Browsing Meteoritics & Planetary Science, Volume 43, Number 12 (2008) by Subjects
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Impact penetration of Europa's ice crust as a mechanism for formation of chaos terrainIce thickness estimates and impactor dynamics indicate that some impacts must breach Europas ice crust; and outcomes of impact experiments using ice-over-water targets range from simple craters to chaos-like destroyed zones, depending on impact energy and ice competence. Firstorder impacts--into thick ice or at low impact energy--produce craters. Second-order impacts punch through the ice, making holes that resemble raft-free chaos areas. Third-order impacts--into thinnest ice or at highest energy--produce large irregular raft-filled zones similar to platy chaos. Other evidence for an impact origin for chaos areas comes from the size-frequency distribution of chaos+craters on Europa, which matches the impact production functions of Ganymede and Callisto; and from small craters around the large chaos area Thera Macula, which decrease in average size and density per unit area as a function of distance from Theras center. There are no tiny chaos areas and no craters 50 km diameter. This suggests that small impactors never penetrate, whereas large ones (berPenetrators: 2.5 km diameter at average impact velocity) always do. Existence of both craters and chaos areas in the size range 2-40 km diameter points to spatial/temporal variation in crust thickness. But in this size range, craters are progressively outnumbered by chaos areas at larger diameters, suggesting that probability of penetration increases with increasing scale of impact. If chaos areas do represent impact sites, then Europas surface is older than previously thought. The recalculated resurfacing age is 480 (-302/+960) Ma: greater than prior estimates, but still very young by solar system standards.
Textural constraints on the formation of impact spherules: A case study from the Dales Gorge BIF, Paleoproterozoic Hamersley Group of Western AustraliaImpact ejecta (about 2.5 Gyr old) in the DS4 layer of the Dales Gorge BIF (Hamersley Group, Western Australia) are so well preserved that many original textures such as vesicles and microlites are faithfully preserved. About 65% of the particles in the layer originated as impact ejecta, of which 81% are splash forms. The remaining 19% are angular, but the splash forms and angular particles have the same composition (mainly diagenetic stilpnomelane and K-feldspar) and share a common suite of internal textures. Some particles contain randomly oriented microlites texturally identical to plagioclase in basalts. Most splash forms have rims of inward-growing crystals that may have formed from the melt (perhaps nucleated by impinging dust) or via thermal devitrification. The rims clearly formed in flight because in broken particles (which make up about 13% of the splash forms) they are generally not present on broken surfaces. The origin of the angular particles is uncertain, but they may represent solid ejecta. Given the large sizes and variable shapes of the splash forms, they are probably droplets of impact melt emplaced ballistically. This is largely by analogy to the K-T boundary layer, but DS4 splash forms differ from K-T spherules in important ways suggesting the K-T model is not universal. The occurrence of basaltic ejecta from a large impact highlights its scarcity in the stratigraphic record despite the areal abundance of oceanic crust. The diverse textures formed via in-flight crystallization suggest particle paths in the plume are more complex than is generally appreciated.
The Dakhleh Glass: Product of an impact airburst or cratering event in the Western Desert of Egypt?Impact cratering is a ubiquitous geological process on the terrestrial planets. Meteorite impact craters are the most visible product of impact events, but there is a growing recognition that large aerial bursts or airbursts should occur relatively frequently throughout geological time. In this contribution, we report on an unusual impact glass--the Dakhleh Glass (DG)--which is distributed over an area of ~400 km^2 of the Dakhleh Oasis, Egypt. This region preserves a rich history of habitation stretching back to over 400,000 years before the emergence of Homo sapiens. We report on observations made during recent fieldwork and subsequent analytical analyses that strengthen previous suggestions that the DG formed during an impact event. The wide distribution and large size of DG specimens (up to ~50 cm across), the chemistry (e.g., CaO and Al2O3 contents up to ~25 and ~18 wt%, respectively), the presence of lechatelierite and burnt sediments, and the inclusion of clasts and spherules in the DG is inconsistent with known terrestrial processes of glass formation. The age and other textural characteristics rule out a human origin. Instead, we draw upon recent numerical modeling of airbursts to suggest that the properties of DG, coupled with the absence of a confirmed crater, can best be explained by melting of surficial sediments as a result of a large airburst event. We suggest that glass produced by such events should, therefore, be more common in the rock record than impact craters, assuming that the glass formed in a suitable preserving environment.