Engineering topics

Mckay, C. P.

Publications and source records attributed to Mckay, C. P..

At least 55 records · Page 3

NASA NTRS1987

Optical efficiencies of lightning in planetary atmospheres

The first simulations of lightning in planetary atmospheres by laser-induced plasmas are reported. These simulations show that the fraction of the energy in lightning discharge channels that is radiated in the visible spectrum is similar for earth, Venus, and Titan, but quite different for Jupiter. One implication of these results is that the amount of trace gases produced by lightning in the Jovian atmosphere must be larger than previously estimated.

Borucki, W. J.↗

NASA NTRS1986

Comment

Frank et al.'s (1986) suggestion that the earth is impacted by small (100-Mg) comets at the rate of 20/min is reconsidered in the light of recent comet models. The possibility of such a swarm of comets is found to be problematical because of the large number of comets required and because thermal stability arguments, while not excluding such a possibility, imply that it is implausible. In a reply, Frank et al. reaffirm their findings that such small comets, following direct elliptical orbits about the sun, are distributed in a disk centered on the sun and lying near the ecliptic plane at heliocentric radial distances 1-10,000 AU. The presence of these comets in the inner solar system is attributed to the gravitational action of the outer planets, passing stars, or interstellar clouds on comets is more distant orbits.

Mckay, C. P.↗

NASA NTRS1986

Methods for computing comet core temperatures

The temperature profile within the comet nucleus provides the key to an understanding of the history of the volatiles within a comet. Certain difficulties arise in connection with current cometary temperature models. It is shown that the constraint of zero net heat flow can be used to derive general analytical expressions which will allow for the determination of comet core temperature for a spherically symmetric comet, taking into account information about the surface temperature and the thermal conductivity. The obtained results are compared with the expression for comet core temperatures considered by Klinger (1981). Attention is given to analytical results, an example case, and numerical models. The formalization developed makes it possible to determine the core temperature on the basis of the numerical models of the surface temperature.

Mckay, C. P.↗

NASA NTRS1986

Experimental basis for a Titan probe organic analysis

The recent Voyager flyby of Titan produced evidence for at least nine organic compounds in that atmosphere that are heavier than methane. Several models of Titan's atmosphere, as well as laboratory simulations, suggest the presence of organics considerably more complex that those observed. To ensure that the in situ measurements are definitive with respect to Titan's atmosphere, experiment concepts, and the related instrumentation, must be carefully developed specifically for such a mission. To this end, the possible composition of the environment to be analyzed must be bracketed and model samples must be provided for instrumentation development studies. Laboratory studies to define the optimum flight experiment and sampling strategy for a Titan entry probe are currently being conducted. Titan mixtures are being subjected to a variety of energy sources including high voltage electron from a DC discharge, high current electric shock, and laser detonation. Gaseous and solid products are produced which are then analyzed. Samples from these experiements are also provided to candidate flight experiments as models for instrument development studies. Preliminary results show that existing theoretical models for chemistry in Titan's atmosphere cannot adequetely explain the presence and abundance of all trace gases observed in these experiments.

Mckay, C. P.↗

NASA NTRS1986

Radiocarbon dating of open systems with bomb effect

The application of radiocarbon dating is extended to include systems that are slowly exchanging carbon with the atmosphere. Simple formulae are derived that relate the true age and the exchange rate of carbon to the apparent radiocarbon age. A radiocarbon age determination does not give a unique true age and exchange rate but determines a locus of values bounded by a minimum age and a minimum exchange rate. It is found that for radiocarbon ages as large as 10,000 years it is necessary to correct for the anthropogenic radiocarbon produced in the atmosphere by nuclear weapons testing. A one-term exponential approximation, with an e-folding time of 14.43 years, is used to model this effect and is shown to be accurate to within 3 percent for exchange time constants of 100 years and greater. The approach developed here is not specific to radiocarbon and can be applied to other radioisotopes in open systems.

Mckay, C. P.↗

NASA NTRS1986

Oxygen budget of a perennially ice-covered Antarctic lake

A bulk O2 budget for Lake Hoare, Antarctica, is presented. Five years of seasonal data show the lake to be persistently supersaturated with O2. Oxygen is carried into the lake in glacial meltstreams and is left behind when this water is removed as ice by ablation and sublimation. A diffusive loss of O2 from the lake through the summer moat is suggested. Measured values of the total O2 in the water column indicate that the time scale of O2 turnover is much longer than a year. Based on these results, it is suggested that the amount of O2 in the water does not change significantly throughout the year and that the lake is also supersaturated with N2.

