Image Credit: Reto Stöckli for the NASA GSFC Earth Observatory

This image of Earth, known as the "Blue Marble" from NASA, is so familiar that most of us consider it to be our planetary self-portrait. But did you know that the Earth does not actually emit any of the light in this famous image? Rather, all of the light seen here is reflected from the Sun. Earth does give off some light, but not in the form we can see with our eyes. Our planet emits some infrared radiation, or heat, which is energy absorbed from the Sun and re-radiated at longer wavelengths. A small amount Earth's infrared radiation–a fraction of a percent–also comes from the radioactive decay of elements in Earth's mantle and crust as well as residual heat left over from when the planet formed. Scientists can detect that infrared light using special instruments and detectors.

Download high-res image file | Download caption as .zip file