Introduction to the Special Issue: Exploration of the Activity of Asteroid (101955) Bennu

Abstract

Near-Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss. The activity manifests itself in the form of ejection events emitting up to hundreds of millimeter- to centimeter-scale particles. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer spacecraft monitored particle activity for a 10-month period that included Bennu's perihelion and aphelion. Novel and classical methods were utilized to detect the particles and characterize their orbital and physical properties. Roughly 30% of the observed particle mass escaped to heliocentric orbit. A majority of particles fell back onto the surface of Bennu after ejection, with the longest-lived particle surviving for 6 days on a temporary orbit. Particle ejection events appear to preferentially take place in the afternoon and evening and from low latitudes, although they can occur at any time or latitude. The reaccumulation of material is biased toward low latitudes resulting in the possible in-fill of craters and growth of Bennu's equatorial bulge. Of the potential mechanisms behind this activity that were investigated in focused studies, meteoroid impacts, thermal fracturing, and ricochet-but not water ice sublimation-were found to be consistent with observations. While phyllosilicate dehydration was not investigated with a focused study, it remains a possible mechanism. These mechanisms are not unique to Bennu, suggesting that many near-Earth asteroids may exhibit activity that has gone undetected thus far. Spacecraft missions with wide-field imagers are encouraged to further characterize this phenomenon.

Figure 1

The large particle ejection event seen on 6 January 2019 at 20:56 UTC. (top) In‐field stray light was removed with a variant of unsharp masking (Hergenrother et al., 2020; Samarasinha & Larson, 2014). The saturated disk of Bennu is the smooth feature in the lower right corner. The ejected particles are the majority of the star‐like objects just above the disk of Bennu as well as all trailed objects. The Earth and Moon are the bright double “stars” near the upper left side of the image. (bottom) A hybrid image combining the same 5‐s exposure shown in the top panel to show the particles but without unsharp masking and a short 0.0014‐s exposure to display the nonsaturated surface of Bennu. The two images have start times separated by ~7 s.

Figure 2

Occurrence of particle ejection events (circles) in the orbit of Bennu (orange). Ejection events of 2 to 19 particles are denoted by small gray circles; events of 20 or more particles are denoted by large black circles. The phases of the OSIRIS‐REx mission that included dedicated particle monitoring (Orbitals A–C) are indicated with blue hatches. The Sun (yellow) and the orbits of Mercury (green), Venus (light blue), Earth (dark blue), and Mars (red) are also shown for reference. Figure is from Hergenrother et al. (2020).

Figure 3

The near‐environment of Bennu shown shortly after the ejection event of 28 August 2019. Particle positions are shown as white dots and labeled with identifiers from Chesley et al. (2020) and Hergenrother et al. (2020). Particle trajectories extend back 18 hr or until the moment of ejection, which ever time is shorter.

Figure 4

Observed translational energy versus local solar time. Local solar time is shown in hours with 0/24 hr corresponding to local midnight and 12 hr to local noon. (top) Local solar time versus Bennu latitude. The area of the circles corresponds to the observed translational energy. (bottom) Observed translational energy versus local solar time in half‐hour bins.

Figure 5

The meteoroid impact flux across the surface of Bennu at different points in its orbit. The scale bars decrease by an order of magnitude between the plot at perihelion and those further from the Sun. Bottke et al. (2020) found that the flux is greatest at perihelion. Impacts skew to the evening on the outbound part of the orbit and to the early afternoon on the inbound part. A preference for ejection from the afternoon and evening side of Bennu matches the findings of Chesley et al. (2020) shown in Figure 4. This figure is from Bottke et al. (2020).

Figure 6

Comparison of the theoretical and observed distribution of particle volume‐equivalent spherical diameters. The theoretical size distribution based on the thermal fracturing model of Molaro, Hergenrother, et al. (2020) is depicted in red. The observed distribution is by Chesley et al. (2020). Figure is from Chesley et al. (2020).