Remote Sensing: A Distant View of Chemistry FOCUS
Image of NASA Ames Research Center acquired by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) aboard the NASA U-2 at an elevation of 20 km. The image is a composite of three 10-nm wide bands centered at 544,633, and 803 nm. The faces of the parallelpiped show the color-coded, relative reflec tance spectra for each of the 358 pixels along the top and 614 pixels along the right-hand edge of the image. Each spectrum covers the 0.4 to 2.5 μτη region from front to back. Black and blue represent low values; red and white represent high values of reflectance. Atmospheric water absorption bands at 1.4 and 1.9 μm are represented by two black stripes. North is to the right. (Processed by the Center for the Study of Earth from Space, University of Colorado, Boulder.)
Imagine an animal that sees hundreds of colors well beyond the reddest red. It views not only the visible universe but also the invisibly colorful displays from molecular motions—an infrared chore ography of stretching, bending, and vi brating atoms. Imagine further that the beast flies high above the Earth and sees at once large areas of the planet's surface. It then would remotely view the chemical signals coming from a forest canopy or mineral outcrop. Individual chemicals, with their own troupe of molecular dancers, would have unique displays of humanly unimaginable colors that might advertise their presence to this beast. You are imagining a remote-sensing beast that already is beyond scientific imagination; it has taken on electronic, glass, and metallic flesh. In its first form, the instrument—the Airborne Imaging Spectrometer (AIS)—flew aboard the National Aeronautics and Space Administration's (NASA) huge C-130 plane between 1983 and 1987. Although not yet ready for routine operation, high-flying spectrometers with spectral and spatial resolution so fine that they can function as geochemical imagers are becoming a novel cate gory of earth-watching tools. The scope of their vision is just what the research community needs for learning how chemical cycles work on a global scale, says Alexander F. H. Goetz, who, as NASA's former imaging spectrometer program chairman, brought AIS and its instrumental progeny into this world. Now mothballed, the AIS served mostly as a testbed for new detector technologies, says Goetz, who now heads the Center for the Study of Earth from Space, a part of the Cooperative Institute for Research in Environmen tal Studies (CIRES). CIRES is run jointly by the National Oceanic and At mospheric Administration and the University of Colorado at Boulder, where Goetz also is professor of geolog-
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FOCUS ical sciences. "With AIS, we were test ing the concept of doing reflectance spectroscopy like you do in a laborato ry, but from an airborne platform in stead," Goetz told ANALYTICAL CHEMISTRY.
AIS begat the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS), which is beginning to gather spectral data from its perch on NASA's super-high-flying ER-2, a modified U-2. The third-generation imagers— High-Resolution Imaging Spectrome ter (HIRIS) and Moderate-Resolution Imaging Spectrometer (MODIS)—will embody lessons learned from their in strumental precursors. They are slated to scan the planet in the mid-1990s from a space platform as part of the Earth Observing System (Eos). "Ideal ly, these instruments will 'see' chemis try of large areas," Goetz said. With more and more urgency, scien tists are striving to develop a global perspective on the distribution of ma terials in the oceans, lands, animals, plants, and skies and determine how materials move within and between these vast reservoirs. Detailed field studies of, say, a small forest here and a desert area there, provide a crucial, though patchwork, picture wrought with vast data holes that are filled by model-guided extrapolation. Reliable remote sensing using imag ing spectrometers could help scientists to fill in the gaps with hard numbers. Such spectrometers would enable re searchers to sample huge contiguous areas of the surface, says Walter Westman, an environmental scientist at the Environmental Policy Analysis Unit of Lawrence Berkeley Lab in Berkeley, CA. Westman, who has worked with the AIS instruments, stresses that the ideal remote-sensing imager is years away. "Research to tie remote-sensing vari ables to global ecological models is now in its early stages," he wrote in a recent article that appeared in Trends in Ecology & Evolution (2). It's one thing to interpret the unperturbed transmis sion spectrum of a liquid in a cuvette, Westman says, but quite another to un derstand the meaning of a reflectance spectrum that has traveled through trees, leaves, soil, and air.
