Resurfacing Asteroids & The Creation Rate of Asteroid Pairs

2019-01-17T00:38:30Z (GMT) by Kevin J. Graves

Many surface and dynamical processes affect the evolution of asteroids in our solar system today. The spectral slopes of S and Q-type asteroids are altered by the weathering of their surfaces due to solar wind interactions and micrometeorite impacts, as well as any processes that work to remove that weathered material. These processes of space weathering and asteroid resurfacing compete with each other to determine the spectral slope of each asteroid, with space weathering raising the spectral slope

and resurfacing lowering it. By considering the distribution of spectral slopes with respect to orbital location and size, we can determine which potential resurfacing processes are the most dominant. I show that the distribution of spectral slopes with respect to size is present in all populations of S and Q-type asteroids in the inner solar system, regardless of orbit. I also show that the spectral slopes of S and Q-type Near-Earth Asteroids (NEAs) decrease with decreasing perihelion, but only for perihelia q < 0.9 AU.

By building Monte Carlo and models N-body simulations of asteroids, I test which resurfacing mechanisms are consistent with these trends in spectral slopes. I find that spin-up and failure from the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect is an important resurfacing mechanism that creates the observed weathering trends with size. I also show that resurfacing asteroids due to close encounters with the terrestrial planets cannot explain the spectral slope vs. perihelion trend at q .

0.9 AU, but that resurfacing asteroids due to thermally induced surface degradation, by assuming a power law relationship between the resurfacing timescale and the solar distance, gives much more consistent results.

I also explore the creation rate of asteroid pairs, which are asteroids that have very similar orbits but are not gravitationally bound. The majority of pairs are formed by YORP spin-up and fission, followed by a separation of the two members. Asteroid pairs are then disassociated over time as their orbits become less similar due to chaos, resonances, and the Yarkovsky effect. I simulate both the formation of asteroid pairs in the inner main belt via YORP and their subsequent disassociation. By comparing the distribution of orbital similarity distances from observations and from our model, I estimate that asteroids fission and create an asteroid pair every 8 − 13 YORP cycles, where a YORP cycle is twice the time it takes the YORP effect to change the spin rate of an asteroid from zero to its critical spin rate. I argue that the rate of fissioning via the YORP effect is not substantially limited by any stagnation or stochastic evolution, and that losing mass via rotational fission is much less effective than collisional disruption, even for small asteroids.

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