Italy’s Etna volcano boiled over on February 19–20, 2013, with three outbursts in 36 hours, after maintaining a low simmer for ten months. The Advanced Land Imager (ALI) on the Earth Observing-1 (EO-1) satellite captured Mount Etna on February 19 at 9:59 a.m. Central European Time, about 3 hours after the end of the first paroxysm. Image © NASA Earth Observatory/ Jesse Allen and Robert Simmon
The false-color image combines shortwave infrared, near-infrared, and green light in the red, green, and blue channels of an RGB picture. This combination makes it easier to differentiate between fresh lava, snow, clouds, and forest.
In the image, fresh lava is bright red, as the hot surface emits enough energy to saturate the instrument’s shortwave infrared detectors but is dark in near-infrared and green light. Snow is blue-green because it absorbs shortwave infrared light, but reflects near-infrared and green light. Clouds made of water droplets (not ice crystals) reflect all three wavelengths of light similarly and appear white. Forests and other vegetation reflect near-infrared more strongly than shortwave infrared and green, and so appear green. Dark gray areas are lightly vegetated lava flows, 30 to 350 years old.