Probing Icy Planetesimals in the Inner Solar System

In addition to the major planets, our solar system is home to millions of smaller bodies. Most of these fall into one of two categories: comets or asteroids. Comets are commonly recognized by their distinctive fuzzy appearances and long, spectacular tails. In contrast, asteroids simply appear as star-like points of light. The physical reason for this observational dichotomy lies in the disparate origins of comets and asteroids. Comets originate in the cold outer solar system (beyond the orbit of Neptune) and thus contain much more ice than asteroids, most of which formed much closer to the Sun (and thus at much warmer temperatures) in the main asteroid belt between Mars and Jupiter. As comets have large, elongated orbits, they experience wide temperature variations. When a comet approaches the Sun, its ice heats up and changes into gas (i.e., sublimates), venting gas and dust into space, giving rise to a tail and a distinctive fuzzy appearance. Away from the Sun, sublimation decreases significantly, and any remaining ice stays frozen until the comet's next close approach to the Sun. In contrast, objects in the main asteroid belt have essentially circular orbits and due to their confinement to the inner solar system, are expected to be baked dry of ice. Recent research, however, has shown that our classical understanding of comets and asteroids is in need of considerable refinement. During the course of my doctoral research, I identified a new class of comets (the main-belt comets, or MBCs) that significantly blurs the conventional distinctions between comets and asteroids. The MBCs are unmistakably cometary in appearance, but are found in the main asteroid belt on circular orbits, which make them orbitally very different from 'normal' comets. Located slightly farther from the Sun, but still within the main asteroid belt, the Hilda asteroids are another population of objects that have been suspected of having cometary properties, though no Hilda asteroid has yet been observed displaying cometary activity. Given the recent discovery of ice on the MBCs, however, the possibility that the more distant Hildas may be icy as well seems more likely, thereby warranting a renewed investigation of these mysterious distant bodies. As an STFC Postdoctoral Fellow at QUB, I will investigate objects that exhibit properties of both asteroids and comets, focusing in particular on MBCs and the Hilda asteroids, and employing both telescope observations and computer simulations. The observational component of my research programme will focus on determining various physical properties of these poorly understood objects (e.g., colours, how dark their surfaces are, and how fast they are spinning), which will enable us to infer their chemical and physical composition. The computational component of my research programme will focus on clarifying the origin of these objects by simulating the ways their orbits may have changed over time due to influences such as the gravity of the major planets and collisions with other asteroids. Combined, these twin lines of inquiry will result in a greatly improved understanding of the murky boundary between asteroids and comets. As leftover debris from the planet formation process, asteroids and comets provide vital information necessary for developing a comprehensive understanding of the early solar system. In addition, our discovery of ice in the MBCs bolsters recent hypotheses that objects from the main asteroid belt played a significant role in the primordial delivery of water to the Earth. Thus, our investigation of the MBCs and other icy objects in the inner solar system also has significant implications for our understanding of the origins of life itself.