Dynamics of granular material on small bodies

The European Space Agency’s ROSETTA spacecraft was launched in 2004 and will rendezvous with comet 67P/Churyumov-Gerasimenko in August 2014. On its route towards the comet, it flew by asteroid (21) Lutetia on 10 July 2010. The surface of (21) Lutetia is covered with a thick layer of regolith. On slopes of several craters this regolith layer collapsed in landslide-like events. A possible trigger mechanism for these low-gravity avalanches is the slow impact of a small mm to cm-sized body. An experimental investigation into this question was conducted where a small mm-sized metal sphere was shot into sloped granular material at velocities of up to 2 m/s. The impact and any event triggered by the impact were recorded using a high-speed high-resolution camera. Subsequent analysis of the data shows that this type of impact can, under certain conditions, trigger a landslide. To gain a better understanding of the microphysical processes governing the distribution of energy in low-energy impacts, a suite of simulations was performed, using a Discrete Element Method (DEM) software called ESyS-Particle. The experiment described above was recreated within the program and the results reinforce the conclusion that low-energy impacts are a viable trigger mechanism for avalanches, both in low and normal gravity.