Fragmenting Active Asteroid 331P/Gibbs

TL;DR

This study analyzes the active asteroid 331P/Gibbs using Hubble data, revealing its fragmentation, binary nature, and debris ejection, providing insights into its formation, rotation, and evolution in the main asteroid belt.

Contribution

The paper presents detailed observations of 331P/Gibbs' fragments, rotation, and debris, offering new evidence for its impact-shattering origin and rotational disruption mechanisms.

Findings

Identified 19 fragments with radii 0.04-0.11 km

Primary nucleus has radius 0.8 km and rapid 3.26-hour rotation

Debris ejected at ~10 cm/s following a specific size distribution

Abstract

We describe active asteroid 331P/Gibbs (2012 F5) using archival Hubble Space Telescope data taken between 2015 and 2018. 331P is an outer main-belt active asteroid with a long-lived debris trail that formed in 2011. Embedded in the debris trail we identify 19 fragments with radii between 0.04 and 0.11 km (albedo 0.05 assumed) containing about 1 percent of the mass of the primary nucleus. The largest shows a photometric range (1.5 magnitudes), a V-shaped minimum and a two-peaked lightcurve period near 9 hours, consistent with a symmetric contact binary (Drahus and Waniak 2016). Less convincing explanations are that 331P-A is a monolithic, elongated splinter or that its surface shows hemispheric 4:1 albedo variations. The debris trail is composed of centimeter sized and larger particles ejected with characteristic 10 cm s$^{-1}$ speeds following a size distribution with index q = 3.7+/-0.1 to 4.1+/-0.2. The HST data show that earlier, ground-based measurements of the nucleus were contaminated by near-nucleus debris, which cleared by 2015. We find that the primary nucleus has effective radius 0.8+/-0.1 km and is in rapid rotation (3.26+/-0.01 hour, range 0.25 magnitudes, minimum density 1600 kg/m3 if strengthless. The properties of 331P are consistent with a) formation about 1.5 Myr ago by impact shattering of a precursor body (Novakovic et al. 2014) b) spin-up by radiation torques to critical rotation c) ejection of about 1 percent of the nucleus mass in mid-2011 by rotational instability and d) subsequent evolution of the fragments and dispersal of the debris by radiation pressure.