T-Matrix and Hapke Modeling of Tir Spectra of Trojan Asteroids

Introduction: Thermal infrared (TIR) observations of the Trojans show prominent 10 um plateaus, which are similar to features observed in comet spectra [1-3]. We present modeling results of TIR spectra of Trojan asteroids (911) Agamemnon, (1172) Aneas, and (624) Hektor [1] using the Multiple Sphere T-Matrix (MSTM) software [4] and Hapke reflectance model [5].

Data and Methods: To calculate the TIR mineral spectra, we used the MSTM software [4] and optical constants [6] with a range of mineral porosities and particle sizes. The MSTM and Hapke-modeled mineral spectra were then combined linearly using a weighted least squares (WLS) model [7] to obtain the best model fit to the Trojan spectra over the wavelength region from ~8.0 to 13.5 um (1250-740 cm^-1). The WLS-computed abundances were used to re-run the MSTM and Hapke models to obtain the final fits.

Results: The modeled abundances resulting in the best fit to the emissivity spectrum for (911) Agamemnon were 100% Fe-rich olivine (fayalite), with 75% porosity, respectively (RMS error 1.35x10^-2), at the 0.5 um particle size. The (1172) Aneas spectrum favored the 1.0 um particle size with abundances of 49% fayalite and 33% forsterite, with 80% porosity, (RMS error 1.67x10^-2). The 0.5 um troilite was also selected in the modeling for (1172) Aneas with an abundance of 18%. The (624) Hektor spectrum was modeled using an abundance of 100% fayalite at the 0.5 um size, with 70% porosity, respectively (RMS error 2.58x10^-2).

Conclusions: Here we show that the 10 um plateau in the Trojan spectra may be modeled using fine particulates with 70-90% porosities, which is similar to lunar regolith porosities [9].

Acknowledgements: V.C. Lowry, K. L, Donaldson Hanna, and H. Campins are supported under NASA Cooperative Agreement 80NSSC19M0214 to the Center for Lunar and Asteroid Surface Science (CLASS) funded by NASA’s Solar System Exploration Research Virtual Institute at UCF. H. Campins also acknowledges support from NASA’s Near-Earth Object Observations program.

References: [1] Emery et al. (2006) Icarus, 182, 496-512. [2] Rivkin (2009) The Trojan Asteroids: Keys to Many Locks. [3] Yang et al. (2012) Icarus, 223, 359-366. [4] Mackowski, D.W. (2013) MSTM version 3.0. [5] Hapke, B. (2012) Theory of Reflectance & Emittance Spectroscopy. [6] n-k data, AIU Jena. [7] Ramsey M. S. & Christensen P. R. (1998) JGR, 103, 577-596. [8] Hapke, B. (2016) Icarus, 273, 75-83.