Possible ice cap on Pluto

This series of New Horizons images of Pluto and its largest moon, Charon, was taken at 13 different times spanning 6.5 days, starting on April 12 and ending on April 18, 2015. During that time, the NASA spacecraft’s distance from Pluto decreased from about 111 million kilometers to 104 million kilometers. 

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

The pictures were taken with the New Horizons Long Range Reconnaissance Imager, or LORRI. Pluto and Charon rotate around a center-of-mass (also called the “barycenter”) once every 6.4 Earth days, and these LORRI images capture one complete rotation of the system.

In the annotated version, a 3x-magnified view of Pluto is displayed in the inset to the lower right, highlighting the changing brightness across the disk of Pluto as it rotates. Because Pluto is tipped on its side (like Uranus), when observing Pluto from the New Horizons spacecraft, one primarily sees one pole of Pluto, which appears to be brighter than the rest of the disk in all the images. Scientists suggest this brightening in Pluto’s polar region might be caused by a “cap” of highly reflective snow on the surface. The “snow” in this case is likely to be frozen molecular nitrogen ice.