Engineering topics

Clark, R. N.

Publications and source records attributed to Clark, R. N..

At least 37 records · Page 2

NASA NTRS1991

New spectral observations of Callisto and leading/trailing hemisphere distinctions

In December 1989 and January 1990, new observations of the leading and trailing edges of Callisto were made from the NASA Infrared Telescope Facility on Mauna Kea in Hawaii. Using the Cool Grating Array Spectrometer, spectral coverage was obtained from 1.89 to 2.46 microns and from 2.8 to 4.2 microns for both the leading and trailing hemispheres. In addition, spectral coverage of the leading hemisphere was obtained from 1.30 to 2.55 microns and from 4.2 to 4.8 microns. Interpretations of the data are given.

Calvin, W. M.↗

NASA NTRS1991

Evidence for ammonium-bearing minerals in Ceres

Evidence for ammonium-bearing minerals was found on the surface of the largest asteroid Ceres. The presence of ammonium-bearing clays suggests that Ceres has experienced a period of alteration by substantial amounts of an ammonium-bearing fluid. The presence of the ammonium-bearing clays does not preclude Ceres maintaining a volatile inventory in the core or in a volatile-rich zone at some distance below the surface. Telescopic observations of Ceres, using the 3.0 meter NASA Infrared telescope facility prompted this reevaluation of its surface mineralogy.

King, T. V. V.↗

NASA NTRS1988

IR-dust observations of Comet Tempel 2 with CRAF VIMS

Measurement strategies are now being planned for using the Visual and Infrared Mapping Spectrometer (VIMS) to observe the asteroid Hestia, and the nucleus, and the gas and dust in the coma of comet P/Tempel 2 as part of the Comet Rendezvous Asteroid Flyby (CRAF) mission. The spectral range of VIMS will cover wavelengths from 0.35 to 5.2 micrometers, with a spectral resolution of 11 nm from 0.35 to 2.4 micrometers and of 22 nm from 2.4 to 5.2 micrometers. The instantaneous field of view (IFOV) provided by the foreoptics is 0.5 milliradians, and the current design of the instrument provides for a scanning secondary mirror which will scan a swath of length 72 IFOVs. The CRAF high resolution scan platform motion will permit slewing VIMS in a direction perpendicular to the swath. This enables the building of a two dimensional image in any or all wavelength channels. Important measurements of the dust coma will include the onset of early coma activity, the mapping of gas and dust jets and correlations with active nucleus areas, observations of the dust coma from various scattering phase angles, coverage of the low wavelength portion of the thermal radiation, and the 3.4 micrometer hydrocarbon emission. A description of the VIMS instrument is presented.

Combi, Michael R.↗

NASA NTRS1986

Atmospheric-water absorption features near 2.2 micrometers and their importance in high spectral resolution remote sensing

Selective absorption of electromagnetic radiation by atmospheric gases and water vapor is an accepted fact in terrestrial remote sensing. Until recently, only a general knowledge of atmospheric effects was required for analysis of remote sensing data; however, with the advent of high spectral resolution imaging devices, detailed knowledge of atmospheric absorption bands has become increasingly important for accurate analysis. Detailed study of high spectral resolution aircraft data at the U.S. Geological Survey has disclosed narrow absorption features centered at approximately 2.17 and 2.20 micrometers not caused by surface mineralogy. Published atmospheric transmission spectra and atmospheric spectra derived using the LOWTRAN-5 computer model indicate that these absorption features are probably water vapor. Spectral modeling indicates that the effects of atmospheric absorption in this region are most pronounced in spectrally flat materials with only weak absorption bands. Without correction and detailed knowledge of the atmospheric effects, accurate mapping of surface mineralogy (particularly at low mineral concentrations) is not possible.

Kruse, F. A.↗

NASA NTRS1986

Use of digital Munsell color space to assist interretation of imaging spectrometer data: Geologic examples from the northern Grapevine Mountains, California and Nevada

Techniques using Munsell color transformations were developed for reducing 128 channels (or less) of Airborne Imaging Spectrometer (AIS) data to a single color-composite-image suitable for both visual interpretation and digital analysis. Using AIS data acquired in 1984 and 1985, limestone and dolomite roof pendants and sericite-illite and other clay minerals related to alteration were mapped in a quartz monzonite stock in the northern Grapevine Mountains of California and Nevada. Field studies and laboratory spectral measurements verify the mineralogical distributions mapped from the AIS data.

Kruse, F. A.↗

NASA NTRS1986

Europa - Characterization and interpretation of global spectral surface units

In the present analysis of the Voyager global multispectral mosaic of the Galilean satellite Europa, which was conducted to map surface units with similar optical properties, color assignments are indicative of the spectral nature of the given unit. The unit maps are noted to exhibit a marked hemispheric asymmetry which is most clearly seen in the UV/violet albedo ratio image. It appears as if the surface has been most darkened at the center of the trailing hemisphere, with a gradual decrease to a minimum at the center of the leading hemisphere as the cosine of the angle from the antapex of motion; this cosine pattern suggests that the darkening is exogenic in origin, and is interpreted as evidence of surface alteration by in bombardment from the Jovian magnetosphere.

