Characterizing Trojan Asteroids Throughout the Solar System

Abstract

This thesis reports on new results on Trojan asteroids throughout the Solar System. Trojan asteroids, which librate around a planet's L4 or L5 point, are a group of small bodies defined by their unique orbital properties and long term stability. Many Trojan asteroid populations are expected to be stable on order the age of the Solar System. Therefore, Trojans are likely relatively pristine remnants of the primordial disk. Therefore, studying their composition can give us a window into the composition of our proto-planetary disk. Since the Lagrange points are dependent on the orbit of the planet, Trojan asteroids are also sensitive to the dynamical history of the Solar System. This effect is particularly relevant for constraining the migration of the giant planets. The sizes and shapes of these objects also tell us what small planetesimals look like and how they coalesced. In short, Trojans are a key population for understanding the formation and evolution of the Solar System.

We currently know the most about Jupiter Trojans (JTs), a population with over 12,000 members. We know that they have rather dark albedos compared to other objects beyond the asteroid belt, but they have similar shapes and sizes to Trans-Neptunian Objects (TNOs) suggesting a common origin. Mars Trojans (MTs) on the other hand are the only stable Trojan population in the inner Solar System, and mostly appear to be covered in olivine, rare in the Solar System. Neptune's Trojans (NTs) appear to have a strange color distribution compared to their presumed source population, TNOs. Finally, Earth Trojans (ETs) are expected to exist based on numerical simulations, and while upper limits have been placed from previous searches, but no stable ones have been found.

All of these previous results leave many open questions about these populations regarding their origin and evolution. I address some of these questions in this thesis. In Chapter II, I present a search for L5 ETs using one hour of time on DECam on the 4m Blanco telescope. We did not discover any new ETs, but did place the most stringent constraints on the population to date. Based on these upper limits I rule out any ETs larger than about one km in size, but there could still be hundreds of undiscovered smaller ETs. A deeper search of this region is necessary to probe the smallest end of the size distribution to test the dynamical stability of this population.

The NT population has been found to be surprisingly lacking in ultra-red members, unlike the nearby Kuiper Belt. In Chapter III, I identify three new ultra-red NTs, bringing the red to ultra-red ratio in line with the TNO ratio. I also find that there is a color-orbit-size correlation in the NT population, which has implications regarding the formation of this population.

In Chapter IV, I present the results of a spectroscopic survey of the NT population. Specifically, I compare the JWST NIRSpec spectra of red NTs to ultra-red NTs, in order to determine if these objects differ in their surface composition. I find that they do indeed have similar but unique surface reflectance properties, where the ultra-red object appears to be covered in Tholins, a reddening agent. This result supports the theory that the NT population evolved into two separate components.