and 1987 before giving way to the newer AVIRIS that now flies aboard the ER-2. "AIS was the first imaging spectrom eter to look at the Earth," said Goetz. Although the orbiting Landsat scan ning systems can view the Earth in sev en spectral bands, the resolution with in the bands is insufficient for distin guishing particular materials in a mix ture. Not so for the AIS instruments. They viewed the spectral ranges from 1.2 to 2.4 μπι in 128 contiguous bands at about a 10-nm resolution—good enough for distinguishing hundreds of geochemically, ecologically, and biolog ically interesting compounds. AIS-I used a 32 X 32 array detector, AIS-II a 64 X 64 array. Each element was made out of infrared-sensitive HgCdTe sandwiched with a silicon charged-coupled device multiplexer. The toasteroven-sized instruments flew in the bel ly of a C-130. A stepping motor nudged the spectrometer gratings through the 128 bands during the time it takes for the plane to fly 1 pixel width on the ground. This translates into a mini mum ground resolution of 12 X 12 m when the plane is flying at 6 km. This resolution is crisp enough for many geological and geobotanical applica tions, researchers say. AVIRIS. The Airborne Infrared Im aging Spectrometer is the next instru ment in the series. It was completed last year and at press time was starting to gather remotely sensed spectral data. Unlike AIS, AVIRIS covers both the infrared and visible regions. In stead of area array detectors, which re-
quire vast computer support, AVIRIS is built with line arrays of silicon and indium antimonide elements. They im age 550 pixels across in 224 contiguous spectral bands ranging from 0.41 to 2.45 μπι, again at 10-nm resolution. Any diagnostic benefit gained by a fin er spectral resolution would be more than offset by a decreased signal-tonoise ratio and an unmanageable del uge of data, Goetz says. The spectral range is analyzed with a set of four spectrometers connected to the scan ning head by fluoride polymer optical fibers (see Figure 1). Because AVIRIS flies in the ER-2, which cruises at 65,000 ft or more, the instrument can see larger swaths than its lower flying predecessors. The AVIRIS "sees" roughly 11 km 2 of ground with a pixel resolution of about 20 m2, fine enough to detect segregated regions of tree death in a forest or mineralogical variations in a region such as Nevada's Cuprite mining district. HIRIS. The High-Resolution Imag ing Spectrometer will be part of the Eos, a multisensor platform designed to obtain Earth sciences data on the global scale that is planned for the mid dle of the next decade. It will have a spatial resolution of 30 m, a purview of 30 km, and a spectral range of 0.4-2.5 μπι at a 10-nm resolution, allowing for roughly 200 discrete bands. Experi ments with the AVIRIS will contribute to the final design of the HIRIS. Getting the hang of remote sensing In a paper published this year in Na ture, Carol Wessman and colleagues (2) reported using spectral data ac-
The new breed of imaging spectrometers AIS. When NASA began funding the imaging spectrometer program in 1981, engineers at the Jet Propulsion Lab oratory (JPL) in Pasadena, CA, built the first two of a series of high-spectralresolution imaging spectrometers: AIS I and AIS II. These flew between 1983
Figure 1. Diagram of the AVIRIS sensor showing four spectrometers connected to the scanning head with optical fibers.
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FOCUS quired with the AIS-II to make "the first accurate estimation of lignin content of whole forest canopies by remote sensing." Lignin, a stable carbohydrate polymer, is a major constituent of plant cell walls. It makes the plants rigid and unpalatable to insects that might otherwise eat them. Lignin also correlates well with a region's fertility, presumably because its decomposition makes the cell wall leaky and allows nitrogencontaining nutrients inside the cells to re-enter the ecosystem. The amount of bioavailable, or mineralized, nitrogen in an area, in turn, relates to the area's potential productivity. By remotely sensing signals linked to the lignin content of a forest canopy, and then hooking up these signals to a model describing the relationship between lignin and nitrogen, the researchers hope to be able to remotely sense nitrogen availability and forest productivity. The results of the study demonstrate the promise of remote sensing of key biogeochemical indicators, according to Goetz and other AIS researchers. But uncertainties about how the detected raw signals relate to canopy lignin demand interpretational caution.. Wessman notes that the "lignin spectrum has not been characterized adequately because of difficulties in extracting lignin (an amorphous polymer) from foliar tissue in a pure, isolated form." In addition, the two spectral lines and one broader spectral region that the researchers used as a lignin signature were empirically selected. "We aren't certain that we are actually seeing lignin features in the spectrum," Wessman said. Walter Westman adds that the "vibrational overtones of the C—OH bond will occur as a result of the abundance of such bonds in cellulose, starch, sugar, lignin, and other carbohydrates in the leaf." The provisional lignin signature that Wessman used in her study also is highly vulnerable to spectral interference by water. Consequently, Westman says he doubts that the remotely sensed signals contain enough information to distinguish lignin from other materials that have similar reflectance spectra. One way to mitigate the danger of such spectral monkey wrenches is to learn how in situ interferences actually affect the reflectance spectrum and then work these findings into the computer program that filters, corrects, and manages data coming from the imaging spectrometer. For example, Westman has stacked dried leaves from different tree species in a cuvette to see how leaf, anatomy, and thickness
BLACKHAHK ISLAND· WISCONSIN
PERCENT FOREST CANOPY LI6NIN
Canopy lignin concentration for Blackhawk Island, WI, as predicted by a linear combination of three transformed Airborne Imaging Spectrometer bands. Changes in concentrations from left to right (west to east) across the island reflect a soil texture gradient resulting from sediment sorting when the island area was an early postglacial flood plain.
