Triton's Surface. This color photo of Neptune's large satellite Triton was obtained on Aug. 24 1989 at a range of 530,000 kilometers (330,000 miles). The resolution is about 10 kilometers (6.2 miles), sufficient to begin to show topographic detail. Credit: NASA
Mercury is the closest planet to our Sun, but oddly enough, not the hottest. Since Mercury has no atmosphere, more of the Sun's heat radiates away from it whereas Venus, even though it's further away from the sun, has a thick greenhouse atmosphere trapping and retaining the Sun's heat and making it hotter.