The Collisional Evolution of the Primordial Kuiper Belt, Its Destabilized Population, and the Trojan Asteroids

Abstract

The tumultuous early era of outer solar system evolution culminated when Neptune migrated across the primordial Kuiper Belt (PKB) and triggered a dynamical instability among the giant planets. This event led to the ejection of ∼99.9% of the PKB (here called the destabilized population), heavy bombardment of the giant planet satellites, and the capture of Jupiter’s Trojans. While this scenario has been widely tested using dynamical models, there have been fewer investigations into how the PKB, its destabilized population, and the Trojans experienced collisional evolution. Here we examined this issue for all three populations with the code Boulder. Our constraints included the size–frequency distributions (SFDs) of the Trojan asteroids and craters on the giant planet satellites. Using this combination, we solved for the unknown disruption law affecting bodies in these populations. The weakest ones, from an impact energy per mass perspective, were diameter D ∼ 20 m. Overall, collisional evolution produces a power-law-like shape for multikilometer Trojans and a wavy-shaped SFD in the PKB and destabilized populations. The latter can explain (i) the shapes of the ancient and younger crater SFDs observed on the giant planet satellites, (ii) the shapes of the Jupiter family and long-period comet SFDs, which experienced different degrees of collision evolution, and (iii) the present-day impact frequency of superbolides on Jupiter and smaller projectiles on Saturn’s rings. Our model results also indicate that many observed comets, the majority which are D < 10 km, are likely to be gravitational aggregates formed by large-scale collision events.

The Properties and Origin of Kuiper Belt Object Arrokoth's Large Mounds

“…Conversely, however, the lack of apparent mound subunits on the surfaces of individual cometary nuclei explored by spacecraft may be a result of differences in formation location and evolutionary history, particularly as all comets observed by spacecraft have undergone rapid, thermally driven, morphological evolution due to entering the inner solar system numerous times. Furthermore, some models predict that most observed comets are collisional fragments or gravitational aggregates formed in collisional events (Bottke et al 2023), processes minimized among the CCKBOs; we therefore discount this latter possibility for Arrokoth. We do, however, recognize that searches for and the study of features that may be formation subunits on cometary nuclei imaged by spacecraft would be of interest, though this is beyond the scope of this Arrokoth paper.…”

Orbital and Absolute Magnitude Distribution of Jupiter Trojans

“…There are four independent parameters in this model, three of which (H b , α 1 , α 2 ) characterize the shape of the magnitude distribution, and H 0 sets the absolute normalization (see Figures 15 and 16). Two more parameters are to be added when a second break at a few kilometers in size has been identified beyond magnitude H b2 ; 15 (Wong & Brown 2015); for H > H b2 the magnitude distribution becomes even shallower (likely because the population has experienced 4.5 Gyr of collisional grinding, e.g., Marschall et al 2022;Bottke et al 2023). Further studies obtained slightly different values, shifting the second break point down to and 6).…”

The Bombardment History of the Giant Planet Satellites

“…A substantial source of bombardment for outer solar system worlds in the post-accretion era is the primordial Kuiper Belt (PKB), a ∼30 Earth mass disk of ice-rock planetesimals located primarily between ∼24 and ∼30 au (e.g., Nesvorný et al 2017;Bottke et al 2023). Dynamical models favor the idea that the giant planets originated on different orbits than we see today, with all of them initially residing within mutual mean motion resonances (MMRs) between ∼5 and ∼17 au (Tsiganis et al 2005;Nesvorný & Morbidelli 2012).…”

“…The timing of this event is constrained by collisional evolution modeling of the PKB and Jupiter Trojans (e.g., Nesvorný et al 2018;Bottke et al 2023). The havoc wreaked by Neptune's passage across the PKB caused numerous Kuiper Belt objects (KBOs) to be ejected onto giant planet-crossing orbits, creating what Bottke et al (2023) called the destabilized population. They argued this population is the probable source of most early impacts on the giant planet satellites.…”

“…Here, we propose to overcome these issues by building on the work of Bottke et al (2023), who modeled the collisional and dynamical evolution of the PKB, destabilized population, and Jupiter Trojans. Their solutions for items #1-#4 will be discussed below.…”