Mercury is the closest planet to the Sun. Because it has virtually no atmosphere (its gravity is so low that it cannot hold gases and liquids to its surface), Mercury has very extreme temperature variations.
The only spacecraft to explore Mercury close-up was Mariner 10 which completed 3 flybys of Mercury in 1974 and 1975.
Reprocessing of the Mariner 10 data has resulted in this beautiful mosaic. Like the Earth's Moon, Mercury's surface shows the scars of impact cratering. The smooth vertical band and patches represent regions where no image information is available.
Does Mercury emit radio waves?