John W. Salisbury

Emissivity of terrestrial materials in the 3–5 μm atmospheric window☆

Remotely sensed infrared radiance emitted by a surface is a function both of its kinetic temperature and its spectral emissivity. Consequently, assumptions are usually made about the emissivity of earth surface materials to allow their temperatures to be determined, or vice versa. To increase the accuracy of these assumptions, the directional hemispherical spectral reflectance of a wide range of natural earth surface materials has been measured and is summarized here. These include igneous, metamorphic, and sedimentary rocks, desert varnish, soils, vegetation, water, and ice. Kirchhoffs Law can be used to predict directional spectral emissivity from these data.

The role of volume scattering in reducing spectral contrast of reststrahlen bands in spectra of powdered minerals

Abstract As particle size decreases, the spectral contrast of reststrahlen bands also decreases. It has been generally agreed that this loss of spectral contrast is due to the increased porosity associated with fine particle size, resulting in formation of photon traps. That is, that the pores acted like small black bodies. However, we show here that the reststrahlen bands change in shape as well as intensity, and this change in shape can only be explained by the occurence of substantial volume scattering at fine particle size, rather than by photon trapping. It appears that the role of porosity is to physically separate 1- 5-μm-diameter particles that are optically thin, even in the reststrahlen bands. When such particles are separated by more than a wavelength, they scatter independently as optically thin, volume-scattering particles. When packed closely together, however, they scatter coherently as if they were large, optically thick, surface-scattering particles. Thus, the loss of spectral contrast of reststrahlen bands for fine particle size materials appears to be due directly to particle size and only indirectly (but critically) to porosity. Coarse particle size materials may also display greatly reduced spectral contrast and altered band shape in their spectra if the larger particles are coated with clinging fines. The practical implication of this finding for remote sensing of the Earth, Moon, Mercury, Mars, and the asteroids is that the spectral features displayed by some particulate materials may be substantially changed from those seen in spectra of solids, requiring the use of a separate spectral search library to identify component minerals.

The mid-infrared reflectance of mineral mixtures (7–14 μm)

Abstract There is growing interest in the mid-infrared spectral region (8–14 μm) as both a laboratory and a remote sensing tool in geology, because this portion of the spectrum contains the characteristic, fundamental, molecular vibration bands for silicates and other mineral groups. However, it is necessary to understand the relationship between the spectra of mineral mixtures and those of individual minerals in the mixture in order to completely interpret and predict mineral abundances from infrared data. Results of this study show quantitatively for the first time that the spectra of particulate mixtures of silicate minerals in this wavelength region combine linearly by volume within a very small error, as long as particles are much larger than the wavelength so that volume scattering is insignificant compared to surface scattering. Results here apply specifically to mineral samples in the 75–250 μm size range. They imply that we can predict the spectral response of a rock if the constituent minerals and their abundances are known. More importantly, our results indicate that the relative quantities of minerals in simple mixtures can be predicted to within 12% in the worst case, and more typically to within 5%. Consequently, geologists should be able to unmix the composite spectra of rocks to determine mineral abundances. This is important for both laboratory rock identification and remote sensing applications. By better understanding how component mineral spectra mix in the spectrum of a rock, we can also better choose spectral band positions and resolutions in infrared remote sensing for compositional identification.

Comparisons of meteorite and asteroid spectral reflectivities

Abstract Laboratory spectral reflectivities have been measured for 41 meteorites, including 22 ordinary chondrites of all chemical and petrologic types. They are compared with available spectral reflectivities for 36 asteroids. We find that absorption band depths, center positions, and breadths are diagnostic of the various types of ordinary chondrites, and that other meteorite types may also be distinguished by reflectivity characteristics. Asteroid spectral reflectivities approximately resemble those for meteorites (especially unequilibrated types), and the range of differences among asteroids is similar to that for meteorites. However, detailed comparisons show that few meteorites exactly match observed asteroids in spectral properties. Impact vitrification and shock can modify the spectral reflectance of materials, but vitrification, at least, is an unimportant process on asteroidal regoliths. Assuming the comparisons can be taken at face value, we conclude that ordinary chondrites either do not come from the main asteroid belt or they come from a few unusual asteroids. Some asteroidal/meteorite matches have been found for enstatite chondrites, a basaltic achondrite, an optically unusual L6 chondrite, and possibly a carbonaceous chondrite. Only 1685 Toro, an Earth-crossing asteroid, resembles typical ordinary chondrites.

