Final answer:
Triton is believed to be a captured moon due to its retrograde orbit and geological characteristics. These include its composition, similar to Kuiper belt objects like Pluto, and its cold, reflective surface.
Step-by-step explanation:
Astronomers believe that Triton is a captured moon primarily due to its retrograde orbit, which is in a direction opposite that of Neptune's rotation. This is unusual for a moon that would have formed from the same disk of material that created the planet, leading scientists to speculate that Triton may have originated from the Kuiper belt before being captured by Neptune's gravity. Additional evidence supporting the capture theory comes from Triton's composition and geological activity. Its high density indicates a composition of approximately 75% rock and 25% water ice, which is similar to Pluto and other Kuiper belt objects, further hinting at its origin from beyond Neptune's orbit. The existence of a polar cap that consists of frozen nitrogen and the extreme cold temperatures measured on Triton, which cause it to reflect about 80% of the solar light it receives, are also distinctive clues about its environment and origin.