Galileo images of asteroid 243 Ida and its satellite Dactyl show surfaces
which are dominantly shaped by impact cratering. A number of observations
suggest that ejecta from hypervelocity impacts on Ida can be distributed far
and wide across the Ida system, following trajectories substantially affected
by the low gravity, nonspherical shape and rapid rotation of the asteroid. We
explore the processes of reaccretion and escape of ejecta on Ida and Dactyl
using 3-dimensional numerical simulations which allow us to compare the
theoretical effects of orbital dynamics with observations of surface
morphology.
The effects of rotation, launch location and initial launch speed are first
examined for the case of an ideal triaxial ellipsoid with Ida's approximate
shape and density. Ejecta launched at low speeds (V << Vesc) reimpact near the
source craters, forming well-defined ejecta blankets which are asymmetric in
morphology between leading and trailing rotational surfaces. The net effect of
cratering at low ejecta launch velocities is to produce a thick regolith which
is evenly distributed across the surface of the asteroid. In contrast, no
clearly defined ejecta blankets are formed when ejecta is launched at higher
initial velocities (V ~ Vesc). Most of the ejecta escapes, while that which is
retained is preferentially derived from the rotational trailing surfaces.
These particles spend a significant time in temporary orbit around the
asteroid, in comparison to the asteroid's rotation period, and tend to be
swept up onto rotational leading surfaces upon reimpact. The net effect of
impact cratering with high ejecta launch velocities is to produce a thinner
and less uniform soil cover, with concentrations on the asteroids' rotational
leading surfaces.
Using a realistic model for the shape of Ida (Thomas et al., this issue), we
find that an extensive color/albedo unit which dominates the northern and
western hemispheres of the asteroid can be explained as the result of
reaccretion of impact ejecta from the large and evidently recent crater
"Azzurra". Initial ejection speeds required to match the color observations
are on the order of a few m/s, consistent with models (e.g., Nolan et al.,
1995; Asphaug et al. this issue) that multikilometer craters on Ida form in
the gravity-dominated regime and are net producers of locally retained
regolith. Azzurra ejecta launched in the direction of rotation at speeds near
10 m/s are lofted over the asteroid and swept up onto the rotational leading
surface on the opposite side.The landing locations of these particles closely
match the distribution of large ejecta blocks observed in high resolution
images of Ida (Lee et al., this issue).
Ida's shape and rotation allow escape of ejecta launched at speeds far below
the escape velocity of a nonrotating sphere of Ida's volume and presumed
density. While little ejecta from Ida is captured by Dactyl, about half of the
mass ejected from Dactyl at speeds of up to 20 m/s eventually falls on Ida.
Particles launched at speeds just barely exceeding Dactyl's escape velocity
can enter relatively long term orbit around Ida, but few are ultimately
reaccreted by the satellite. Because of its low gravity, erosion of Dactyl
would take place on exceedingly short timescales if unconsolidated materials
comprise the satellite and crater formation is in the gravity regime. If
Dactyl is a solid rock, then its shape has evolved from a presumably irregular
initial fragment to its present remarkably rounded figure by collision with a
population of impactors too small to be detected by counting visible craters.
As the smallest solar-system object yet imaged by a spacecraft, the morphology
of Dactyl is an important clue to the asteroid population at the smallest
sizes.