Mercury's orbital period is just shy of 88 days, but its rotation around its axis was for a long time a mystery: sometimes its surface markings (to the extent telescopes managed to resolve this small planet) repeated in a way suggesting that, like the Earth's Moon, one of it sides always faced the Sun. But at other times, they did not. The puzzle was solved in 1965 by Han-Shou Liu and John A. O'Keefe, who showed the planet rotated in a strange resonance--3 rotations for every 2 orbits.
NASA's Mariner 10 mission flew by Mercury on March 29, 1974, and twice more in September and in March 1975. It observed a cratered surface resembling that of the Moon, and as expected, no trace of any atmosphere. Because the Sun is so close, the sunlit side of Mercury is hot enough to melt lead.
An unexpected feature of Mercury was its magnetism, apparently due to a magnetic core like Earth's. The field is weak, too weak to create a symmetric sheltered "magnetosphere" around the planet where charged particles may be trapped ("radiation belt"). However, its field lines are "dragged out" by the solar wind to create a long magnetic tail somewhat like the Earth's, and when Mariner 10 passed that region it unexpectedly observed a sudden impulsive acceleration event of charged particles.
The above image was taken on 14 January 2008 by the Messenger spacecraft operated for NASA by the Applied Physics Lab of the Johns Hopkins University in Baltimore (while an acronym for "Messenger" exists, the name also hints at Mercury's role as messenger of the ancient gods). Messenger is scheduled for two more encounters with Mercury, each bleeding off some of its velocity, enabling it to be captured in 2011 in an orbit around the planet
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