The Active Asteroids

Abstract

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.

The Main Belt Comets and ice in the Solar System

“…We use T J = 3.05 instead of T J = 3.00 because small drifts in T J beyond 3.0 are possible due to non-gravitational forces or terrestrial planet interactions (e.g., Levison et al 2006;Hsieh and Haghighipour 2016). Other authors (e.g., Jewitt et al 2015c) have used T J = 3.08 as the boundary for similar reasons; the resulting list of MBC candidates is the same. For the purposes of this paper, we use the term MBC to refer to an active asteroid that exhibits activity determined to likely be due to sublimation (e.g., from dust modelling results, or confirmation of recurrent activity near perihelion with intervening periods of inactivity).…”

“…These include debris released by impacts (either cratering events or catastrophic disruption)-e.g., (596) Scheila Jewitt et al 2011;Ishiguro et al 2011a, b;Moreno et al 2011b) and P/2012 F5 (Gibbs) (Stevenson et al 2012;Moreno et al 2012)-and rotational disruption-e.g., 311P (Jewitt et al 2015b)-thought to be an outcome of YORP spin-up for small asteroids (Scheeres 2015). There are other hypothesised effects that have yet to be conclusively demonstrated to explain observed 'activity', such as thermal cracking, electrostatic levitation of dust, or radiation pressure accelerating dust away from the surface (Jewitt et al 2015c). We also exclude dynamically asteroidal objects (i.e., with T J > 3.05) outside of the asteroid belt for which activity has been detected, e.g., (3200) Phaethon (Jewitt and Li 2010;Li and Jewitt 2013;Hui and Li 2017) and 107P/(4015) Wilson-Harrington (Fernandez et al 1997).…”

“…We summarise discovery circumstances of the currently known MBCs in Table 1. The discovery rate has increased Jewitt et al (2015c): P/2015 X6 (PANSTARRS), and P/2016 J1 (PANSTARRS), both discovered by the Pan-STARRS survey at Haleakala, Hawaii. P/2016 J1 is particularly interesting as it was observed to split into two pieces Moreno et al 2017), and its current activity perhaps was triggered by a small impact, whereby formerly buried ice started sublimating and the torque rapidly drove the parent to fragment .…”

“…We also exclude dynamically asteroidal objects (i.e., with T J > 3.05) outside of the asteroid belt for which activity has been detected, e.g., (3200) Phaethon (Jewitt and Li 2010;Li and Jewitt 2013;Hui and Li 2017) and 107P/(4015) Wilson-Harrington (Fernandez et al 1997). See Jewitt et al (2015c) and references therein for more extensive discussion of active asteroids beyond our definition of 'MBC'. MBCs are of special interest within the broader class of active asteroids as they indicate the possible presence of water in bodies of a size that are very common in the asteroid belt, implying that there is potentially a large population of icy bodies there.…”

“…Previous reviews on the subject of MBCs cover the more general topic of 'active asteroids', including considering how non-icy bodies can eject dust and therefore exhibit comet-like appearances (Bertini 2011;Jewitt et al 2015c). We briefly discuss this below, but concentrate on questions related to MBCs and ice in the Solar System.…”