Shape, regolith size and thickness, SMFe^0 content, and spectral type of Tianwen-2 target asteroid (469219) Kamo'oalewa

Abstract

China's Tianwen-2 spacecraft will return samples from the near-Earth asteroid (469219) Kamo'oalewa. We previously reported that Kamo'oalewa develops an LL-chondrite-compositional, highly space-weathered surface. This study aims to estimate Kamo'oalewa's shape, regolith grain size and thickness, sub-micrometer iron (SMFe0) content, and spectral type. Using the lightcurve data and the Cellinoid model, we modeled Kamo'oalewa's shape, rotation period, and pole orientation. We then estimated its global distribution of regolith critical size using the balance method of gravity, cohesive force, and centrifugal force. Furthermore, in the temperature range of 253.15 to 473.15 K, we measured the thermal parameters of laser-irradiated LL chondrite powder that best matches Kamo'oalewa's spectrum, estimating Kamo'oalewa's thermal inertia and skin depth (lower limit of regolith thickness). Using the radiative transfer mixing model, we also estimated the content of SMFe0 in Kamo'oalewa's regolith. Finally, using the MIT online spectral classification tool for the laser-irradiated LL chondrite powder, we obtained a virtual spectral type of Kamo'oalewa. Our model gives a size of 68 m x 46 m x 39 m, a rotation period of 27.66 minutes, and a pole orientation of 134.7 degrees longitude and -11.4 degrees latitude for Kamo'oalewa. Regolith grains with a size <2 cm can remain stable over 93.8% of the global surface area of Kamo'oalewa. Laser-irradiated LL chondrite powder shows a low thermal inertia (95.5 to 135.1 J m^-2 K^-1 s^-1/2), corresponding to a thermal skin depth of 3 to 3.5 mm on Kamo'oalewa. An SMFe0 content of 0.29 +- 0.05 wt.% is required to match Kamo'oalewa's spectrum. The virtual spectral type of Kamo'oalewa is given as "Sqw".

Figures (8)

• Figure 1: Modeled parameters change with the number of iterations. (A) Mean square error $\chi^2$ changes with the number of iterations. (B) Rotation period changes with the number of iterations. (C) Longitude changes with the number of iterations. (D) Latitude changes with the number of iterations. When running the 877th iteration, $\chi^2$ is minimum, and the rotation period, longitude, and latitude converge to 0.461 h, 134.7$^{\circ}$, and $-$11.4$^{\circ}$, respectively.
• Figure 2: Global distribution of regolith critical size of modeled Kamo'oalewa shape viewed from different views (A--D). Kamo'oalewa’s shape was modeled as 68.15 m $\times$ 45.66 m $\times$ 38.89 m, and the pole orientation was 134.7$^{\circ}$ in longitude and $-$11.4$^{\circ}$ in latitude (ecliptic coordinate system). The blue to red colors represent the increasing critical size of the regolith grains. Regolith grains with a critical size < 2 cm can stably remain on 93.83% of the global surface area of Kamo'oalewa, of which grains < 1 cm in diameter occupy 67.16% of global surface area. Two white zones that appear around poles (occupy 3.4% of the global surface area) could remain the grains with a critical size > 4 cm.
• Figure 3: Thermal parameters of space-weathered LL chondrite powder and skin depth of the regolith of Kamo'oalewa. (A) Thermal conductivity changes with temperature. (B) Thermal inertia changes with the temperature. (C) In the temperature range of 253.15 to 473.15 K, the skin depth ranges from 3.04 to 3.53 mm.
• Figure 4: Estimation of SMFe$^0$ content in the regolith of Kamo'oalewa. The spectrum of Kamo'oalewa was best matched to a model of fresh Kheneg Ljouâd (a LL5/6 chondrite) powder with 0.29 $\pm$ 0.05 wt.% SMFe$^0$. Spectra are normalized at 0.55 $\mu$m.
• Figure 5: Spectral evidence for identifying the composition of Kamo'oalewa. (A) Band parameters of Kamo'oalewa and meteorites. (B) Reflectance spectra of fresh and laser-irradiated LL chondrite powder. (C) 0.55 $\mu$m-normalized spectra of Kamo'oalewa, three lunar samples, and laser-irradiated LL chondrite powder. (D) Band parameters of Kamo'oalewa, three lunar samples, and laser-irradiated LL chondrite powder. (E) Reflectance spectra of Kamo'oalewa, Itokawa, Flora family, and the lunar Giordano Bruno crater. (F) Band parameters of Kamo'oalewa, Itokawa, the Flora family, and the lunar Giordano Bruno crater.