Non-impact origin of crater-like features on top-shape near-Earth asteroids

Introduction:

To understand the long-term history and age of a planetary surface, it is important to analyze the distribution of impact craters, including the case of fast-spinning asteroids. Two of such asteroids have recently been visited by space probes such as the Hayabusa2 and the OSIRIS-REx spacecraft that targeted the C-type and B-type asteroids (162173) Ryugu and (101955) Bennu, respectively. These NEAs have been found to share similar physical and morphological characteristics, including low bulk density, surface characterized by large blocks and boulders, and top-shape profiles [1, 2]. Moreover, on the surface of both small bodies, several geological features can be identified in detail, such as crater-like features, boulders, linear features, fossae, etc [e.g., 1, 3]. Many crater-like depressions can be identified over the surface strongly skewed towards low equatorial latitudes. This uneven global distribution suggests that different evolutionary and impact history took place at the equatorial ridge with respect to the rest of the body.

Such bodies likely aggregated gravitationally right after the catastrophic collision that originated them [4]. In addition, collisions in the main belt and lately the YORP effect may have contributed to modify their spin rate. For instance, they have been found to be currently slowly spinning up (Bennu) and down (Ryugu) by YORP [5, 6]. The past spinning history of such bodies is unknown, however they may have had spin rates fast enough to cause the detachment of boulders from their surface eventually falling back onto them. Under such conditions, the formation of currently observed crater-like depressions may not be necessarily related to impact events and may rather be the outcome of repeated boulder landing and taking off cycles.

Here, we focus on checking the potential correlation between the formation of depressions and crater-like features with the dynamical behaviour of boulders on Ryugu and Bennu like-asteroids. 

Methodology:

We use the global mosaic of images available for Ryugu and Bennu asteroids created by using data collected from OCAMS and ONCs cameras suites overlapped to shape models [2, 7] to identify and map the characteristic features which we are interested in.

To check our hypothesis on the origin of crater-like features, we developed a two-stage model research, based on: (1) dynamical study of lifted particles due to fast spin, and (2) modelling the movement of big boulders using a N-body Discrete Element Method (PKDGRAV. [8,9]). In the first stage, we analyze the dynamics of particles that can detach from the surface of Ryugu and Bennu at a time they were spinning fast enough so that apparent centrifugal acceleration is sufficient to overcome local gravity. We therefore integrate the equation of motion of sample particles in a non-inertial rotating reference frame [10]. Sample particles -representing blocks and boulders- are initially placed at the center of the triangular facets in the shape model of each asteroid [2, 7]. Their dynamical evolution in time is followed and their trajectory and behaviour is tracked. Particle detachment from the surface at the critical spin state, and the landing latitude on the asteroid surface is also analyzed. In the second stage, we use PKDGRAV to place boulders on the asteroid surface and follow their dynamical evolution at an increasing spin rate.

Summary:

Our model of boulder motion on asteroids Bennu and Ryugu under fast spin conditions shows the formation of depressions and crater-like features due to boulder detaching, landing and/or bouncing off their surface is possible. Also, final landing of boulders at high latitudes is found. This model suggests a potential explanation for the crater abundance observed around the equatorial region in Ryugu and Bennu. In near future, the DART (NASA, 2022) and Hera (ESA, 2026) space missions are scheduled to approach the NEA binary system (65803) Didymos. The primary of such system is also a top-shape fast-spinning asteroid for which we may expect similar features as those studied in Bennu and Ryugu.

Acknowledgements:

LMP, NET and ACB acknowledge funding from the NEO-MAPP project (H2020-EU-2-1-6/870377). LMP acknowledges support from the Margarita Salas UCM postdoctoral grants funded by the Spanish Ministry of Universities with European Union funds - NextGenerationEU.

References:

[1] Lauretta, D. S., et al. (2019). Nature 568, 55–60. [2] Watanabe, S., et al. (2019). Science 364, 268–272. [3] Sugita, S., et al. (2019). Science, 364(6437), 252. [4] Campo Bagatin, A., et al. (2018). Icarus 302, 343–359. [5] Kanamaru, M., et al. (2021). JGR: Planets 126. [6] Hergenrother, C.W. et al. (2019). Nat. Commun. 10, 1291. [7] Barnouin, O.S. et al. (2019). Nature Geoscience 12, 247-252. [8] Richardson, D. C. (2000). Icarus, 143, 45. [9] Schwartz, S. (2012). Granular Matter 14, 263. [10] Trógolo N., et al., (2021). EPSC2021-676.