Nearly three months after Tolbachik began erupting, lava continues to flow from the Kamchatkan volcano. Over time, the lava flows change location and shift across the landscape. In this image, infrared data is superimposed on a natural-color image to highlight active flows. The image was collected by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite on February 14, 2013.
The infrared light detectable by some satellite sensors provides information that is invisible to the human eye. Humans see a spectrum of light in a range of colors from violet to red. The colors are related to the wavelength of the light, which also corresponds to energy. Violet light has the shortest wavelength and highest energy; red light the longest wavelength and lowest energy. Infrared light has a longer wavelength, and even lower energy than red light. Like the individual colors of the visible spectrum, infrared light is also subdivided into categories. From shortest to longest wavelength: near infrared, shortwave infrared, and thermal infrared.
Most of the light we see (and detected by satellites) is reflected light from the Sun or another source; that light bounces off a surface and into our eyes, a camera, or a satellite instrument. In addition to reflecting light, all surfaces also emit light, at a wavelength determined by temperature. The hotter the object, the shorter the wavelength and higher the energy. At room temperature, most emitted light is long wavelength thermal infrared and therefore invisible to humans. Only very hot objects emit visible light (like an electric burner or the filament of an incandescent lightbulb).
Lava hotter than 900° C (1,700° F) glows red. As it cools, lava stops glowing visibly, but will continue to glow in lower energy wavelengths of near infrared and shortwave infrared light.
On Tolbachik volcano, the Advanced Land Imager detected the infrared light emitted from recent and ongoing lava flows. Near infrared light detected by ALI is colored yellow, and shows the erupting vent and an active lava flow, open to the air. Areas glowing in shortwave infrared light, including the fringes of the active flow, are tinted red. These surfaces are hotter than the frigid winter air of Kamchatka, but cooler than the channelized flowing lava. To the east, scattered red pixels reveal residual heat on a flow formed in early February.
NASA Earth Observatory image by Jesse Allen and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team. Caption by Robert Simmon.