weighted by wavelength intervals and reflectance errors, whereas panels (c) and (d) use unweighted analysis due to more uniform data point distribution. Data points with error bars in (a) and (c) represent Kamo'oalewa's original measurements, while its spectral curves in (c) and (d) are smoothed using a Savitzky-Golay filter (window length 11, polynomial order 3; SGFM). The smoothed full-band spectrum of Kamo'oalewa in panel (b) consists of the smoothed VNIR spectrum and the original H and K data points.
• Figure 2: BC1 analysis of spectra from different samples. (a) Spectrum of Kamo'oalewa; (b) spectrumspectra of laser-irradiated olivine (25 mJ, 4 pulses; 2017AA...597A..50Y); (c) spectrumspectra of olivine and OPX mixtures (90 wt.% olivine, 10 wt.% OPX; RELABdb). The spectrum in (a) extends only to 1.25 $\mathrm{\mu m}$, with the BC1 fitted using the method of 2014Icar..234..132H being 1.00 $\mathrm{\mu m}$. In (b) and (c), both truncated spectra up to 1.25 $\mathrm{\mu m}$ and full-wavelength spectra were used. Truncating the spectrum at 1.25 $\mathrm{\mu m}$ causes the calculated BC1 to shift toward shorter wavelengths; thus, Kamo'oalewa's actual BC1 is at least 1.00 $\mathrm{\mu m}$ or (more likely) longer, suggesting a high olivine content. The fitting range refers to the region around the absorption band center fitted with a fourth-order polynomial, following the method of 2014Icar..234..132H.
• Figure 3: Scatter plot of R950/R750 versus R750/R550 for various sample spectra. The blue data points represent LSCC and RELAB lunar samples 2006Icar..184...83P2012Icar..220...51B, exhibiting a progression from the lower left to the upper right, which corresponds to increasing Is/FeO values; point size indicates the corresponding FeO content. No variation associated with FeO content is observed. These blue points define the range of spectral changing trend induced by lunar-style space weathering, with the Kamo'oalewa data point positioned at the upper left edge of this range. Refer to the legend for the interpretation of the remaining data points.
• Figure 4: Empirical model of Is/FeO versus NOMAT, fitted using RELAB and LSCC lunar sample data. Blue dots represent the original data points used for the fit. The red shaded region indicates the 1$\mathrm{\sigma}$ prediction interval of the model. Data points from the Chang’e-5 sample lie outside the 1$\mathrm{\sigma}$ prediction interval but within the 2$\mathrm{\sigma}$ prediction interval. The orange shaded region denotes the Is/FeO value range for S-type asteroid Ida. The fitted Is/FeO value for Kamo'oalewa is 25.97 ± 19.62.
• Figure 5: Additional spectral comparisons of the Kamo'oalewa spectrum. (a) Direct comparison with laser-irradiated olivine spectra 2017AA...597A..50Y; (b) Comparison of NIR colors using the method of 2018AA...617A..12P. In panel (a), the morphology of the 1-$\mathrm{\mu m}$ absorption band in the Kamo'oalewa spectrum resembles that of the olivine spectrum after 3-4 irradiations with a 25 mJ laser. In panel (b), the data points for Kamo'oalewa fall within the region associated with A-type asteroids (see Figure 1 in 2018AA...617A..12P for details).