Erosion and Ejecta Reaccretion on 243 Ida and Its Moon

Using a realistic model for the shape of Ida (P. Thomas, J. Veverka, B. Carcich, M. J. S. Belton, R. Sullivan, and M. Galileo images of Asteroid 243 Ida and its satellite Dactyl Davies 1996, Icarus 120, 000–000), we find that an extensive show surfaces which are dominantly shaped by impact cratercolor/albedo unit which dominates the northern and western ing. A number of observations suggest that ejecta from hemispheres of the asteroid can be explained as the result hypervelocity impacts on Ida can be distributed far and wide of reaccretion of impact ejecta from the large and evidently across the Ida system, following trajectories substantially recent crater ‘‘Azzurra.’’ Initial ejection speeds required to affected by the low gravity, nonspherical shape, and rapid match the color observations are on the order of a few meters rotation of the asteroid. We explore the processes of reaccreper second, consistent with models (e.g., M. C. Nolan, E. tion and escape of ejecta on Ida and Dactyl using threeAsphaug, H. J. Melosh, and R. Greenberg 1996, Icarus, dimensional numerical simulations which allow us to compare submitted; E. Asphaug, J. Moore, D. Morrison, W. Benz, the theoretical effects of orbital dynamics with observations and R. Sullivan 1996, Icarus 120, 158–184) that multikilomeof surface morphology. ter craters on Ida form in the gravity-dominated regime and The effects of rotation, launch location, and initial launch are net producers of locally retained regolith. Azzurra ejecta speed are first examined for the case of an ideal triaxial launched in the direction of rotation at speeds near 10 m/ ellipsoid with Ida’s approximate shape and density. Ejecta sec are lofted over the asteroid and swept up onto the launched at low speeds (V ! Vesc) reimpact near the source rotational leading surface on the opposite side. The landing craters, forming well-defined ejecta blankets which are asymlocations of these particles closely match the distribution metric in morphology between leading and trailing rotational of large ejecta blocks observed in high resolution images surfaces. The net effect of cratering at low ejecta launch of Ida (P. Lee, J. Veverka, P. Thomas, P. Helfstein, velocities is to produce a thick regolith which is evenly M. J. S. Belton, C. Chapman, R. Greeley, R. Pappalardo, distributed across the surface of the asteroid. In contrast, no R. Sullivan, and J. W. Head 1996, Icarus 120, 87–105). clearly defined ejecta blankets are formed when ejecta is Ida’s shape and rotation allow escape of ejecta launched at launched at higher initial velocities (V p Vesc). Most of the speeds far below the escape velocity of a nonrotating sphere ejecta escapes, while that which is retained is preferentially of Ida’s volume and presumed density. While little ejecta from derived from the rotational trailing surfaces. These particles Ida is captured by Dactyl, about half of the mass ejected from spend a significant time in temporary orbit around the Dactyl at speeds of up to 20 m/sec eventually falls on Ida. asteroid, in comparison to the asteroid’s rotation period, and Particles launched at speeds just barely exceeding Dactyl’s tend to be swept up onto rotational leading surfaces upon escape velocity can enter relatively long-term orbit around Ida, reimpact. The net effect of impact cratering with high ejecta but few are ultimately reaccreted by the satellite. Because of launch velocities is to produce a thinner and less uniform its low gravity, erosion of Dactyl would take place on exceedsoil cover, with concentrations on the asteroids’ rotational ingly short time scales if unconsolidated materials compose the leading surfaces. satellite and crater formation is in the gravity regime. If Dactyl is a solid rock, then its shape has evolved from a presumably 1 Permanent address: Observatoire de Nice, B.P. 229 06304, Nice Cedex irregular initial fragment to its present remarkably rounded 4, France. figure by collision with a population of impactors too small to 2 Permanent address: Arecibo Observatory, P.O. Box 995, Arecibo, Puerto Rico 00613. be detected by counting visible craters. As the smallest solar