Nelson, M. L.↗

NASA NTRS1985

First Direct Detection of Clay Minerals on Mars

Magnesian clays or clay-type minerals were conclusively detected in the martian regolith. Near-IR spectral observations of Mars using the Mauna Kea 2.2-m telescope show weak but definite absorption bands near microns. The absorption band positions and widths match those produced by combined OH stretch and Mg-OH lattice modes and are diagnostic of minerals with structural OH such as clays and amphiboles. Likely candidate minerals include serpentine, talc, hectorite, and sponite. There is no spectral evidence for aluminous hydroxylated minerals. No distinct band occurs at 2.55 microns, as would be expected if carbonates were responsible for the 2.35 micron absorption. High-albedo regions such as Elysium and Utopia have the strongest bands near 2.35 microns, as would be expected for heavily weathered soils. Low-albedo regions such as Iapygia show weaker but distinct bands, consistent with moderate coatings, streaks, and splotches of bright weathered material. In all areas observed, the 2.35-micron absorption is at least three times weaker than would be expected if well-crystallized clay minerals made up the bulk of bright soils on Mars.

Singer, R. B.↗

NASA NTRS1985

Spectral Units on Europa and Ganymede

Comparisons of Europa and Ganymede multispectral data show that Ganymede is less spectrally variable than Europa. Four major spectral units dominate Ganymede, corresponding to the ancient cratered terrain and the grooved terrain in the leading and trailing hemispheres. A hemispheric asymmetry in UV absorption definitely exists on Ganymede, although it is not so strong as that on Europa. Comparison of normalized spectra for the four major units shows that the sense of the asymmetry (more absoption toward shorter wavelengths on the trailing hemisphere) is also the same on the two bodies. This hemispheric asymmetry is interpreted as evidence of alteration of the surface by magnetospheric bombardment or micrometorite bombardment. It is concluded that the pattern observed represents a steady state involving both of these exogenic modifying agents. The spectral changes which could be produced by these two processes are grain size alteration and changes in composition. The spectral effects of variation in water ice grain size are fairly well known. Laboratory experiments are being conducted to study the spectral effects of sulfur irradiation on water ice.

Mccord, T. B.↗

NASA NTRS1985

Determination of Mineral Type, Abundance, and Grain Size with Remotely Obtained Reflectance Spectra

To derive the abundances of minerals in a surface composed of more then one mineral in an intimate mixture, the bidirectional reflectance spectrum must be measured at several angles of incidence and emission. From these measurements, the scattering parameters may be derived, and the effective single scattering albedo can be computed from the models. If the scattering parameters are not derived, and the particles scatter anisotropically, then the computed effective single scattering albedo has been computed, the solution to the model is a nonlinear least squares problem with three unknowns per particle type. The solution is feasible when the complex indices of refraction and the densities of each mineral component are known. The unknowns are: the grain size of each particle type, the mass fraction of each particle type, and a grain internal scattering coefficient that describes grain imperfections.

Clark, R. N.↗

NASA NTRS1985

Spectral properties of water ice and contaminants

For remote sensing studies of ices in the solar system, it is important to understand the optical properties of water ice, and mixtures of ice and particulate materials. The present paper has the objective to review the current understanding of the spectral properties of ice, and mixtures of ice and particulates. The review is to provide a basis for the understanding of the remote sensing of ice. It is found that reflectance spectra of ice-soil intimate mixtures are complex, nonlinear functions of the optical and physical properties of the components which comprise the surface.

Lucey, P. G.↗

NASA NTRS1984

Reflectance spectroscopy - Quantitative analysis techniques for remote sensing applications

The empirical methods and scattering theories that are important for solving remote sensing problems are among the methods for remotely sensed reflectance data analysis presently compared. In the case of the photon mean optical path length concept's implications for reflectance spectra modeling, it is shown that the mean optical path length in a particulate surface is in roughly inverse proportion to the square root of the absorption coefficient. Absorption bands, which are Gaussian in shape when plotted as true absorptance vs photon energy, are also Gaussians in apparent absorptance, although they have a smaller intensity. An apparent continuum in a reflectance spectrum is modeled as a mathematical function that is used to isolate a particular absorption feature for analysis, and it is noted that this continuum should be removed by dividing it into the reflectance spectrum.

Clark, R. N.↗

NASA NTRS1984

Spectral properties of ice-particulate mixtures and implications for remote sensing. I - Intimate mixtures

From laboratory spectra of many ice-particulate mixtures with high, medium, and low albedo particulates, the band depths, continuum reflectance, and ratio of band depth to continuum reflectance are derived as a function of the logarithm of thef particulate weight fraction in the sample. Ice band depths are dependent on the particulate albedo, and increase with smaller weight fractions of particulates until the bands saturate and their depths decrease. While the continuum reflectance is a complex function of the particulate albedo and wavelength of light, the band depth-to-continuum reflectance ratios appear independent of particulate albedo in such a fashion that, for a given grain size of particulate, a calibration curve to the abundance of ice and particulates is derived. Theoretically produced frost spectra show similar curves of growth for band depth, continuum reflectance, and corresponding ratio vs the photon mean optical path length.

