Hovering on the Horizon

The limb of the Earth is a work of awesome beauty and a gift to science. When observed from space, the palette of gaseous layers of atmosphere reminds us of the fragility and tenuousness of the cocoon that shelters life from cold, harsh space. That same view also allows scientists to detect the gases and particles that make up our different layers of our atmosphere. Astronauts aboard the International Space Station captured a bit of both in this digital photograph from July 31, 2011. They threw in the Moon as an extra gift.

Closest to Earth's surface, the orange-red glow reveals Earth's troposphere—the lowest, densest layer of the atmosphere, and the one we live within. A brown transitional layer is the upper edge of the troposphere, known as the tropopause. A milky white and gray layer sits above that, likely a slice of the stratosphere with perhaps some noctilucent clouds thrown in. The upper reaches of the atmosphere - the mesosphere, thermosphere, and exosphere - fade from shades of blue to the blackness of space.

The different colors occur because the dominant gases and particles in each layer act like prisms filtering out certain colors of light. Instruments carried on satellites and on craft such as the space shuttle have allowed scientists to decipher characteristics of the ozone layer and the climate-altering effects of aerosols.

A thin crescent of the Moon is illuminated by the Sun below the horizon of the Earth. Though the Moon is more than 384,400 kilometers (238,855 miles) away, the perspective from the camera makes it appear to be a part of our atmosphere.