New perspectives on the Popigai impact structure

The record of large-scale cratering on Earth is scant, and the only currently 'proven' 100-km-class impact structure known to have formed within the Cenozoic is Popigai, located in the Siberian Arctic at 71.5 deg N, 111 deg E. Popigai is clearly a multiringed impact basin formed within the crystalline shield rocks (Anabar) and platform sediments of the Siberian taiga, and estimates of the volume of preserved impact melt typically exceed 1700 cu km, which is within a factor of 2-3 of what would be predicted using scaling relationships. We present the preliminary results of an analysis of the present-day topography of the Popigai structure, together with refined absolute age estimates, in order to reconstruct the pre-erosional morphology of the basin, as well as to quantify the erosion or sediment infill rates in the Popigai region

K/T age for the popigai impact event

The multi-ringed POPIGAI structure, with an outer ring diameter of over 100 km, is the largest impact feature currently recognized on Earth with an Phanerozoic age. The target rocks in this relatively unglaciated region consist of upper Proterozoic through Mesozoic platform sediments and igneous rocks overlying Precambrian crystalline basement. The reported absolute age of the Popigai impact event ranges from 30.5 to 39 Ma. With the intent of refining this age estimate, a melt-breccia (suevite) sample from the inner regions of the Popigai structure was prepared for total fusion and step-wise heating Ar-40/Ar-39 analysis. Although the total fusion and step-heating experiments suggest some degree of age heterogeneity, the recurring theme is an age of around 64 to 66 Ma

Where’s the Wisdom in Service Learning?

No abstract

Civil Liberties During War: History\u27s Institutional Lessons. Book Review Of: All the Laws but One: Civil Liberties in Wartime. by William H. Rehnquist

Book review of: All the Laws But One: Civil Liberties in Wartime. By William H. Rehnquist. New York: Alfred A. Knopf, Inc., 1998. Pp. 254. Reviewed by: Margaret A. Garvi

A Study of Selected Antipoverty Interventions Among the United States Poor

This thesis has several components. Its review of antipoverty literature leads-in to a theoretical perspective of the causes, effects, and remediation of poverty in the United States. A small minority of powerful individuals and corporations who monopolize decision- making apparatuses consistently make decisions which nurture unjust, inequitable allocations of the nation\u27s resources. As they protect their vested interests, the socio- economic and political systems which perpetuate the gross economic imbalance between rich and poor are nurtured . The poor (and increasingly , the middle class) are left virtually powerless, even over the decisions which affect their own lives. In order to explore various approaches to poverty, a survey of 65 agencies was conducted . It studies the goals, scope, modus operandi, staffing, funding, eligibility requirements, and other features of primarily nonprofit organizations. These agencies play a vital role in the reduction of poverty. They provide material aid, crisis intervention, advocacy, and supportive relationships to the poor. Their services are a partial solution to the problem of poverty , but the sheer volume of clients acts as a deterrent to their effectiveness. They cannot continue the struggle against poverty alone

Geological remote sensing signatures of terrestrial impact craters

Geological remote sensing techniques can be used to investigate structural, depositional, and shock metamorphic effects associated with hypervelocity impact structures, some of which may be linked to global Earth system catastrophies. Although detailed laboratory and field investigations are necessary to establish conclusive evidence of an impact origin for suspected crater landforms, the synoptic perspective provided by various remote sensing systems can often serve as a pathfinder to key deposits which can then be targetted for intensive field study. In addition, remote sensing imagery can be used as a tool in the search for impact and other catastrophic explosion landforms on the basis of localized disruption and anomaly patterns. In order to reconstruct original dimensions of large, complex impact features in isolated, inaccessible regions, remote sensing imagery can be used to make preliminary estimates in the absence of field geophysical surveys. The experienced gained from two decades of planetary remote sensing of impact craters on the terrestrial planets, as well as the techniques developed for recognizing stages of degradation and initial crater morphology, can now be applied to the problem of discovering and studying eroded impact landforms on Earth. Preliminary results of remote sensing analyses of a set of terrestrial impact features in various states of degradation, geologic settings, and for a broad range of diameters and hence energies of formation are summarized. The intention is to develop a database of remote sensing signatures for catastrophic impact landforms which can then be used in EOS-era global surveys as the basis for locating the possibly hundreds of missing impact structures. In addition, refinement of initial dimensions of extremely recent structures such as Zhamanshin and Bosumtwi is an important objective in order to permit re-evaluation of global Earth system responses associated with these types of events

Ion beam micromachining of integrated optics components

Thin film integrated optics components such as light guides, modulators, directional couplers, and polarizers demand high quality edge smoothness and high resolution pattern formation in dimensions down to submicrometer size. Fabrication techniques combining holographic and scanning electron beam lithography with ion beam micromachining have produced planar phase gratings with intervals as small as 2800 Å, guiding channel couplers in GaAs, and also wire- grid polarizers for 10.6-µm radiation

Channel Optical Waveguides and Directional Couplers in GaAs -- Imbedded and Ridged

Two-channel imbedded directional couplers were fabricated with proton implantation, yielding complete light transfer in 2 mm. Ridged channel guides were fabricated by ion-micromachining epitaxial layers, and a method of directional coupling was demonstrated

Optically pumped GaAs surface laser with corrugation feedback

A GaAs distributed-feedback laser was fabricated and pumped optically. A narrow stimulated spectrum was obtained around 0.83 µ with threshold pumping power of ~2 × 10^5 W/cm^2