Biological Terrorism, Emerging Diseases, and National Security

Examines the extent to which bioterrorist attacks have proven or may prove difficult to distinguish from outbreaks of emerging diseases. Makes recommendations for how the U.S. could better prepare to meet the threat of biological terrorism

The pre- and post-accretion irradiation history of cometary ices

Comets Halley and Wilson exhibited similar 3.4 micron emission features at approx. 1 AU from the Sun. A simple model of thermal emission from organic grains fits the feature, provides optical depths in good agreement with spacecraft measurements, and explains the absence of longer-wavelength organic features as due to spectral heliocentric evolution (Chyba and Sagan, 1987). The model utilizes transmission spectra of organics synthesized in the laboratory by irradiation of candidate cometary ices; the authors have long noted that related gas-phase syntheses yield polycyclic aromatic hydrocarbons, among other organic residues (Sagan et al., 1967). The authors previously concluded (Chyba and Sagan, 1987) that Halley's loss of several meters' depth with each perihelion passage, combined with the good fit of the Halley 3.4 micron feature to that of comet Wilson (Allen and Wickramasinghe, 1987), argues for the primordial - but not necessarily interstellar - origin of cometary organics. The authors examine the relative importance to the formation of organics of the variety of radiation environments experienced by comets. They conclude that there is at present no compelling reason to choose any of three contributing mechanisms (pre-accretion UV, pre-accretion cosmic ray, and post-accretion radionuclide processing) as the most important

Time-Minimal Control of Dissipative Two-level Quantum Systems: the Generic Case

The objective of this article is to complete preliminary results concerning the time-minimal control of dissipative two-level quantum systems whose dynamics is governed by Lindblad equations. The extremal system is described by a 3D-Hamiltonian depending upon three parameters. We combine geometric techniques with numerical simulations to deduce the optimal solutions.Comment: 24 pages, 16 figures. submitted to IEEE transactions on automatic contro

Constraints on the Orbital Evolution of Triton

We present simulations of Triton's post-capture orbit that confirm the importance of Kozai-type oscillations in its orbital elements. In the context of the tidal orbital evolution model, these variations require average pericenter distances much higher than previously published, and the timescale for the tidal orbital evolution of Triton becomes longer than the age of the Solar System. Recently-discovered irregular satellites present a new constraint on Triton's orbital history. Our numerical integrations of test particles indicate a timescale for Triton's orbital evolution to be less than $10^5$ yrs for a reasonable number of distant satellites to survive Triton's passage. This timescale is inconsistent with the exclusively tidal evolution (time scale of $>10^8$ yrs), but consistent with the interestion with the debris from satellite-satellite collisions. Any major regular satellites will quickly collide among themselves after being perturbed by Triton, and the resulting debris disk would eventually be swept up by Triton; given that the total mass of the Uranian satellite system is 40% of that of Triton, large scale evolution is possible. This scenario could have followed either collisional or the recently-discussed three-body-interaction-based capture.Comment: 10 pages, 4 figures, accepted for ApJ

Terrestrial production vs. extraterrestrial delivery of prebiotic organics to the early Earth

A comprehensive treatment of comet/asteroid interaction with the atmosphere, ensuring surface impact, and resulting organic pyrolysis is required to determine whether more than a negligible fraction of the organics in incident comets and asteroids actually survived collision with Earth. Results of such an investigation, using a smoothed particle hydrodynamic simulation of cometary and asteroidal impacts into both oceans and rock, demonstrate that organics will not survive impacts at velocities approx. greater than 10 km s(exp -1), and that even comets and asteroids as small as 100m in radius cannot be aerobraked to below this velocity in 1 bar atmospheres. However, for plausible dense (10 bar CO2) early atmospheres, there will be sufficient aerobraking during atmospheric passage for some organics to survive the ensuing impact. Combining these results with analytical fits to the lunar impact record shows that 4.5 Gyr ago Earth was accreting at least approx. 10(exp 6) kg yr(exp 1) of intact cometary organics, a flux which thereafter declined with a approx. 100 Myr half-life. The extent to which this influx was augmented by asteroid impacts, as well as the effect of more careful modelling of a variety of conservative approximations, is currently being quantified. These results may be placed in context by comparison with in situ organic production from a variety of terrestrial energy sources, as well as organic delivery by interplanetary dust. Which source dominated the early terrestrial prebiotic inventory is found to depend on the nature of the early terrestrial atmosphere. However, there is an intriguing symmetry: it is exactly those dense CO2 atmospheres where in situ atmospheric production of organic molecules should be the most difficult, in which intact cometary organics would be delivered in large amounts

Questionnaires from the National Health Interview Survey, 1985-89

This series report includes the questions used in the National Health Interview Survey from 1980 to 1984. The report contains the basic health and demographic questionnaires and current health topic queWonnaires.By Michele M. Chyba, Division of Health Intetview Statistics, and Linda R. Washington, Division of Data Services."August 1993."Also available via the World Wide Web.Includes bibliographical references (p. 289)

Organic synthesis in the outer Solar System: Recent laboratory simulations for Titan, the Jovian planets, Triton and comets

We tabulate the most abundant gases and their radiation yields, for two experimental pressures: 0.24 mb, more relevant to upper atmosphere excitation, and 17 mb, more relevant to tropospheric, cosmic ray excitation. The yields computed in the 0.24 mb experiment combined with measured electronic fluxes and a simple, eddy diffusion model of Titan's atmosphere predict abundances of detected molecules in agreement with those found by Voyager and for heavier products, in somewhat better agreement with observation than photochemical absolute reaction rate kinetics models. All Voyager organics are accounted for and no detectable products are found that Voyager did not detect. A striking increase of products with multiple bonds is found with decreasing pressure. Hydrocarbon abundances decline slowly with increasing carbon number. Additionally, we list preliminary estimates for the yield of the heteropolymer, which seems to be produced in a quantity comparable (in moles of C+N consumed) to the total amount of gaseous product. The production rate required to sustain Titan's haze against sedimentation also indicates yields of this order. As can be seen from the table, over 10(exp 9) years substantial amounts of these products can accumulate on the surface -- ranging from cm thickness for the (C+N equals 4) species to a meter or more for HCN and C2H2; we also expect a meter or more of tholins. Similar analyses have been or are being done for the Jovian planets and Triton. Charged particle irradiation of hydrocarbon clathrates or mixed hydrocarbon/water ices produces a range of organic products, reddening and darkening of the ices and characteristic infrared spectra. From such spectra, the predicted emission by fine particles in cometary comae well-matches the observed 3.4 micron emission spectra of Comet Halley and other recent comets. Heliocentric evolution of organic emission features in comets is predicted. Organic products of such ice irradiation may account for colors and albedos on some of the satellites in the outer solar system, especially Triton and Pluto, where solid methane is known to exist