(596) Scheila in Outburst: A Probable Collision Event in the Main Asteroid Belt

Abstract

Images of asteroid (596) Scheila have been acquired at various dates after the discovery of the 2010 outburst. Assuming a short-duration event scenario, as suggested by the quick vanishing of the dust tail brightness with time, and integrating numerically the equation of motion of individual particles ejected from the surface, we have developed a tail model from which we estimate the parameters associated to the geometry of the ejection, the size distribution, and the velocity distribution of the ejected particles, as well as the total mass ejected. We found a weak inverse power-law dependence of ejection velocity versus particle radius, with velocities ranging from 50 to 80 m s −1 for particle radii in the range 5 cm to 8×10 −5 cm, respectively. These velocities are very different from those expected from ice sublimation at the asteroid heliocentric distance (∼3 AU), and suggest a collision scenario as a likely cause of the outburst. We found that the ejected particles are distributed in size following a power law of index -3, and, based on the ejecta mass and scaling laws, the impactor size is estimated at 30-90 m in radius, assuming an impact velocity of ∼5 km s −1 , and the same density (1500 kg m −3 ) for the asteroid as for the projectile. We have inferred an asymmetry in the ejecta along the axis normal to the asteroid orbit plane, a likely indicator of an oblique impact. The impact is estimated to have occurred on November 27th, with an accuracy not better than ±3 days.

The Active Asteroids

“…The brightening of Scheila was sudden, the scattering cross section declined smoothly with time, and the morphology evolved in a way consistent with the expansion of an impulsively ejected coma under the action of radiation pressure (Bodewits et al 2011;Jewitt et al 2011b;Moreno et al 2011b;Hsieh et al 2012;Ishiguro et al 2011). All these signatures are consistent with impact production but are difficult or impossible to reconcile with the other mechanisms of Section 3.…”

“…(596) Scheila, a 113 km diameter object with red geometric albedo ∼0.04 (Table 2), developed a coma in late 2010. Over the course of a month, this coma expanded with a characteristic speed ∼60 m s −1 and faded in response to the action of solar radiation pressure (Bodewits et al 2011;Jewitt et al 2011b;Moreno et al 2011b), apparently without any continued replenishment of particles from the nucleus. The gas production from the nucleus was reportedly limited to Q OH 10 25 s −1 (Moreno et al 2011b) to Q OH 10 26 s −1 (Howell & Lovell 2011), corresponding to water production rates 0.3-3 kg s −1 (the meaning of these limits is unclear given the non-steady nature of the mass-loss event from Scheila).…”

“…Over the course of a month, this coma expanded with a characteristic speed ∼60 m s −1 and faded in response to the action of solar radiation pressure (Bodewits et al 2011;Jewitt et al 2011b;Moreno et al 2011b), apparently without any continued replenishment of particles from the nucleus. The gas production from the nucleus was reportedly limited to Q OH 10 25 s −1 (Moreno et al 2011b) to Q OH 10 26 s −1 (Howell & Lovell 2011), corresponding to water production rates 0.3-3 kg s −1 (the meaning of these limits is unclear given the non-steady nature of the mass-loss event from Scheila). The mass of dust in micron-sized grains was 4 × 10 7 kg ), while more model-dependent attempts to account for mass in larger particles gave 6 × 10 8 kg (Bodewits et al 2011) to 2 × 10 10 kg (Moreno et al 2011b).…”

“…The gas production from the nucleus was reportedly limited to Q OH 10 25 s −1 (Moreno et al 2011b) to Q OH 10 26 s −1 (Howell & Lovell 2011), corresponding to water production rates 0.3-3 kg s −1 (the meaning of these limits is unclear given the non-steady nature of the mass-loss event from Scheila). The mass of dust in micron-sized grains was 4 × 10 7 kg ), while more model-dependent attempts to account for mass in larger particles gave 6 × 10 8 kg (Bodewits et al 2011) to 2 × 10 10 kg (Moreno et al 2011b). No ice was observed in the coma (Yang & Hsieh 2011 (Hsieh et al 2011a), little is yet known about this object.…”

The Main Belt Comets and ice in the Solar System

“…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).…”