Mercury Impact Ejecta’s Contribution to its Own Meteoroid Population
Type of DegreeMaster's Thesis
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Mercury’s position in the inner Solar System and lack of an atmosphere subjects it to the greatest mean velocity impacts of the inner Solar System. Ejecta launched at speeds less than 4.25 km/s are retained by the planet and contribute to Mercury’s dust environment, secondary crater population, or regolith gardening. However, simple-crater forming impact events can accelerate regolith to velocities high enough to escape Mercury’s Hill sphere, some of which may eventually come back to Mercury. It is not well understood how much of this ejected material returns to Mercury. This thesis seeks to understand how much of Mercury’s meteoroid population is composed of material originating from itself. A host of impacts into Mercury’s surface are simulated using the ejecta-scaling relationships of Housen & Holsapple (2011) and Richardson et al. (2007). Properties of launched particles are such as ejection speed and direction are quantified so their trajectories can be calculated. Ejecta that escapes Mercury have their orbital trajectories propagated by an N-body code for thousands of years under the influence of gravity, solar radiation pressure, and Poynting-Robertson drag. They are monitored for entering a planet’s Hill sphere or approaching too close to the Sun. The likelihood of a particle returning to Mercury is dependent on its size (or rather, its value of β, the ratio of radiation force to solar gravity). Only 22% of 10 µm radius particles come back because of their susceptibility to spiral into the Sun due to the decelerating effects of Poynting-Robertson drag. On the other hand, grains with radii of 50, 100, 500, and 1000 µm are not as readily perturbed by non-gravitational forces. The percentages of these particles that return to Mercury are approximately 50.5%, 68.7%, 85.7%, and 88.6%, respectively. Consequently, it is estimated that 39% of particles and 87% of mass that escapes Mercury via an impact event will return within 10,000 years. Grains with radii on the order of 10s of micron, by number, contribute about 80% of returning particles but only 0.2% of returned mass. Alternatively, particles with radii greater than a few hundreds of micron compose less than 20% of returning particles, but over 99.5% of the returning mass. These results imply that Mercury’s meteoroid population is almost certainly is composed partially of material originating from itself and that unhindered grains will re-impact Mercury’s surface and further contribute to surface processes such as boulder decimation, regolith breakup and gardening, melt production, volatile vaporization and topographic diffusion.