This image is stitched together from thousands of observations made over the past 4 years by the MASCS/UVVS instrument, which measures sunlight scattered off of Mercury's tenuous atmosphere. Scattered sunlight gives the sodium a bright orange glow. This scattering process also gives sodium atoms a push - this "radiation pressure" is strong enough, during parts of Mercury's year, to strip the atmosphere and give Mercury a long glowing tail. Someone standing on Mercury's nightside at the right time of year would see a faint orange similar to a city sky illuminated by sodium lamps!
Instrument: Mercury Atmospheric and Surface Composition Spectrometer (MASCS)/Ultraviolet and Visible Spectrometer (UVVS)
The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. In the mission's more than four years of orbital operations, MESSENGER has acquired over 250,000 images and extensive other data sets. MESSENGER's highly successful orbital mission is about to come to an end, as the spacecraft runs out of propellant and the force of solar gravity causes it to impact the surface of Mercury in April 2015.
For information regarding the use of images, see the MESSENGER image use policy.