may account for their fresh appearance. This mechanism of groove formation requires an interior which coherently trans-Asteroids respond to impact stresses differently from either mits elastic stress. While this precludes a classic ''rubble laboratory specimens or large planets. Gravity is typically pile'' asteroid, it does allow well-joined fault planes, and so small that seismic disturbances of a few cm s Ψ1 can welded blocks or pores smaller than the stress pulse. Β© 1996 devastate unconsolidated topography. Yet the presence of Academic Press, Inc.
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
I. INTRODUCTION solids to examine the initial stage of crater formation on asteroid 243 Ida, up to the cessation of fracture and the
The physical geology of asteroid 243 Ida can be exestablishment of the cratering flow; at this stage we can infer plained largely in terms of a history dominated by several final crater diameters but not profiles. We find that a modified strength scaling applies for craters up to a few 100 m in collisions of global consequence, plus a fusillade of smaller diameter forming in rock subject to Ida's gravity, and that impacts pocking and gardening the surface and launching gravity controls all craters larger than Θ1 km. ''Bright seismic waves through the local rock. The pre-history of annuli'' around a number of intermediate craters may be Ida, probably as part of a larger parent body from which the result of low-velocity surface disturbances, rather than the entire Koronis family derived, is not considered here, bright proximal ejecta deposits. We also consider large imalthough Ida's irregular shape may suggest a collisional pactors, to which Ida presents a curved, finite target surface genesis. In this paper we study the mechanics of impact in with irregular gravity. These can excavate asymmetrical conan effort to establish a substantive correlation between cavities. Stresses from large events can refocus and cause theoretical predictions and the Galileo observations. As a fracture far from the crater; using the shape of Ida as a basis for 3D hydrocode simulations, we show that impact result we hope to offer insights into the mechanical propergenesis of the Vienna Regio concavity can cause fracture in ties of Ida (whether it is intact or has deep regolith, etc.), Pola Regio, where grooves are observed in spacecraft images. together with a clearer general understanding of impact Other simulations indicate that the formation of the Θ10 cratering on small bodies.
km crater Azzurra might have reopened these fractures, which Part of our research is aimed at non-local effects of large craters in finite targets. The idea that certain landforms on small objects might be due to distal effects from large