Captured Moon - The pink hue of Neptune's largest moon, Triton, is thought to result from a slowly evaporating layer of nitrogen ice. Triton is an oddity among moons in that its orbit is highly tilted to the plane of Neptune's equator, and it is in a retrograde orbit. These facts have led scientists to believe that Triton formed independently of Neptune and was later captured by Neptune's gravity.
Image and text from JPL/NASA