Mechanical and geological effects of impact cratering on Ida

Asteroids respond to impact stresses differently from either laboratory specimens or large planets. Gravity is typically so small that seismic disturbances of a few cm s^-1 can devastate unconsolidated topography. Yet the presence of regolith and the likelihood that many asteroids are gravitational assemblages tell us that gravity cannot generally be ignored. We use numerical models for impact fracture in solids to examine the initial stage of crater formation on asteroid 243 Ida, up to the cessation of fracture and the establishment of the cratering flow; at this stage we can infer final crater diameters but not profiles. We find that a modified strength scaling applies for craters up to a few 100 m in diameter forming in rock subject to Ida's gravity, and that gravity controls all craters larger than ∼1 km. "Bright annuli" around a number of intermediate craters may be the result of low-velocity surface disturbances, rather than bright proximal ejecta deposits. We also consider large impactors, to which Ida presents a curved, finite target surface with irregular gravity. These can excavate asymmetrical concavities. Stresses from large events can refocus and cause fracture far from the crater; using the shape of Ida as a basis for 3D hydrocode simulations, we show that impact genesis of the Vienna Regio concavity can cause fracture in Pola Regio, where grooves are observed in spacecraft images. Other simulations indicate that the formation of the ∼10 km crater Azzurra might have reopened these fractures, which may account for their fresh appearance. This mechanism of groove formation requires an interior which coherently transmits elastic stress. While this precludes a classic "rubble pile" asteroid, it does allow well-joined fault planes, and welded blocks or pores smaller than the stress pulse.