THE BARRINGER-AWARD-ADDRESS - PRESENTED 1996 JULY-25, BERLIN, GERMANY- IMPACT EXPERIMENTS RELATED TO THE EVOLUTION OF PLANETARY REGOLITHS

Impact-induced comminution of planetary surfaces is pervasive througho ut the solar system and occurs on submillimeter to global scales, resu lting in comminution products that range from fine-grained surface soi ls, to massive, polymict ejecta deposits, to collisionally fragmented objects. Within this wide range of comminution products, we define reg oliths in a narrow sense as materials that were processed by repetitiv e impacts to dimensional scales comparable to or smaller than that of component minerals of the progenitor rock(s). In this paper, we summar ize a wide variety of impact experiments and other observations that w ere primarily intended to understand the evolution of regoliths on lun ar basalt flows, and we discuss some of their implications for asteroi dal surfaces. Cratering experiments in both rock and noncohesive mater ials, combined- with photogeologic observations of the lunar surface, demonstrate that craters <500 m in diameter contribute most to the exc avation of local bedrock for subsequent processing by micrometeorites. The overall excavation rate and, thus, growth rate of the debris laye r decreases with time, because the increasingly thicker fragmental lay er will prevent progressively larger projectiles from reaching bedrock . Typical growth rates for a 5 m thick lunar soil layer are initially (similar to>3 Ga ago) a few mm/Ma and slowed to <1 mm/Ma at present. T he coarse-grained crater ejecta are efficiently comminuted by collisio nal fragmentation processes, and the mean residence time of a 1 kg roc k is typically approximate to 10 Ma. The actual comminution of either lithic or monomineralic detritus is highly mineral specific, with feld spar and mesostasis comminuting preferentially over pyroxene and olivi ne, thus resulting in mechanically fractionated fines, especially at g rain sizes <20 mu m. Such fractionated fines also participate preferen tially in the shock melting of lunar soils, thus giving rise to ''aggl utinate'' melts. As a consequence, agglutinate melts are systematicall y enriched in feldspar components relative to the bulk composition of their respective host soil(s). Compositionally homogeneous, impact der ived glass beads in lunar soils seem to result from micrometeorite imp acts on rock surfaces, reflecting lithic regolith components and assoc iated mineral mixtures. Cumulatively, experimental and observational e vidence from lunar mare soils suggests that regoliths derive substanti ally from the comminution of local bedrock; the addition of foreign, e xotic components is not necessary to explain the modal and chemical co mpositions of diverse grain size fractions from typical lunar soils. R egoliths on asteroids are qualitatively different from those of the Mo on. The modest impact velocities in the asteroid belt, some 5 km s(-1) , are barely sufficient to produce impact melts. Also, substantially m ore crater mass is being displaced on low-gravity asteroids compared t o the Moon; collisional processing of surface boulders should therefor e be more prominent in producing comminuted asteroid surfaces. These p rocesses combine into asteroidal surface deposits that have suffered m odest levels of shock metamorphism compared to the Moon. Impact meltin g does not seem to be a significant process under these conditions. Ho wever, the role of cometary particles encountering asteroid surfaces a t presumably higher velocities has not been addressed in the past. Unf ortunately, the asteroidal surface processes that seemingly modify the spectral properties of ordinary chondrites to match telescopically ob tained spectra of S-type asteroids remain poorly understood at present , despite the extensive experimental and theoretical insights summariz ed in this report and our fairly mature understanding of lunar surface processes and regolith evolution.