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At least 721 records · Page 40

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NASA NTRS1978

Planetary gamma-ray spectroscopy

The measured intensities of certain gamma rays of specific energies escaping from a planetary surface can be used to determine the abundances of a number of elements. The fluxes of the more intense gamma-ray lines emitted from 32 elements were calculated using current nuclear data and existing models for the source processes. The source strengths for neutron-capture reactions were modified from those previously used. The fluxes emitted form a surface of average lunar composition are reported for 292 gamma-ray lines. These theoretical fluxes were used elsewhere to convert the data from the Apollo gamma-ray spectrometers to elemental abundances and can be used with measurements from future missions to map the concentrations of a number of elements over a planet's surface. Detection sensitivities for these elements are examined and applications of gamma-ray spectroscopy for future orbiters to Mars and other solar-system objects are discussed.

Reedy, R. C.↗

NASA NTRS1977

Crater evolutionary tracks

The technique proposed in the present paper for studying planetary surface processes is based on the measurement of crater rim height, h, depth, d, and diameter, D. The h/d and d/D ratios provide a quantitative description of crater morphology as well as a quantitative method for assessing the relative importance of competing crater modification processes (since h, d, and D change as a crater is degraded by surface processes, and h/d and d/D change with time). Different classes of processes produce distinctive evolutionary tracks on an h/d versus d/D diagram. Tracks for three general classes of crater modification (processes adding material to the crater; processes redistributing the material within the crater vicinity; and processes removing the material from the crater vicinity) are calculated, and h/d and d/D ratios for craters on the earth, moon, and Mars are compared.

Schubert, G.↗

NASA NTRS1977

Spontaneous magnetic field generation in hypervelocity impacts

Hypervelocity impacts of meteoroids onto early planetary surfaces may have generated short-lived magnetic fields. The high specific power densities of the impacts, plasma production in the ejecta clouds, and the chemically layered targets of the meteoroids are analyzed in describing the evolution of the magnetic fields. Durations from about one millionth of a minute to one minute, as well as strengths up to 100 tesla, are posited for the impact-generated magnetic fields. The analogy of magnetic-field generation in laser-target experiments is also mentioned. The acquisition of shock remanence and thermoremanence by the ejecta and nearby rock following impact is discussed.

Srnka, L. J.↗

NASA NTRS1977

Impact-induced energy partitioning, melting, and vaporization on terrestrial planets

The reported investigation was conducted in connection with the desirability to obtain information regarding the response of a solidified planetary surface to the primary impact of a meteoroid whose dimensions are small compared to that of the planet. The investigation is an extension of studies conducted by Ahrens and O'Keefe (1977) concerned with a calculation via finite-difference techniques of the entire flow field, the energy partitioning, and the quantity of melt and vapor produced upon impact of an iron object on a gabbroic anorthosite half-space at 15 km/sec. The assumptions used in these calculations are summarized. Calculations based on an equation of state for the low- and high-pressure mineral assemblage of gabbroic anorthosite, initial density 2.94 g/cu cm are considered. An algorithm is formulated for the description of the phase transformation upon shock loading. It is shown that the relative volume of melt and the equivalent volume of shock-induced vapor is proportional to the meteoroid kinetic energy (at high speeds) using the similarity scaling of projectile and target properties as suggested by Dienes and Walsh (1970).

Okeefe, J. D.↗

NASA NTRS1977

Application of high explosion cratering data to planetary problems

The present paper deals with the conditions of explosion or nuclear cratering required to simulate impact crater formation. Some planetary problems associated with three different aspects of crater formation are discussed, and solutions based on high-explosion data are proposed. Structures of impact craters and some selected explosion craters formed in layered media are examined and are related to the structure of lunar basins. The mode of ejection of material from impact craters is identified using explosion analogs. The ejection mode is shown to have important implications for the origin of material in crater and basin deposits. Equally important are the populations of secondary craters on lunar and planetary surfaces.

Oberbeck, V. R.↗

NASA NTRS1977

Crater morphometry from bistatic radar

In the present paper, bistatic radar techniques are discussed which can be used to identify and measure the dimensions of anomalously scattering regions on planetary surfaces. The morphometric potential of the bistatic radar is demonstrated by an example in which spectral features in Apollo 14 echoes are correlated with parts of the lunar crater Lansberg. Crater dimensions, including width of interior walls and extent of the ejecta blanket, are determined from a quantitative analysis. It is shown that the technique can be generalized for scattering areas at arbitrary positions with respect to the ground track, and could easily be inverted for characterization of a 'blind' target, such as Venus.