Midinfrared (2.5–13.5 μm) reflectance spectra of powdered stony meteorites

Abstract Reflectance spectra (2.5 to 13.5 μm) of 60 powdered meteorite samples representing 50 different meteorites are presented for comparison with spectral measurements of asteroids. Powdered samples were used as analogues of asteroidal regoliths. These spectra show that most powdered meteorite samples have undergone alteration, even if only exposed to water vapor in the air, and many have been contaminated by volatile hydrocarbons characterized by absorption bands near 3.45 μm. In contrast, primary macromolecular hydrocarbons do not display the 3.45-μm bands, or, in fact, any other detectable spectral features in these reflectance spectra. However, powdered meteorites display a wide variety of other spectral features that can be used for their identification. These include residual reststrahlen features, which occur as reflectance peaks, absorption bands due to overtone/combination tone bands, which occur as reflectance troughs, and the Christiansen feature, which also occurs as a trough in reflectance. The most prominent reststrahlen peaks are those of olivine and pyroxene, and the wavelength position of the olivine features is sensitive to the Mg/Fe ratio. Olivine and pyroxene also show strong overtone/combination tone absorption bands that shift position with the Mg/Fe ratio, and calcite can be identified in the spectra of some carbonaceous chondrites from such overtone/combination tone bands. These spectral features can be used independently to help determine mineralogy and meteorite type, but using the entire spectrum in a digital search library is preferred for successfull identification. The spectral effect of the vacuum environment should be relatively small for meteorites compared to that found in previous work for granites, but confident interpretation of the spectra of asteroids will require a better understanding of the effects of the space environment on meteorite spectra and of the spectral mixing model for meteorite minerals.

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earths surface and atmosphere in the 3-5-µm and 8-14-µm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-µm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-µm and 8-14-µm atmospheric windows.

Infrared (8-14 μm) remote sensing of soil particle size

Abstract Particle size of soils plays a significant role in erosion potential and other mechanical properties. Most soils are dominated by the residual mineral quartz, which displays prominent reststrahlen bands in the 8–14 μm atmospheric window. The Earth Observing System (EOS) will likely provide world-wide multispectral imagery in the 8–14 μm region via the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument. The ratio of ASTER bands 10/14 can be used to estimate particle size in soils, if other ASTER bands are used to minimize the confusion factors provided by soil moisture, vegetation cover, soil organic content, and the presence of abundant minerals other than quartz. Use of band ratios minimizes the effects of poor surface temperature estimates, but maximizes the need for high signal-to-noise data.

First use of an airborne thermal infrared hyperspectral scanner for compositional mapping

In May 1999, the airborne thermal infrared hyperspectral imaging system, Spatially Enhanced Broadband Array Spectrograph System (SEBASS), was flown over Mormon Mesa, NV, to provide the first test of such a system for geological mapping. Several types of carbonate deposits were identified using the 11.25-μm band. However, massive calcrete outcrops exhibited weak spectral contrast, which was confirmed by field and laboratory measurements. Because the weathered calcrete surface appeared relatively smooth in hand specimen, this weak spectral contrast was unexpected. Here we show that microscopic roughness not readily apparent to the eye has introduced both a cavity effect and volume scattering to reduce spectral contrast. The macroroughness of crevices and cobbles may also have a significant cavity effect. The diminished spectral contrast is important because it places higher signal-to-noise ratio (SNR) requirements for spectroscopic detection and identification. This effect should be factored into instrumentation planning and interpretations, especially interpretations without benefit of ground truth. SEBASS had the required high SNR and spectral resolution to allow us to demonstrate for the first time the ability of an airborne hyperspectral thermal infrared scanner to detect and identify spectrally subtle materials.

Mars: Components of infrared spectra and the composition of the dust cloud

Abstract Infrared spectra of Mars are made up of three separate components, each of which may dominate the spectrum under different Martian meteorological and observational conditions. By means of laboratory examples we show that both the shape and spectral contrast of the spectral curves change dramatically, depending on which component is dominant. Each experimental condition has been experienced during either the Mariner 69 or 71 observations. Comparing the preliminary Mariner 71 radiance data with laboratory transmission spectra, we suggest that the clay mineral montmorillonite could be the major component of the Martian dust cloud.

Measurements of thermal infrared spectral reflectance of frost, snow, and ice

Because much of Earths surface is covered by frost, snow, and ice, the spectral emissivities of these materials are a significant input to radiation balance calculations in global atmospheric circulation and climate change models. Until now, however, spectral emissivities of frost and snow have been calculated from the optical constants of ice. We have measured directional hemispherical reflectance spectra of frost, snow, and ice from which emissivites can be predicted using Kirchhoff s law (e = 1-R). These measured spectra show that contrary to conclusions about the emissivity of snow drawn from previously calculated spectra, snow emissivity departs significantly from blackbody behavior in the 8–14 μm region of the spectrum; snow emissivity decreases with both increasing particle size and increasing density due to packing or grain welding; while snow emissivity increases due to the presence of meltwater.