A Productive History
To improve his odds, Anemone turned to Charles “Jay” Emerson, a geography professor who is also based at Western Michigan University. Emerson specializes in interpreting satellite images, including those from the Enhanced Thematic Mapper Plus (ETM+) on the Landsat 7 satellite.
Since 1972, Landsat satellites have observed our planet’s forests, deserts, cities, farms—and badlands. The few paleontologists who have been willing to take advantage of the technology have benefited because, even though Landsat can’t see fossils, it can see the kinds of rocks where they are likely to reside. This is partly because Landsat can see wavelengths of light that human eyes can’t detect.
Anything in the universe that is warmer than absolute zero (-273 degrees Celsius) emits electromagnetic radiation. All of that energy—from gamma rays to X rays to radio waves—is referred to as the electromagnetic spectrum. “The retina of the human eye responds to a limited portion of the electromagnetic spectrum,” Emerson explains. The radiation just beyond the human range of vision includes ultraviolet light (shorter wavelengths than blue light) and infrared light (longer than red light).
In addition to blue, green, and red wavelengths, the ETM+ sensor on Landsat 7 can detect infrared radiation, which enables it to distinguish—sometimes better than human eyes—between different rock types. The minerals that make up Earth’s rock layers have distinct chemical compositions and crystalline structures, and tend to reflect different wavelengths in unique ways. The differences occur in visible and infrared wavelengths, or what remote sensing scientists refer to as spectral bands. “Using the seven discrete spectral bands in the Landsat ETM+ sensor,” Emerson says, “image analysts can use different combinations of reflectance to identify different land cover materials.”
Anemone and Emerson aren’t the first scientists to turn to remote sensing for fossil hunting. One of their colleagues at the Denver Museum of Nature and Science, Richard Stucky, started working with NASA to explore Landsat’s potential in the mid-1980s. Stucky and colleagues produced a series of “false terrain” images designed to show contrast between different rock types.
“Suppose you have a landscape with rock layers from different geologic periods and very clear separations—what we call ‘unconformities’—between them,” Stucky says. “And suppose you know that one rock layer has high silica content and another has high clay content.” By acquiring satellite images of the unconformities and then confirming the rock types through on-site human inspection, Stucky and other researchers thought they could create computer software to distinguish between the rocks.
In the early days, Stucky had doubts about whether the approach would work. “I remember one image that showed a black dot in the middle of this big exposure, and we couldn’t imagine what it might be,” he says. He was worried that Landsat wasn’t showing the land surface accurately, and if so, it wouldn’t be much use in locating fossil-rich sites. “We went out and looked, and it turned out to be a trash dump. Landsat picked up on the iron content in the abandoned cars.” The ground-truth exercise allayed Stucky’s early fears.
Detecting an iron-rich trash dump isn’t the same as distinguishing between different types of rock, but it did show that Landsat sensors could take advantage of the different ways that minerals reflect light. Stucky was impressed with the early results. “They weren’t perfect, but they were pretty good. It was a promising start.”