Simpson, R. A.↗

NASA NTRS1976

OAST Space Theme Workshop. Volume 3: Working group summary. 9: Aerothermodynamics (M-3). A: Statement. B: Technology needs (form 1). C. Priority assessment (form 2). D. Additional assessments

Twelve aerothermodynamic space technology needs were identified to reduce the design uncertainties in aerodynamic heating and forces experienced by heavy lift launch vehicles, orbit transfer vehicles, and advanced single stage to orbit vehicles for the space transportation system, and for probes, planetary surface landers, and sample return vehicles for solar system exploration vehicles. Research and technology needs identified include: (1) increasing the fluid dynamics capability by at least two orders of magnitude by developing an advanced computer processor for the solution of fluid dynamic problems with improved software; (2) predicting multi-engine base flow fields for launch vehicles; and (3) developing methods to conserve energy in aerothermodynamic ground test facilities.

Source record↗

NASA NTRS1977

MARS: The Viking discoveries

The Viking spacecraft are described as well as the instruments carried to accomplish the combined goal of studying the atmosphere and geology of the entire planet, and to analyze the Martian soil and search for life in two specific locations. Imagery received from the spacecraft illustrate discussions of the planetary surface, composition, and winds. Suggestions for further reading are included along with a list of available NASA film. Experiments and activities for classroom use are described.

French, B. M.↗

NASA NTRS1978

Planetary gamma-ray spectroscopy

Reedy, R. C.↗

NASA NTRS1979

Detailed geometrical modeling of Jupiter's 10-related decametric radiation

The paper compares observational results of Jupiter's decametric radiation with the beaming directions predicted by emission of a wide-angle conical sheet of radiation. Using the conical beaming model, beaming directions were calculated assuming a source region above the planetary surface along the Io-affected magnetic flux tube at a point where the electron gyrofrequency is equal to the radiated frequency. The analysis is based on the octupolar magnetic field models from the Pioneer 11 measurements, and an apparent displacement of the flux tube 20 deg ahead of the Io orbital position produces agreement between the calculated and observed positions of the sources. The longitudinal extents of the three main sources are approximated by limiting radiation to points where the angle between the magnetic field vector at the point of emission and the Zenographic equatorial plane is at a minimum. For lower frequencies the Io-dependent radiation is emitted at all longitudes, and the beaming model concurs with this conclusion when all cutoffs are eliminated.

Thieman, J. R.↗

NASA NTRS1980

Reports of planetary geology program, 1979 - 1980

Abstracts of 145 reports are compiled addressing the morphology, geochemistry, and stratigraphy of planetary surfaces with some specific examinations of volcanic, aeolian, fluvial, and periglacial processes and landforms. In addition, reports on cartography and remote sensing of planet surfaces are included.

Wirth, P.↗

NASA NTRS1979

Wave tilt sounding of multilayered structures

The relationship between the wave tilt and the electrical parameters of a multilayered structure is investigated. Particular emphasis is placed on the inverse problem associated with the sounding planetary surfaces. An inversion technique, based on multifrequency wave tilt, is proposed and demonstrated with several computer models. It is determined that there is close agreement between the electrical parameters used in the models and those in the inversion values.

Warne, L.↗

NASA NTRS1980

The role of fallback ejecta in the modification of impact craters

The physical processes responsible for the deposition of fragments of target material ejected from the transient cavity of an impact crater within the crater are discussed and quantities of fallback ejecta deposited in impact craters formed on different planetary surfaces are estimated. Consideration is given to the relative importance of in-flight particle collisions and atmospheric particle deceleration, and a model of the aerodynamic deceleration of fallback ejecta is developed. When applied to Meteor Crater in Arizona, the model predicts 5% of the primary ejecta, with an average fragment size of 1.4 cm, to have fallen back into the crater, in agreement with empirical studies of the crater fallback deposit, thus indicating atmospheric deceleration to be the principal mechanism for fallback deposition on earth. The model also predicts the proportion of primary ejecta to be deposited as fallback to increase with excavation cavity diameter on the earth. Model estimates also suggest that more than half of the primary ejecta from large-scale Venusian cavities should be redeposited as fallback, while relatively small amounts of fallback are expected for lunar and Martian craters.

