Asteroids IV

1. INTRODUCTION Small solar system bodies are conventionally labeled as either asteroids or comets, based on three distinct properties: (1) Observationally, small bodies with unbound atmospheres (“comae”) are known as comets, while objects lacking such atmospheres are called asteroids. (2) Dynamically, comets and asteroids are broadly distinguished by the use of a dynamical parameter, most commonly the Tisserand parameter measured with respect to Jupiter (Kresak, 1982; Kosai, 1992). It is defined by T a a e a a i J J J = + − ( )       2 1 2 1 2 / cos( ) (1) where a, e, and i are the semimajor axis, eccentricity, and inclination of the orbit (relative to Jupiter’s orbit), while aJ = 5.2 AU is the semimajor axis of the orbit of Jupiter. This parameter , which is conserved in the circular, restricted threebody problem, provides a measure of the close-approach speed to Jupiter. Jupiter itself has TJ = 3. Main-belt asteroids have a 3, while dynamical comets (from the Kuiper belt and Oort cloud) have TJ 3.08, and (3) show evidence for mass loss, e.g., in the form of a resolved coma or tail. In defining the critical Tisserand parameter separating asteroids from comets as TJ = 3.08, rather than 3.0, we avoid many ambiguous cases caused by deviations of the real solar system from the circular, restricted three-body approximation. This definition also excludes Encke-type comets (2P/Encke has TJ = 3.02), and the quasi-Hilda comets (TJ ~ 2.9–3.04). The orbital distribution of the currently known active asteroids is shown in Fig. 2. Three objects occupy planet-crossing orbits while the remaining 15 orbit in the main belt. 221 Jewitt D., Hsieh H., and Agarwal J. (2015) The active asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 221–241. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch012. The Active Asteroids David Jewitt University of California at Los Angeles Henry Hsieh Academia Sinica Jessica Agarwal Max Planck Institute for Solar System Research Some asteroids eject dust, producing transient, comet-like comae and tails; these are the active asteroids. The causes of activity in this newly identified population are many and varied. They include impact ejection and disruption, rotational instabilities, electrostatic repulsion, radiation pressure sweeping, dehydration stresses, and thermal fracture, in addition to the sublimation of asteroidal ice. These processes were either unsuspected or thought to lie beyond the realm of observation before the discovery of asteroid activity. Scientific interest in the active asteroids lies in their promise to open new avenues into the direct study of asteroid destruction, the production of interplanetary debris, the abundance of asteroid ice, and the origin of terrestrial planet volatiles. 222   Asteroids IV The active asteroids are remarkable for being an entirely new population located in one of the closest and most intensively studied regions of the solar system. Their activity is driven by a surprisingly diverse set of physical processes. Reported examples include hypervelocity impact [e.g., (596) Scheila], rotational instability probably driven by Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) torques (e.g., 311P, P/2013 R3), thermal disintegration driven by intense solar heating of minerals [(3200) Phaethon] and the sublimation of ice [(1) Ceres, 133P, 238P, 313P, and 324P are the strongest examples]. Impact and rotational disruptions are primary sources of meteorites and larger planet impactors . Observations promise to improve our understanding of the physics, and of the rates, of both asteroid destruction processes and meteoroid production. Analogous disruptions occurring around other stars are likely responsible for the creation of debris disks (e.g., Shannon and Wu, 2011). A common dynamical end-fate of planet-crossing bodies is to strike the Sun; a better understanding of the role of thermal disintegration will be important both in the solar system and in the context of photospheric impactors in polluted white dwarf systems (Jura and Xu, 2013). Finally, the survival of primordial ice in the asteroids may offer the opportunity to sample volatiles from a region of the protoplanetary disk different from that in which the Kuiper belt and Oort cloud comets formed. The outer asteroid belt is also a likely source region for the volatile inventory of Earth (Morbidelli et al., 2000; O’Brien and Greenberg, 2005), giving new relevance to the origin of the oceans. The active asteroids were called “main-belt comets” by Hsieh and Jewitt (2006), while Hsieh et al. (2012a) employed the term “disrupted asteroids” to refer to objects that exhibit comet-like activity believed to be non-sublimationdriven...