It was a surprise to run across this photo on NASA's Flickr stream, showing the Amazon rain forest — something we expect to look like an emerald jewel. Instead, it looks like rust. Why? There's a fascinating explanation:

The image is a view of South America and portions of North America and Africa from MESSENGER’s Dual Imaging System’s. The wide-angle camera records light at 11 different wavelengths, including visible and infrared light. Combining blue, red, and green light results in a true-color image from the observations.

The image above substitutes infrared light for blue light in the three-band combination. The resulting image is crisper than the natural color version because our atmosphere scatters blue light. Infrared light, however, passes through the atmosphere with relatively little scattering and allows a clearer view. That wavelength substitution makes plants appear red. Why? Plants reflect near-infrared light more strongly than either red or green, and in this band combination, near-infrared is assigned to look red.

Apart from getting a clearer image, the substitution reveals more information than natural color. Healthy plants reflect more near-infrared light than stressed plants, so bright red indicates dense, growing foliage. For this reason, biologists and ecologists occasionally use infrared cameras to photograph forests. 

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