Wharton, R. A., Jr.↗

NASA NTRS1986

Space Station gas-grain simulation facility - Application to exobiology

The technical issues involved in performing experiments on the behavior and properties of aerosols in a microgravity environment provided by the Space Station are reviewed. The displacement of a particle resulting from g-jitter for ballistic, Knudsen, and Stokes flow regimes is examined in detail, and the radiation, acoustic, electrostatic, and electromagnetic mechanisms for the control of this motion are described. The simulation of organic haze production on Titan has been selected as an example experiment for detailed study. The purpose of this experiment was to simulate the photolysis of methane and the subsequent formation of the organic haze particles in the Titan upper atmosphere.

Mckay, C. P.↗

NASA NTRS1985

Temperatures within comet nuclei

Knowledge of the temperature distribution within comet nuclei is critical to the understanding of the state and behavior of comets. The present study is concerned with the temperature profile below the surface of the nucleus and below any dust mantle which may be present. With respect to the physics, the considered model is in some respects similar to that discussed by Weissman and Kiefer (1981, 1984). However, the new model is improved in various respects. It represents the first attempt to investigate directly the detailed nature of thermal profiles within a comet nucleus. Included in the investigation are calculations which consider the effects of variations in latitude, nucleus spin axis orientation, surface albedo, and thermal emissivity. It is shown that the application of the model, together with in situ measurements of temperatures within a cometary nucleus, could significantly constrain the range of possible subsurface physical properties.

Squyres, S. W.↗

NASA NTRS1985

A calculation of the Rosseland mean opacity of dust grains in primordial solar system nebulae

The Rosseland mean opacity owing to grains was calculated as a function of temperature and density for nebulae having solar elemental abundances. The values of the mean opacity were evaluated with a generalized formulation allowing for anisotropic scattering. The values of the mean opacity do not depend sensitively on the choice of the particle size distribution function, provided that there are few particles having sizes in excess of several tens of microns. The results indicate that thermal convection in primordial nebulae occurs over broader ranges of altitudes at low temperatures than at high temperatures, and for size distributions for which extensive aggregation has not yet occurred.

Pollack, J. B.↗

NASA NTRS1985

Spectra of simulated lightning on Venus, Jupiter, and Titan

Laser-induced plasmas in various gas mixtures were used to simulate lightning in other planetary atmospheres. This method of simulation has the advantage of producing short-duration, high-temperature plasmas free from electrode contamination. The laser-induced plasma discharges in air are shown to accurately simulate terrestrial lightning and can be expected to simulate lightning spectra in other planetary atmospheres. Spectra from 240 to 880 nm are presented for simulated lightning in the atmospheres of Venus, earth, Jupiter, and Titan. The spectra of lightning on the other giant planets are expected to be similar to that of Jupiter because the atmospheres of these planets are composed mainly of hydrogen and helium. The spectra of Venus and Titan show substantial amounts of radiation due to the presence of carbon atoms and ions and show CN Violet radiation. Although small amounts of CH4 and NH3 are present in the Jovian atmosphere, only emission from hydrogen and helium is observed. Most differences in the spectra can be understood in terms of the elemental ratios of the gas mixtures. Consequently, observations of the spectra of lightning on other planets should provide in situ estimates of the atmospheric and aerosol composition in the cloud layers in which lightning is occurring. In particular, the detection of inert gases such as helium should be possible and the relative abundance of these gases compared to major constituents might be determined.

Borucki, W. J.↗

NASA NTRS1985

On the mean particle size and water content of polar mesospheric clouds

Results from the ultraviolet spectrometer experiment (UVS) aboard the Solar Mesospheric Explorer, which measures ultraviolet sunlight scattered from air molecules in the mesosphere, are reported and discussed. The UVS experiment, its observing modes, and details of the observing sequence are described. The data are compared with predictions of Mie scattering theory, assuming the size distribution is parameterized by a mean radius and a width parameter. The water ice content, mean particle radius, and column number are calculated as a function of cloud brightness, and in terms of the unknown width parameter. It is concluded that polar mesospheric cloud particles are quite small, having average volume-effective radii less than 70 mm, and that clouds differ in their optical properties mainly as a result of their mean particle size.

