Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Abstract

Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.

Fig. 1. Reaccumulated aggregates can have a wide range of shapes.

For four different conditions that range in impact energy (represented by the impact energy relative to the calculated catastrophic disruption threshold, Q/Q*), we show the axial ratios of the reaccumulated remnants, represented by a single point in each plot. From a to d, the impact energy Q is increased from below Q* (a) to above Q* (b–d). The marginal distributions of the minor-to-major axis ratios (c/a) and the intermediate-to-major axis ratios (b/a) are also presented. The relative radius of each aggregate is represented by the radius of its data point. The axial ratios of Ryugu and Bennu are shown as blue and red points, respectively (Note: the sizes of these points do not reflect the relative size of Ryugu and Bennu to the aggregates formed in these simulations. The maximum resolution attained in our simulations results in aggregates that are larger by a factor of 4–8 than Ryugu and Bennu). The mean, $\overline{\mu }$, and standard deviation, $\overline{\sigma }$, of the axis ratios of all reaccumulated aggregates in each run are shown in the legends of the panels. For these impact conditions, we find that the higher the impact energy, the more spherical the average aggregate.

Fig. 2. Reaccumulated aggregates can resemble Bennu and Ryugu.

a–h Comparison of the Bennu (a, b) and Ryugu (e, f) shape models^, and the shape profiles of two reaccumulated aggregates (c, d, g, h) from simulation 3 (Table 1). The panels a, c, e, g show the profile of each shape from a polar view, and the panels (b, d, f, h) show the profiles from an equatorial view. The shapes of the reaccumulated remnants are traced in black and are compared to the traced shapes of Bennu (blue dashed line) and Ryugu (red dotted line). i The temperature change, ΔT (y-axis), the original depth in the parent body (x-axis), and the degree of compaction (color, unitless) of each particle that makes up the simulated aggregate shown in (c) and (d). j The same properties as in i are shown for the simulated aggregate shown in (g) and (h). i, j The particles that experience the most heating and compaction originate near the surface, close to the impact point. Those experiencing the least heating and compaction also originate near the surface, but closer to the antipode of the impact. Although the two reaccumulated remnants (shown in c, d, g, h) have very similar shapes, they apparently have different thermal histories. Comparing i and j, we find a clear difference in the peak and average temperature change and the degree of compaction experienced by the material that formed these two aggregates. Furthermore, the aggregate shown in g and h is composed of material that is sampled from a larger maximal depth within the parent body.

Fig. 3. A catastrophic disruption can be at the origin of thermal alteration.

a Stereoscopic pair at time t = 0 of the gravitational phase of the disruption of the parent body for simulation 3 (see Supplementary Movie 1). The peak temperature change for each particle from the impact is shown (see also Fig. 5) with colors scaling from 10 K (blue) to 1000 K (red). b Stereoscopic pair of the gravitational reaccumulation of the largest remnant (see Supplementary Movie 2) at t = 4.75 hours, exhibiting filamentary structure that will eventually collapse into a single aggregate. Other smaller filaments will escape and collapse to form smaller aggregates. This stereoscopic pairs in the Supplementary Movies can be viewed in ‘parallel’ stereo mode simply by relaxing the eye convergence. We suggest to view the pair of images from a foot or so away, and look through the screen to infinity, allowing the two images to float across each other. Where the two central pictures exactly overlap, the ‘fused’ 3-D image is to be found; all that is then necessary is to gently adjust the focus of the eyes, while the convergence remains relaxed, to obtain a clear stereoscopic image. This technique is called ‘Free Viewing’ of stereo pairs. For a more authentic stereo effect, use a Brewster format stereoscope — The London Stereoscopic Company OWL or similar. Detailed instructions may be found at LondonStereo.com.

Fig. 4. Multiple dynamical paths lead to the formation of oblate spheroids.

