Jeffrey Roy Johnson

Remote sensing of potential lunar resources: 1. Near‐side compositional properties

New telescopic CCD multispectral imaging of the lunar near side and 330–870 nm spectroscopy of selected regions are presented as aids in estimating compositional differences relevant to locating potential lunar resources such as ilmenite (FeTiO3) and solar wind-implanted 3He and H. Conversion of 400/560 nm CCD ratio images to weight percent TiO2 using a version of the Charette et al. (1974) empirical relation allows construction of a new TiO2 abundance map for the lunar maria accurate to ±2 wt % HO2. This map provides a consistent view of TiO2 distribution among mature mare soils and can be used to estimate areas potentially rich in ilmenite. Western Mare Tranquillitatis exhibits the highest TiO2 abundances (>8 wt %), followed by regions near Flamsteed P and in northern Oceanus Procellarum. A 950/560 nm CCD ratio mosaic of the full Moon provides estimates of relative surface maturity on local scales as defined by the degree of agglutinate production due to micrometeorite impacts. Since high 3He concentrations are correlated with mature ilmenite-rich soils, a combination of relative surface maturity maps and TiO2 abundance maps may be useful for estimating 3He (and possibly H) distribution on local scales. While dark mantle materials are also potential sources of ilmenite, 3He, and H, their compositional differences from mare soils prevent accurate remote mapping of their TiO2 abundance and relative maturity. Reflectance spectra (relative to a more rigorously defined MS-2 standard region) of dark mantle materials and high-TiO2 mare areas exhibit previously undocumented near-UV (<350 nm) changes in spectral slope apparently related to compositional variations. Determination of the primary lunar sites for resource utilization will be dictated by the future goals and priorities of the lunar resource program.

Remote sensing of potential lunar resources: 2. High spatial resolution mapping of spectral reflectance ratios and implications for nearside mare TiO2 content

High spatial resolution maps illustrating variations in spectral reflectance 400/560 nm ratio values have been generated for the following mare regions: (1) the border between southern Mare Serenitatis and northern Mare Tranquillitatis (including the MS-2 standard area and Apollo 17 landing site), (2) central Mare Tranquillitatis, (3) Oceanus Procellarum near Seleucus, and (4) southern Oceanus Procellarum around Flamsteed. We have also obtained 320–1000 nm reflectance spectra of several sites relative to MS-2 to facilitate scaling of the images and provide additional information on surface composition. Inferred TiO2 abundances for these mare regions have been determined using an empirical calibration which relates the weight percent TiO2 in mature mare regolith to the observed 400/560 nm ratio. Mare areas with high TiO2 abundances are probably rich in ilmenite (FeTiO3) a potential lunar resource. The highest potential TiO2 concentrations we have identified in the nearside maria occur in central Mare Tranquillitatis. Inferred TiO2 contents for these areas are >9 wt % and are spatially consistent with the highest-TiO2 regions mapped previously at lower spatial resolution. We note that the morphology of surface units with high 400/560 nm ratio values increases in complexity at higher spatial resolutions. New telescopic spectra of landing sites successfully reproduce the Charette relation, although we find that the 400/560 nm value is strongly a fimction of the sample area size. The increased spectral contrast of the 400/730 nm ratio over the 400/560 nm ratio demonstrates the potential increased precision with which the 400/730 nm ratio might be used to estimate TiO2 abundances. Comparisons have been made with previously published geologic maps, Lunar Orbiter IV, and ground-based images, and some possible morphologic correlations have been found between our mapped 400/560 nm ratio values and volcanic landforms such as lava flows, mare domes, and collapse pits.

A reevaluation of spectral ratios for Lunar Mare TiO2 mapping

The empirical relation established by Charette et al. [1974] between the 400/560 nm spectral ratio of mature mare soils and weight percent TiO2 has been used extensively to map titanium content in the lunar maria [e.g., Johnson et al., 1977; Johnson et al., 1991b]. Relative reflectance spectra of mare regions show that a reference wavelength further into the near-IR, e.g., > 700 nm, could be used in place of the 560 nm band to provide greater contrast (a greater range of ratio values) and hence a more sensitive indicator of titanium content. An analysis of 400/730 nm ratio values derived from both laboratory and telescopic relative reflectance spectra suggests that this ratio provides greater sensitivity to TiO2 content than the 400/560 nm ratio. The increased range of ratio values is manifested in higher contrast 400/730 nm ratio images compared to 400/560 nm ratio images. This potential improvement in sensitivity encourages a reevaluation of the original Charette et al. [1974] relation using the 400/730 nm ratio.

