Unfortunately, few of these features are visible to the naked eye. If a geologist could look down on the mineral-rich area of Chile around Carrera Pinto from space, she would see the drab gray landscape shown in the top picture of the three-photo sequence on the left (click on icon). But unlike humans, the U.S. Landsat satellite that recorded these images in 1985 also perceives light in the near-infrared and mid-infrared bands of the spectrum. These bands detect differences in the way light is reflected by pasture land, forests, and different kinds of soil and rock. By assigning an artificial color (green in this case) to the portion of the infrared band that detects vegetation, image-processing specialists at Earth Satellite Corp. produced the middle image. By assigning several other contrasting colors to light reflected by different types of rock, they produced the third image (bottom picture), which emphasizes the differences in the area's lithology.

If the geologist knows that a spot that shows up as bright pink on the map is the site of a rich copper mine, she can look for other spots with the same color "signature"-and use this information to zero in on new deposits.