Numerical Simulation of Regolith Production on 951 Gaspra
Abstract
In spite of extensive Earth-based observation and theoretical modeling, the physical state of asteroids is still not understood. The debate continues over whether asteroids have significant regolith, and whether they are solid bodies or "rubble piles". These questions are important to the issue of meteorite delivery, as they influence the nature of the material which can be removed from the surfaces of asteroids by collisions. We use a numerical hydrodynamic model of a projectile impacting a body, which allows for fracture of the body during impact, at scales smaller than the cell size (Melosh, Ryan, and Asphaug 1992) to explore the physical state of asteroids, using asteroid 951 Gaspra as a test case. Our model is a sphere with the volume of Gaspra, with a 6400 m radius. We modeled the asteroid both as solid basalt, and as almost completely fragmented basalt. We assumed an impact with relative velocity 5300 m/s (the RMS relative velocity) for other asteroids with 951 Gaspra. Table 1 shows the results of several different impacts. The projectiles chosen would create a 4 km crater (a) in a half-space under gravity-scaling (Melosh 1986), (b) in this spherical numerical model, and (c) in a half-space under strength-scaling (Vickery 1986). Because the target is spherical, neither of the half-space models predict the crater radius correctly. We find that in all cases, the "Transport Distance" is of order 100 m. This is the distance that unconsolidated surface regolith (not crater ejecta) will be moved by the "jolt" of the impact, assuming no cohesion. This transport will tend to obliterate small craters. The (crater) Ejecta Escaping and Regolith Deposited are also shown, both for our results, and for the scaling-law model of Housen, Schmidt, and Holsapple (1983). The scaling-law results are for a half-space: the fraction escaping would be larger for a sphere. The results are comparable, and suggest that for all but the largest impacts (which essentially destroy the target), regolith is added to the surface. This also works towards obliterating small craters. The largest impact modeled showed marginal catastrophic disruption: 20% of the target was accelerated to escape velocity (7.9 m/s). The interior of the body was also significantly fractured: the largest remaining piece had a size of 1 km. Thus this impact rubblized the approximately 80% of the body which did not escape. This modeling suggests that an impact large enough to remove (rather than generate) regolith rubblizes the entire body, in effect creating a body which is entirely regolith. These results suggest that asteroids should have substantial regolith, such that small cratering impacts would tend to eject multiply disrupted regolith material. The central pressure in Gaspra is 40 kPa (0.4 bar), so that regolith or a low strength rubble pile would be unlikely to anneal into a solid body with significant strength (more than a few kPa) again. This implies that the "texture" of meteorites generated by cratering should be distinct from those resulting from catastrophic disruption. References Melosh H. J., Ryan, E. V. and Asphaug, E. (1992) Submitted to J. Geophys. Res. Housen, K. R., Schmidt, and Holsapple, K. A. (1983) J. Geophys. Res. 88. 2485-2499. Melosh, H. J. (1986) Impact cratering: A geologic process. Oxford. New York. 245 Vickery, A. M. (1986) Icarus 67, 224-236. Table 1. (a) (b) (c) Crater Radius 2km 4km 6km Projectile Radius 76m 150m 284m Transport Distance 100m 500m >=7 km Ejecta Escaping (code/scaling law) 30%/15% 65%/32% 99%/68% Regolith Deposited (code/scaling) 24m/28m 40m/180m 5m/280m Final State Intact Mostly Intact Rubble
- Publication:
-
Meteoritics
- Pub Date:
- July 1992
- Bibcode:
- 1992Metic..27Q.270N