Multispectral Imaging from Mars PATHFINDER

The Imager for Mars Pathfinder (IMP) was a mast-mounted instrument on the Mars Pathfinder lander which landed on Mars Ares Vallis floodplain on July 4, 1997. During the 83 sols of Mars Pathfinders landed operations, the IMP collected over 16,600 images. Multispectral images were collected using twelve narrowband filters at wavelengths between 400 and 1000 nm in the visible and near infrared (VNIR) range. The IMP provided VNIR spectra of the materials surrounding the lander including rocks, bright soils, dark soils, and atmospheric observations. During the primary mission, only a single primary rock spectral class, Gray Rock, was recognized; since then, Black Rock, has been identified. The Black Rock spectra have a stronger absorption at longer wavelengths than do Gray Rock spectra. A number of coated rocks have also been described, the Red and Maroon Rock classes, and perhaps indurated soils in the form of the Pink Rock class. A number of different soil types were also recognized with the primary ones being Bright Red Drift, Dark Soil, Brown Soil, and Disturbed Soil. Examination of spectral parameter plots indicated two trends which were interpreted as representing alteration products formed in at least two different environmental epochs of the Ares Vallis area. Subsequent analysis of the data and comparison with terrestrial analogs have supported the interpretation that the rock coatings provide evidence of earlier martian environments. However, the presence of relatively uncoated examples of the Gray and Black rock classes indicate that relatively unweathered materials can persist on the martian surface.

Evaluation of the sensitivity of reflectance ratios to mafic minerals in the lunar regolith

The 950/560 nm spectral ratio has been traditionally used as a qualitative indicator or lunar soil maturity because the 950 nm band falls within the major Fe2+ mineral absorption bands near 1 µm characteristic of relatively immature, crystalline surfaces. Since there are several other processes unrelated to maturation that may affect this ratio value, we have evaluated the 950/730 nm ratio as a potentially better measure of the relative 1-µm band strength. We find that the 950/730 nm ratio appears to be much more sensitive to the 1 µm band depth, but feel that other unknown variables preclude using these single ratios to provide meaningful quantitative results at this time. Because of problems with near-IR scattering in vidicon detectors used in previous studies, we have measured the scattering properties of our two CCDs and conclude that any scattering is negligible.

Modeling of fluidized ejecta emplacement over digital topography on Venus

The FLOW computer model of McEwen and Malin (1989) modified for application to the study of Venus fluidized ejecta blankets (FEBs) demonstrates that relatively low viscosities, yield strengths, and initial velocities are required to duplicate the observed flow paths of the outflow materials. The model calculates the velocities and simulated flow paths of gravity flows over Magellan topography. The model is formulated to determine flow movements from initial conditions, gravitational acceleration, and resistance to motion as described by Coulomb, viscous, and turbulent resistance forces. Successful duplication of observed FEB flow paths has been achieved for the FEB craters Addams, Isabella, and Cochran. When used as a simple energy-line model, the model requires low coefficients of friction to extend FEBs to near their observed termini in the synthetic aperture radar (SAR) imagery, although the resulting straight flow lines do not follow the observed flow paths well. For Bingham flow, the model requires low values of viscosity and yield strength which are more similar to pyroclastic or debris flows than basaltic lavas. Flows of 100-m depth require 1 to 2 orders of magnitude higher values of both viscosity and yield strength than 10-m-deep flows. The complicated nature of the flow lines for the low velocity model suggests that FEBs were probably emplaced under variably laminar and turbulent flow conditions, where underlying topography influenced both the direction and energy of flow materials.

Using Observer Models to Predict Softcopy Performance With Digital Mammography

The goal of this study was to evaluate reader performance viewing softcopy mammograms as the type of phosphor in the monitor faceplate changes from PI04 to P45. The human observer results were then compared to the output from a human visual system model that was developed by the Sarnoff Corporation for assessing the discriminability of targets embedded in complex scenes. The results indicate that performance for the human observer is better (for the detection of microcalcifications in mammograms) using the P45 monitor compared to the PI 04 monitor. Physical evaluation of the monitors indicates that the signal-to-noise ratio for the P45 is higher than for the PI04, which likely explains the difference in performance. The human visual stem model was effective in predicting the performance of the human observers.

SU‐C‐217A‐05: The Design of An Institution Wide Comprehensive Technique Chart for Size‐ Specific Radiography from Pediatrics to Adults

Purpose: A properly designed projection radiography technique chart enables adherence to maximum Entrance Skin Exposure (ESE) standards, avoidance of improper exposures, and optimization of image quality across devices and patient sizes. The goal of this project was to develop and “field test” a technique chart designed to accomplish these tasks. Methods: All radiographic anatomical examinations at our institution were characterized in terms of average body‐part thickness, kVp, grid use, and field size, across a group of eight patient sizes ranging from neonates to bariatric adults. Wide‐angle transmission fractions (TFs) were measured across a range of thickness, field sizes, and kVp, from which multi‐dimensional fits enabled TF to be estimated for any examination type. For each examination type, target ESE values necessary to deliver manufacturer‐specified detector exposure were determined. The maximum and target ESE values for patients other than average adults were propagated from the average adult values to those for each of the patient size groups using the estimated TFs. For a given x‐ray system, the maximum and the target mAs values were then estimated from the measured mR/mAs for that system for each of the anatomical examinations and patient sizes. Results: Generated mAs values compared reasonably well with empiric values used by technologists for average adult. However, there were notable discrepancies across devices and patient sizes. For large patients, the generated values were in general substantially larger than empiric values. The new technique chart is currently implemented across our institution. Conclusions: Using a universal technique chart enables a reduction in variability of acquisition across the imaging operation at a large medical center. This can substantially aid in improving the consistency of medical imaging and provide important guidance as how image quality and dose can be optimized across patient sizes and age groups.