Snapshots of three cases of the gravitational reaccumulation of those smaller aggregates are in panels (a–c). Simulations snapshots at time steps 1 min, 0.75 h, 2 h, and 5 h after the collision, separated by red lines, are given for each dynamical path presented in panels (a–c). The first panel of each case shows the immediate distribution of the particle that reaccrete to form the final aggregate shown in the last panel of the same case. a Path 1 shows the ejection of a material stream that forms an elongated disk (blue arrows) with a dense core (red arrow). The disk of material then accretes onto the equator, spinning up the core through conservation of angular momentum and building up a ridge (green arrow). Paths 2 and 3 (b, c) show the formation of oblate spheroids through the collapse of multiple cores that subsequently coalesce. b For path 2, the aggregates have a high enough relative speed that the smaller bodies break up on impact and individual particles accrete onto the larger body (blue arrow) isotropically. c For path 3, a kind of nucleation occurs, where a large primary aggregate quickly forms out of the merger (blue arrow) and becomes the focus point of a gradual deposition of smaller aggregates that accrete isotropically, slowly building up an oblate spheroid (see Supplementary Movies 3–5 of these three paths).

Fig. 5. Super-catastrophic disruptions produce a large diversity of thermally altered rubble-piles.

From a to d, the impact energy Q is increased from below the impact energy threshold for disruption Q* (a) to above Q* (b–d). For each simulation, we show the peak temperature change of each particle (y-axis), its distance from the impact point (x-axis), and its postimpact compaction (color scale). Particles closer to the impact point are more heated and compacted, and particles that experience a change in temperature of $\gtrsim$200 K are almost fully compacted. The vertical black dashed lines highlight different fractions of escaping material that originates from within a given distance of the impact point. For increasingly higher impact energies, escaping material originates at further distances from the impact point (for example, c shows an impact where Q/Q* = 1.481, which results in 50% of the escaping material originating from >60 km away from the impact point). Three-dimensional visualizations of the peak temperature changes and compaction of material in the parent body are presented in Supplementary Figs. 3 and 4.

Fig. 6. A catastrophic impact thermally alters material that forms rubble-pile asteroids.

From a to d, the impact energy Q is increased from below the impact energy threshold for disruption Q* (a) to above Q* (b–d). For every reaccumulated aggregate in a simulation, we calculate the mean change in temperature by averaging the peak temperature changes of each component particle. We plot this value as a function of the minor-to-major axis ratio (c/a), which tracks the sphericity of each represented object. We find that the aggregates are able to experience mean temperature changes that likely altered the material from its original state within the parent body. The degree of alteration is independent of the final asteroid shapes, and different spheroidal or top-shaped asteroids can have varying degrees of thermal alteration driven by impact-induced heating (as shown for the two aggregates in Fig. 2). Unintuitively, rubble piles generated from less energetic impacts are more likely to be thermally altered, and super-catastrophic impacts produce a more diverse population of objects. Assuming an initial isothermal target with a temperature of 150 K, we highlight the change in temperature thresholds for ice sublimation (180 K, blue dashed line), ice melt (273 K, orange dashed line), CI peak heating (423 K, green dashed line), the thermal decomposition of free organic matter (520 K, red dashed line), and the dehydration of CM and CI chondrites (773 K, purple line).

Fig. 7. Microporosity can be a predictor of the material provenance within the parent body.

The mean compaction of the particles that make up an individual rubble-pile aggregate in our simulations is a measure of the microporosity of that aggregate. We show the mean compaction for all four cases described in Table 1. The color of each scatter point represents its associated simulation: purple, green, yellow, and red circles are aggregates from the Q/Q* = 0.64, 1.07, 1.48, and 1.98 cases, respectively. We find that the change in a rubble pile’s microporosity relative to that of the parent body material (y-axis) is an indicator of that material’s original location from within a parent body (x-axis, parameterized by the mean source region of all particles making up the rubble pile, ${\overline{R}}_{\mathrm{source}}$, normalized by the parent body radius, $R_{\mathrm{parent}}$), regardless of the energy of the family forming impact. This is illustrated by the black dashed line, which is a linear fit to data from all simulations, and has the form C = (1.68 ± 0.02) ${\overline{R}}_{\mathrm{source}}/R_{\mathrm{parent}}$.