It may not be a Double Rainbow, but it's a Lunar Rainbow! How could this be possible? it doesn't rain there. The moon doesn't even have an atmosphere... so how could the Lunar Reconnaissance Orbiter's camera capture this phenomenal phenomenon?
The LROC's Wide Angle Camera uses filters to capture images, capturing three different wavelengths: a 689nm filter for red, a 643nm filter for green and a 604nm filter for blue. To get one image, the spacecraft's camera shoots three quickly consecutive images, which are then combined by the computer to create one single image.
When the sun is directly overhead the Lunar surface with the LRO in between, a phenomenon called "opposition surge" happens - a sudden increase in the surface brightness, with no shadows on the surface. As the light reaches the LROC's camera, it interferes with itself. And as this happens, the filters change, capturing the shining at sightly different moments. When the software combines the resulting images, you can observe the light shifting in the form of a rainbow.
According to Brett Denevi at the LROC blog, the images "provide a huge new dataset for studying how light interacts with a particulate surface at different wavelengths. Perhaps an esoteric-sounding field of study, but this data can help us understand the reflectance images and spectra we have of the moon and other bodies throughout the Solar System."
OK, Brett. I believe you, but I was really expecting an explanation that included unicorns. Or at least, lunarprechauns. [LROC]