Settle, M.↗

NASA NTRS1981

On the polarity of cyclostrophic flow in planetary atmospheres

Fluids which are completely inviscid in the mathematical sense do not exist. Thus, the concepts of gradient flow and cyclostrophic balance are interpreted as approximate solutions of a boundary value problem for small but finite viscosity. Large scale phenomena such as the superrotation of Venus and cyclones are effectively bounded by the rigidly rotating planetary surface. This polarizes the circulation and excludes so-called anomalous motions from the flow regime. With scale phenomena such as dust devils, both directions are observed which is attributed to the stochastic nature of wind systems surrounding the disturbance.

Mayr, H. G.↗

NASA NTRS1981

Radar investigation of asteroids

Radar investigations were conducted of selected minor planets, including: (1) observations during 1981-82 of 10 potential targets (2 Pallas, 8 Flora, 12 Victoria, 15 Eunomia, 19 Fortuna, 22 Kalliope, 132 Aethra, 219 Thusnelda, 433 Eros, and 2100 Ra-Shalom); and (2) continued analyses of observational data obtained during 1980-81 for 10 other asteroids (4 Vesta, 7 Iris, 16 Psyche, 75 Eurydike, 97 Klotho, 216 Kleopatra, 1685 Toro, 1862 Apollo, 1865 Cerberus, and 1915 Quetzalcoatl). Scientific objectives include estimation of echo strength, polarization, spectral shape, spectral bandwidth, and Doppler shift. These measurements: (1) yield estimates of target size, shape, and spin vector; (2) place constraints on topography, morphology, and composition of the planetary surface; (3) yield refined estimates of target orbital parameters; (4) reveal the presence of asteroidal satellites.

Ostro, S. J.↗

NASA NTRS1981

The magnetopause of the earth and planets

The intrinsic magnetic fields of Mercury, the earth, Jupiter and Saturn, all deflect the solar wind well above the planetary surface. The current layer or magnetopause which flows between the magnetized solar wind and magnetospheric plasmas should play an important role in determining the strength of the interaction but has only been investigated at the earth where the interplanetary magnetic field direction is found to exert a primary control on the structure of the magnetopause and the global dynamics of the magnetosphere. The solar wind interaction with Venus is quite different than that with the earth because Venus has at most a very weak magnetic field. Nevertheless, the current layer on the Venus ionopause has many similarities to that of the earth, in particular the presence of small scale structure.

Russell, C. T.↗

NASA NTRS1980

Synthetic aperture radar range - Azimuth ambiguity design and constraints

Problems concerning the design of a system for mapping a planetary surface with a synthetic aperture radar (SAR) are considered. Given an ambiguity level, resolution, and swath width, the problems are related to the determination of optimum antenna apertures and the most suitable pulse repetition frequency (PRF). From the set of normalized azimuth ambiguity ratio curves, the designer can arrive at the azimuth antenna length, and from the sets of normalized range ambiguity ratio curves, he can arrive at the range aperture length or pulse repetition frequency. A procedure based on this design method is shown in an example. The normalized curves provide results for a SAR using a uniformly or cosine weighted rectangular antenna aperture.

Mehlis, J. G.↗

NASA NTRS1982

Application of Kubelka-Munk theory of diffuse reflectance to geologic problems - The role of scattering

An understanding of the reflectance spectra of scattering media is vital for the appropriate interpretation of the reflectance spectra of planetary surfaces. When the absorption coefficient (k) and the mean size of the scattering centers are small, the Kubelka-Munk (K-M) theory of diffuse reflectance is valid. Since small values of k are characteristic of a wide variety of geologically important materials over a significant range of wavelength, the K-M theory should be applicable to appropriate portions of the reflectance spectra of these media if the dimensions of the scattering centers are sufficiently small. To test the utility of the K-M theory, a comparison is conducted of a set of theoretically generated spectra with a set of independently measured experimental spectra. The similarities found in the behavior of the two sets of spectra demonstrate the applicability of the K-M theory to the understanding of physical phenomena. Aspects of wavelength-dependent scattering are investigated.

Morris, R. V.↗