Mid-Infrared Spectral Studies of Jovian Trojan Asteroids and the Effects of Regolith Porosity

Abstract

Jupiter’s Trojan asteroids (hereafter Trojans) make up a large group of primitive bodies that carry insight into the formation mechanisms that shaped our Solar System. Mid-infrared (MIR; 5-35 µm) spectra of Trojans exhibit puzzling silicate emission features, similar to spectra of optically thin comet comae – an odd finding because asteroid regoliths are optically thick. This discovery led to the hypothesis that Trojan surfaces may consist of a fine grained, ‘fluffy’ regolith of silicates (hereafter ‘regolith porosity’). In this dissertation, I investigate the Trojan surface composition as a window into their formation and subsequent evolution. By constraining the effects of regolith porosity on MIR spectra, Trojan surface composition can be more accurately interpreted, leading to a deeper understanding of formation mechanisms.

I began this investigation with laboratory experiments designed to constrain regolith porosity, a parameter that greatly affects MIR spectra. To simulate regolith porosity, I began by selecting two olivine, seven pyroxenes, and one mixture of olivine and pyroxene. Then, I ground the samples into the following grain sizes: 0-20 µm, 20-45 µm, and 45-63 µm, and mixed them with potassium bromide (KBr; a MIR-transparent salt) from 0%-90% with 10% intervals by weight. Finally, I measured the MIR spectra of each silicate sample mixture with a Fourier transform infrared spectrometer at every regolith porosity interval. Results indicate a gradual transition from primarily surface scattering (typical for optically thick material) to primarily volume scattering (typical for optically thin material) spectral regime, supporting my hypothesis. Using these results to analyze Hektor, the largest Trojan, indicates Hektor’s regolith is highly porous (at least 84% void space).

To complete my investigation of Trojan compositions, I reduced and analyzed eleven MIR Trojan spectra taken with the Spitzer Space Telescope. I compared the Trojan spectra to spectra of meteorite powder, comets, and mineral endmember spectra. My results indicate Trojans are likely covered in a highly porous regolith of fine-particulate crystalline olivine (primarily forsterite), enstatite, diopside, and amorphous silicates. The minerals that are likely on the Trojan surface are consistent with the Nice model, which predicts the Trojans originally formed in the primordial Kuiper belt.