Thomas, G. E.↗

NASA NTRS1985

Cryoconite holes on glaciers

Cryoconite holes are water-filled depressions on the surface of glaciers. They contain microbial communities and may contribute to glacial wastage and biological colonization of ice-free areas.

Wharton, R. A., Jr.↗

NASA NTRS1985

Noctilucent cloud formation and the effects of water vapor variability on temperatures in the middle atmosphere

To investigate the occurrence of low temperatures and the formation of noctilucent clouds in the summer mesosphere, a one-dimensional time-dependent photochemical-thermal numerical model of the atmosphere between 50 and 120 km has been constructed. The model self-consistently solves the coupled photochemical and thermal equations as perturbation equations from a reference state assumed to be in equilibrium and is used to consider the effect of variability in water vapor in the lower mesosphere on the temperature in the region of noctilucent cloud formation. It is found that change in water vapor from an equilibrium value of 5 ppm at 50 km to a value of 10 ppm, a variation consistent with observations, can produce a roughly 15 K drop in temperature at 82 km. It is suggested that this process may produce weeks of cold temperatures and influence noctilucent cloud formation.

Mckay, C. P.↗

NASA NTRS1985

Thickness of ice on perennially frozen lakes

A simple energy-balance model for the perenially frozen lakes of Antarctica's southern Victoria Land is presented which, using the measured ablation rate of 30 cm/yr, can explain the observed ice thickness. Some speculations are presented on the ice cover that could have existed on possible former lakes in the equatorial regions of Mars.

Mckay, C. P.↗

NASA NTRS1985

The impact of polar stratospheric clouds on the heating rates of the winter polar stratosphere

The perturbation affecting IR radiative heating rates of the lower stratosphere that are prompted by the occurrence of polar stratospheric clouds (PSCs) during the Arctic and Antarctic winter are presently calculated by means of a multispectral radiative transfer code that allows for scattering, absorption, and thermal emission by particles and gases. Attention is given to perturbations arising from both the particulate opacity of the PSCs and the decrease of H2O vapor accompanying their formation. For plausible values of model parameters, the former, direct effect is always one of increased radiative cooling, while the indirect effect is always one of decreased cooling.

Pollack, J. B.↗

NASA NTRS1985

Comet nucleus gamma-ray spectrometer penetrator - An in situ nucleus composition experiment

An engineering design study of a comet nucleus penetrator system for in situ determinations of comet composition is presented. The system is designed for participation in the Comet Rendezvous/Asteroid Flyby mission to comet Kopff in 1996. The instrumentation payload of the penetrator system will include a passive gamma-ray spectrometer; a high frequency response accelerometer for measurements of the impact deceleration profile of the system as it penetrates to the nucleus of the comet; and several thermocouples mounted along the length of the penetrator to measure temperature and temperature gradients. The mechanical, electrical and thermal interfaces between the penetrator and the Mariner Mark II spacecraft are described. A series of line drawings is provided which illustrates the proposed configuration of the system.

Swenson, B. L.↗

NASA NTRS1985

The case for Mars II

Among the topics discussed are: the political aspects of the exploration of Mars, preparatory mission scenarios in advance of manned Martian exploration, and scientific programs for a base on Mars. Consideration is also given to: mission strategies and spacecraft design for a Mars base program; concepts for the realization of a manned mission to Mars; dirigible airship applications to Martian surface exploration; and the power requirements of several Martian base designs. Additional topics include: the impact of propellant manufacturing on early manned exploration of the solar system; water supply issues for a manned Mars base; and psychological and interpersonal adaptation to life on Mars.

Mckay, C. P.↗

NASA NTRS1985

Antarctica - Lessons for a Mars exploration program

The history of exploration and the international system of control of Antarctica has often been cited as a paradigm for the exploration of space. The small isolated research stations have been used to model the psychological stresses of future space missions. In addition, the programmatic structure of the U.S. Antarctic Research Program provides several possible analogs to future Mars Programs presently under discussion. These are: (1) Continued presence; (2) Civilian, military and private sector involvement; (3) Scientific activities; (4) Risk assessment and logistical support; (5) Accessibility for non-specialists; (6) Political and strategic motivations; (7) International cooperation/competition. Survival in Antarctica is contingent on advanced technology and the active transport of supplies. The scientific exploration of this remote and barren expanse without, of course, the aid and guidance of indigenous people certainly provides one of the closest analogs available to future science activities on the Martian surface.

Mckay, C. P.↗