NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
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A JPL infrared experiment flown on NASA's space shuttle last November individually identified minerals on Earth's surface from an altitude of l50 miles -- apparently even differentiating between two types of clays that are so similar they can only be seperately identified through laboratory testing.
The initial results of the Shuttle Multispectral Infrared Radiometer (SMIRR) were announced today at the annual meeting of the American Society of Photogrammetry in Denver, Colo.
The success of the experiment promises dramatic improvements in future satellites for geological mapping and for locating hidden mineral and petroleum resources.
In some cases, SMIRR surpassed field techniques in identifying minerals. For example, fine-grained clay minerals like montmorillonite and kaolinite, found in weathered rocks and widely distributed on Earth's surface, are so similar that they can only been differentiated from one another through laboratory testing. The SMIRR experiment, however, appears to have been able to indiv- idually recognize the two clays from the shuttle payload bay, l50 miles above Earth's surface. (Verification of the kaolinite identification was made by collecting surface sample. However, no samples have yet been obtained in areas containing montmorillonite.)
The experiment's ability to differentiate between different kinds of clays is one of the most important results for geologists, because mapping clays in sedimentary rocks is important for locating petroleum deposits, and clays are surface clues for buried metal deposits like copper, gold and silver.
SMIRR was planned by Principal Investigator Dr. Alexander F.H. Goetz of JPL and Co-Investigator Dr. Lawrence C. Rowan of the U.S. Geological Survey at Reston, Va. The instrument was developed and built at JPL.
The SMIRR experiment successfully sampled 80,000 kilometers (50,000 miles) of Earth's surface in the eastern U.S., southern Europe, north Africa, the Middle East and China.
SMIRR identified minerals on the basis of their spectral reflectance. Each mineral reflects and absorbs visible and infrared light in unique way. This reflectance or "signature" can be determined with spectrometer in the field or on satellite. Past satellite measurements, however, have been unable to definitively identify most minerals, particularly clays and carbonates, that are important for geologic studies.
Data from the SMIRR experiment will help determine which bands from the infrared portion of the spectrum should be used in future Earth-observational satellite systems.
SMIRR's global sample of different geological types in variety of climates may reveal unknown relationships that exist between surface and subsurface materials. The results will also show the effect of varying climatic environments on the signatures of similar rock types, and show how different amounts of water vapor in the atmosphere affect the quality of SMIRR data.
The SMIRR experiment began operating about four-and-a-half hours after launch and was controlled throughout the flight by commands from the ground.
SMIRR acquired three hours and six minutes of data over Africa, Asia, and the Middle East, Europe, Mexico and the U.S. Two hours were found to have been taken over cloudy or partly cloudy areas, while one hour of prime data was obtained over totally cloud- free land areas.
As the SMIRR data is processed, computer-assembled maps will be produced, and each l00-meter (328-foot) diameter area will be tagged with color according to its predominant rock type. Because the SMIRR is not an imaging device, photographs are necessary to geographically locate the instruments' readings. Two l6-millimeter cameras, one color and one black-and-white, were aligned with the SMIRR telesocpe to provide accompanying images to the SMIRR data.
Analysis has shown that the cameras remained perfectly aligned after launch stresses.
Scientists at JPL and the U.S. Geological Survey gathered spectral data with field spectrometer to determine the best spectral bands for use on future orbiting multispectral scanners (like the Landsat geological satellite series), to distinguish different classes of rock.
These studies indicated that bands in the l.0 to 2.5 micrometer region could perceive subtle differences between similar geological units. SMIRR tested ten bands within that region to determine their effectiveness in identifying surface materials from orbit, which will aid in building orbital systems to produce global maps showing areas where mineral deposits are likely to be found.
Due to delay in the launch of the shuttle and its shortened flight, data over Australia, southern Africa and South America were missed. the experiment was to have collected data primarily in the Western U.S., where the majority of ground-based field reflectance measurements were obtained, but clouds covered the area during most of the flight. SMIRR team members will collect ground-based reflectance measurements in other areas where SMIRR obtained data.
The SMIRR system consists of l7.8 centimeter (seven-inch) diameter telescope (a modified version of the telescope used on the Mariner l0 mission to Venus and Mercury in l973), filter wheel, two detectors, two film cameras and supporting electronics.
The unit weighs 99 kilograms (2l8 pounds), measures 56-by 94-by-ll7 centimeters (22-by-37-by-46 inches), and carries an opaque cover which was rotated over the top of the telescope to protect the optics when the experiment was not in operation.
The filter wheel contains l5 evenly spaced positions. Every third position is opaque to provide zero base for the detector electronics. The remaining l0 positions contain filters to sample the spectral bands of interest.
Two mercury-cadium-telluride detectors convert photons to electrons, which comprise the transmission signal. Timing and control electronics coordinate the filter wheel, the cameras and the detector readout.
In the time that an individual filter is in the telescope's optical path (the filter wheel spins at l00 revolutions per second), the detector electronics assembly amplifies and integrates the signal, then converts the signal from analog to digital form for recording on the payload recorder.
The SMIRR experiment was developed by JPL for NASA's Office of Space and Terrestrial Applications (OSTA).