Composition and origin of L5 Trojan asteroids of Mars: Insights from spectroscopy

Abstract

We investigate the mineralogical makeup of L5 Martian Trojan asteroids via reflectance spectroscopy, paying special attention to (101429) 1998 VF31, the only L5 Trojan that does not belong to the Eureka family (Christou, 2013). We find that this asteroid most likely belongs to the Bus-Demeo S-complex, in agreement with Rivkin et al. (2007). We compare it with a variety of solar system bodies and obtain good spectral matches with Sq- or S-type asteroids, with spectra of the lunar surface and of Martian and lunar meteorites. Mixture fitting to spectral endmembers suggests a surface abundance of Mg-rich orthopyroxene and iron metal or, alternatively, a combination of plagioclase and metal with a small amount of Mg-poor orthopyroxene. The metallic component may be part of the intrinsic mineral makeup of the asteroid or an indication of extreme space weathering. In light of our findings, we discuss a number of origin scenarios for (101429). The asteroid could be genetically related to iron-rich primitive achondrite meteorites (Rivkin et al 2007), may have originated as impact ejecta from Mars - a scenario proposed recently for the Eureka family asteroids (Polishook et al., 2017) - or could represent a relic fragment of the Moon's original solid crust, a possibility raised by the asteroid's close spectral similarity to areas of the lunar surface. If, on the other hand, (101429) is a relatively recent addition to the Martian Trojan clouds (Christou et al., 2020), its origin is probably traced to high-inclination asteroid families in the Inner Main Belt. For the olivine-dominated Eureka family, we find that the two smaller asteroids in our sample are more spectrally similar to one another than to (5261) Eureka, the largest family member. Spectral profiles of these three asteroids are closely similar shortward of ~0.7 μm but diverge at longer wavelengths. For the two smaller asteroids in particular, we find the spectra are virtually identical in the visible region and up to 0.8 μm. We attribute spectral differences in the near-IR region to differences in either: degree of space weathering, olivine chemical composition and/or regolith grain size.