Compositional Constraints for Lucy Mission Trojan Asteroids via Near-infrared Spectroscopy

Abstract

We report near-infrared (0.7-2.5 µm) reflectance spectra for each of the six target asteroids of the forthcoming NASA Discovery-class mission, Lucy. Five Jupiter Trojans (the binary (617) Patroclus system, (3548) Eurybates, (21900) Orus, (11351) Leucus, and (15094) Polymele) are well-characterized, with measurable spectral differences. We also report a survey-quality spectrum for main belt asteroid (52246) Donaldjohanson. We measured a continuum of spectral slopes including "red" (Orus, Leucus), "less red" (Eurybates, Patroclus-Menoetius) and intermediate (Polymele), indicating a range of compositional end-members or geological histories. We perform radiative transfer modeling of several possible surface compositions. We find that the mild-sloped spectra and low albedo of Patroclus and Eurybates imply similar compositions. Eurybates (~7 wt.% water ice) and Patroclus (~4 wt.% water ice) are consistent with a hydrated surface. Models for Orus and Leucus are consistent with each other and require a significantly more reddening agent (e.g. iron-rich silicates or tholin-like organics). Polymele has a linear spectrum like Patroclus, but a higher albedo more closely aligned with Orus/Leucus, defying simple grouping. Solar system formation models generally predict that the Jovian Trojans accreted in the outer solar system. Our observations and analysis are generally consistent with this expectation, although not uniquely so.

Stability of Jovian Trojans and their collisional families

“…In the case of the Jovian Trojans, simulations of hypothetical objects have indicated that at small sizes (<1 km), the Yarkovsky effect could impact the stability of the objects (Wang & Hou 2017;Hellmich et al 2019). As we are simulating known Jovian Trojans, the majority of the objects are greater than several kilometres in size (Emery et al 2015), and have unknown or highly uncertain thermal properties (Slyusarev & Belskaya 2014;Sharkey et al 2019). For these reasons, we have not included the Yarkovsky effect in our simulations.…”

Complex Water Ice Mixtures on NII Nereid: Constraints from NIR Reflectance

“…In all models, a single grain size for all materials is assumed. As with Sharkey et al (2019), we find that allowing for unique grain sizes for each component produces model degeneracies and slows convergence without improving fits. We adopt a single grain size bounded over the range of 0.5-1000 µm.…”

“…In this data set, that means fitting band shapes, centers, depths, and the continuum. To determine what physical constraints our data can place on Nereid's surface properties, we utilize the Markov-chain Monte Carlo (MCMC) Hapke modeling methodology after Sharkey et al (2019), which utilizes the emcee python package (Foreman-Mackey et al 2013) and makes use of methodologies developed by Emery & Brown (2004) and Emery et al (2011) for interpreting models of featureless asteroid spectra. Our aim is to explore the types of materials which constitute plausible analogs to a given surface.…”

Investigating the Relationship between (3200) Phaethon and (155140) 2005 UD through Telescopic and Laboratory Studies

“…The observations followed the procedure of Kareta et al 2018, whereby observations of the target were 'bookended' by observations of a local G-type star for immediate telluric correction and later slope-corrected further using a wellcharacterized solar analog G2V star (SAO 93936) observed near-zenith. This method has been used to effectively and reproducibly characterize faint Solar System small bodies on many occasions (see, e.g., Sharkey et al 2019. ) The reduction was performed partially within the 'spextool' environment (Cushing et al 2004) and partially within custom-written scripts in Python (see 'Software' after the Acknowledgements section).…”