Clark, R. N.↗

NASA NTRS1984

Saturn's satellites - Near-infrared spectrophotometry (0.65-2.5 microns) of the leading and trailing sides and compositional implications

Water ice absorptions at 2.0, 1.5, and 1.25 microns are noted in near-IR spectra of Tethys, Dione, Rhea, Iapetus, and Hyperion, and the weak 1.04-micron ice absorption, which is detected for Rhea and Dione, is studied to establish band depth upper limits. The leading-trailing side 1.04-micron ice band depth differences on Saturn's satellites are similar to those for the Galilean satellites, indicating possible surface modification by magnetospheric charged particle bombardment. Limits are obtained for the amounts of particulates, trapped gases, and ammonium hydroxide on the surface. With the exception of the dark side of Iapetus, the surfaces of all of Saturn's satellites are nearly pure ice water.

Steele, A.↗

NASA NTRS1984

Surface of Miranda - Identification of water ice

Near-infrared spectrophotometry at 5-percent resolution shows Miranda to have a water-ice surface. Estimates of Miranda's albedo made from the depth of its 2.0-micron absorption band suggest that its visual geometric albedo is likely to be between 10 and 70 percent which when combined with the satellite's visual magnitude, yields a diameter of 500 + or 225 km. There is some evidence that suggests the visual geometric albedo of Miranda may be greater than or equal to 0.3, which implies that its diameter may lie near the lower end of the estimated range. With these results all the Uranian satellites are now known to have water-ice surfaces.

Brown, R. H.↗

NASA NTRS1984

Nitrogen on Triton

The tentative identification of liquid nitrogen as the source of the 2.16-micron absorption band in the spectrum of Triton has suggested a surface model in which nitrogen, together with the previously identified methane, occur in some kind of chemical association. Water frost may be another surface chemistry component, and dark, photochemically-produced particles occur as a minor contaminant of the ices or liquid. Temperature is noted to be a critical factor in the surface model, because the calculated subsolar surface temperature of Triton lies near nitrogen's melting-freezing temperature. The present modeling of Triton suggests nitrogen's occurrence in the form of fine grains.

Clark, R. N.↗

NASA NTRS1984

Ice-Soil Mixtures: Visual and Near-Infrared Remote Sensing Techniques

The spectral properties of ice soil mixtures are reviewed. Water ice has an absorption coefficient which varies by several orders of magnitude in the visual and near infrared, and has several prominent overtone absorptions. Different wavelengths can be used to probe to different depths in the surface as well as for different mineral impurity concentrations. Empirical and/or theoretical models might be used to derive characteristics grain size of the ice or of the impurity minerals and to derive abundance of the ice and rock or soil components. Quantitative analysis of remotely obtained reflectance spectra can only be performed by using absorption features in the spectra of ice and soil, and not by broadband response. It is suggested that this might be done by special selection of several narrow band filters in the near infrared which will adequately define the ice absorptions.

Clark, R. N.↗

NASA NTRS1983

Spectral properties of mixtures of montmorillonite and dark grains - Implications for remote sensing minerals containing chemically and physically adsorbed water

The spectral properties from 0.4 to 3 microns of montmorillonite plus dark carbon grains (called opaques) of various sizes are studied as a function of the weight fraction of opaques present. The reflectance level and band depths of the 1.4-, 1.9-, 2.2-, and 2.8-micron water and/or OH absorption features are analyzed using derived empirical relationships and scattering theory. It is found that the absorption band depths and reflectance level are a very nonlinear function of the weight fraction of opaques present but can be predicted in many cases by simple scattering theory. The 2.8-micron bound water fundamental band is the most difficult absorption feature to suppress. The overtone absorptions are suppressed a greater amount than the fundamental but are still apparent even when 10-20 wt pct opaques are present. The relationships observed and the simple scattering theory presented show that quantitative compositional remote sensing studies are feasible for surfaces containing complex mineral mixtures.

Clark, R. N.↗

NASA NTRS1983

Frost grain size metamorphism - Implications for remote sensing of planetary surfaces

The effective grain size of a material on a planetary surface affects the strength of absorption features observed in the reflectance of a particulate surface. In the case of a planetary surface containing volatile ices, the absorption characteristics can change in connection with processes leading to a change in the grain size of the material. The present investigation is concerned with an evaluation regarding the occurrence of such processes and the implications for remote sensing applications. It is found that quantitative modeling of the kinetics of grain growth and destruction by thermal and nonthermal processes can provide a means to reconcile apparent optical paths in the volatile portions of planetary surfaces with the physical history of those surfaces. Attention is also given to conditions in the case of the Pluto/Triton system, Uranus and Saturnian satellites, and the Galilean system.

Clark, R. N.↗