Rates of annual nitrogen mineralization—a measure of how much nitrogen is bioavailable—for Blackhawk Island, WI, as estimated from AIS-predicted canopy lignin concentrations. affect the spectra. "There is a desperate need for qualified spectroscopists to get involved in this area," Westman stressed. Although he portrays the complexities of remotely measuring chemical constituents as daunting, Westman suspects that most of the problems can be
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solved. "And if we can develop a reliable tool for detecting canopy chemistry remotely, it would have enormous implications for ecology, agriculture, and global chemistry management," he said. For example, forest managers and farmers would benefit by having chemical information about the entire region
FOCUS they are responsible for—not just sam pled regions that may not be represen tative of the whole. Also, hard-to-reach tropical forests for which little data ex ist would be easily monitored with the AVIRIS or HIRIS. "Our goal is to develop methodolo gies and approaches to global ecology," said Wessman. "We want to measure productivity on a global scale." Her canopy lignin/nitrogen study is a first step toward that goal, she says. Her co workers on that project were John Aber of the University of Wisconsin, David Peterson of NASA Ames Research Center (Moffet Field, CA), and Jerry Melillo of the Ecosystems Center at the Marine Biological Laboratory (Woods Hole, MA). In another preliminary study, Nancy A. Swanberg and Pamela Matson of NASA Ames Research Center altered small areas of canopy chemistry to see if the AIS could notice any difference from unperturbed cohorts. They pre sented their study last year at the Third Airborne Imaging Spectrometer Data Analysis Workshop held at JPL. The scientists compared AIS data from four groups of 25 m X 25 m plots of Douglas fir trees near Mt. Taylor in New Mexico. Test plots were fertilized, irrigated, or treated with sawdust. Con
trol plots were left alone. Laboratory analyses of nitrogen, phosphorus, amino acids, lignin, starch, cellulose, and proteins of fresh foliar samples, which were collected monthly, corrobo rate that canopy chemistry does reflect the different treatments. For instance, leaves from fertilized plots contain "markedly higher concentrations of ni trogen than those from other treat ments," according to Swanberg and Matson (3). Researchers admit that signs of these chemical differences are difficult to discern in AIS data. But they expect that a better understanding of the dy namics of reflectance spectra and of spectral interferences will enable them to include correcting factors. Remote sensing with an imaging spectrometer will also help Earth scientists find and monitor specific minerals, according to Goetz. Several years ago, Goetz and co workers (4) used AIS data to demon strate that they could use remotesensing data to identify specific miner als. Flying over the Cuprite mining dis trict, the AIS gathered data that the scientists interpreted as signs of kaolinite [Al2(Si205)(OH)4] and alunite [KA1 3 (S0 4 )(0H) 6 ]. The scientists used laboratory reflectance spectra of field
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samples to validate the remotely sensed spectra. "Direct identification of Earth sur face materials on the basis of their unique spectral reflectance features is now possible with imaging spectrome ter systems in aircraft and eventually in spacecraft," the researchers con cluded (4). Since then, imaging spectroscopy has even made its debut in the com mercial world. Geophysical Environ mental Research, a small New Yorkbased company, flies an imaging spec trometer aboard a light aircraft that is used mostly for mineral and oil ex ploration. Hurdles ahead Without remote sensing, researchers end up looking at tiny areas of the sur face in detail and then extrapolating to larger areas or even to global scales. "With remote sensing, you can do glob al sampling," Goetz said. Models of global chemical cycles will become far more specific and robust because far fewer assumptions and guesses will go into their construction. In addition, from the orbital perspective of the HIRIS, scientists will be able to moni tor global chemical cycles over time. "But we're a long way from being there," Goetz warned. Remote sensing rarely generates data that are easily matched to the data acquired in lab oratory analyses. Scientists must bridge this gap if remote sensing of Earth chemistry is to become a reality, Westman stressed. Models of how raw reflectance spectra detected by imag ing spectrometers hook up with the spectral sources will make or break this technology, he said. "We need interdisciplinary teams of scientists to do this," added Goetz. "When we started in 1980, we had nei ther the optics, nor the detectors, nor the computer capability to deal with data." Since then, these remote sensing beasts have been refining their chemi cal eyesight. "We now have the optics, pretty much the computer muscle, and the detectors are nearly there. It is all coming together," Goetz said. Ivan Amato References (1) Westman, W. E. Trends in Ecology & Evolution 1987,2(11), 333-37. (2) Wessman, C. Α.; Aber, J. D.; Peterson, D. L.; Melillo, J. M. Nature 1988, 335(6186), 154-56. (3) Swanberg, Ν. Α.; Matson, P. A. In Pro ceedings of the Third Airborne Imaging Spectrometer Data Analysis Workshop; Jet Propulsion Laboratory: Pasadena, CA, 1987; pp. 70-74. (4) Goetz, A.F.H.; Vane, G.; Solomon, J. B.; Rock, Β. Ν. Science 1985, 228(4704), 1147-53.
