A large bright spot on Titan, Saturn's largest moon, has scientists and astronomers perplexed.

The international Cassini spacecraft has flown by Titan four times since December, and on each pass its imaging equipment - spectrometers and cameras - has recorded a bright, semicircular area on the moon's surface up to 550 kilometers wide.

The spot is the brightest area ever observed on Titan. Scientists think it might be a "hot spot" - possibly caused by a recent asteroid impact, or evidence that Titan has a warm interior. Others say the spot could be a reflection from some unusual material on the moon's surface, or a large mass of clouds.

Cassini's next pass by Titan on July 2 will be at night. Scientists will be watching for a glow that would tell them the mysterious area is hot.