Diverse evolutionary pathways of spheroidal asteroids driven by rotation rate

Area of Science:

• Planetary Science
• Asteroid Evolution
• Solar System Dynamics

Background:

• Asteroids provide insights into early solar system evolution and exhibit diverse shapes reflecting their history.
• Near-Earth asteroids (NEAs) like Bennu and Ryugu offer unique opportunities to study asteroid formation and evolution.
• Despite common characteristics, asteroids may follow distinct evolutionary pathways.

Purpose of the Study:

• To investigate the evolutionary pathways of two near-Earth asteroids, (101955) Bennu and (162173) Ryugu, despite their similar spheroidal shapes.
• To analyze the impact of rotation period on surface material distribution and gravitational fields of asteroids.
• To understand the diversity of asteroid evolution leading to observed shapes and structures.

Main Methods:

• Utilized high-resolution imagery from NASA's OSIRIS-REx and JAXA's Hayabusa2 missions.
• Mapped approximately 200,000 boulders on Bennu and Ryugu to analyze surface features.
• Compared particle size distribution and boulder spatial patterns in relation to asteroid rotation periods.

Main Results:

• Identified latitudinal particle size sorting on both Bennu and Ryugu, indicating surface material movement.
• Observed opposite directions of material transport: toward the equator on Bennu (4.3 hr rotation) and toward the poles on Ryugu (7.6 hr rotation).
• Inferred a past slower rotation for Bennu (>5 hr) based on large boulder distribution, suggesting an evolutionary path similar to Ryugu.

Conclusions:

• Asteroid rotation periods, even differing by a few hours, significantly alter gravitational fields and surface dynamics.
• Small variations in rotation can reverse local gravity, driving diverse asteroid evolutionary pathways.
• The study highlights that similar shapes do not imply identical evolutionary histories for asteroids like Bennu and Ryugu.