An Observational Study of Small Bodies Evolving in the Inner Solar System

Abstract

We present observational studies of small Solar System bodies, which have been evolving in the inner Solar System (inside the orbit of Jupiter) via solar radiative heating, mutual impact, and other processes such as space weathering and rotational spin-up. The main objective of this PhD dissertation is to figure out the two primary evolutionary processes by means of observations, which consist of "solar radiative heating" and "impacts", which have modified the primordial distribution of small bodies to create the present ones. Accordingly, the thesis consists of the following two major research.<br/> First, we study the evolution of icy small bodies in the inner Solar System, and investigate how these icy bodies trace evolution under the solar radiation field. They had been stored in the outer Solar System (beyond the orbit of Neptune) until recently (within <1-10 million years), and injected into the inner Solar System to be observed as 'comets'. It is expected that some comets should have been observed as asteroids once they lose icy volatiles by solar heat (so-called, dormant comets). However, the population of dormant comets was not examined substantially because the appearances are quite similar to asteroids. We made use of the brand-new infrared asteroid catalogs (including their sizes and albedos) to extract dormant comet population masquerading as asteroids on the basis of their orbital elements and albedos. As a result, we identify ~100 potential dormant comets. With the largest samples of potential dormant comets, we find that they exhibit a size distribution shallower than that of ordinary active comet nuclei. From this result, we conjecture that larger cometary nuclei tend to accumulate inert dust mantles which could insulate cometary volatiles beneath their surfaces.<br/> In addition, we also find that some small (<3 km) asteroids in comet-like orbits are originated from the asteroid belt rather than the outer Solar System via thermal perturbation force (known as the Yarkovsky effect).<br/> The second subject is the impact. We examine a possible impact phenomenon through observations. Such impact phenomena have been investigated through past records (e.g., craters and asteroidal families). From the previous research, it is suggested that there are two major types of impact phenomena known as 'cratering' and 'catastrophic disruption'. However, there was only a witness report regarding the cratering (one type of impacts) occurred on asteroid (596) Scheila in 2010 before this thesis work. We investigate an unusual dust ejection from asteroid P/2010 A2 (LINEAR) in the innermost asteroid belt, by performing a dynamical analysis of dust grains under the solar gravitational and radiation fields. After a comprehensive parameter search for the model, we succeed in the reproduction of any observational images taken after the discovery. We find that the spatial distribution of the remnant dust cloud is best explained by our best-fit model that implies a catastrophic disruption as the result of an asteroid-asteroid impact. We estimate the specific disruption energy of 350 J/kg and conjecture that the parent asteroid could have a porous rubble-pile structure like asteroid (25143) Itokawa. In addition, we made a new observation with Gemini/GMOS-N in 2017 and detected 11 fragments. We derive the rotational period of the largest fragment. The new observational results also support the idea of the catastrophic disruption. For these reasons, we insist that the P/2010 A2 event could be the first event of the catastrophic disruption that we human beings witnessed with modern astronomical instruments.