As seen from Earth, Mercury is never far from the Sun. It is therefore observable only in twilight, when it is low in the sky but the sky is dark, or in daylight, when it is high but contrast is poor. Also, it's very tiny. Observers mapped a few faint markings on Mercury and concluded that because of solar tidal effects, Mercury always showed the same face to the Sun. Below is one map of the surface markings on Mercury as mapped from Earth.
This misconception was heightened by a double numerical coincidence: Mercury and Earth line up every 116 days, almost exactly two rotations of Mercury. Mercury is highest in the evening sky in winter and in the dawn sky in summer for Northern Hemisphere observers, and three alignments of Mercury and the Earth take 348 days, very nearly a year. Thus, an astronomer observing Mercury at its most favorable would see it for years on end at the same time of year and always showing the same face to the sun! An observer who spent years seeing the same predictable markings on Mercury would be justified in concluding it always showed the same face to the Sun.
If Mercury always kept the same face to the Sun, the sunward face of Mercury should be the hottest place in the Solar System, but the opposite face should be the coldest. So in 1965, when astronomers tried to calibrate the giant Arecibo radio telescope, they aimed it at the night side of Mercury, which should be emitting zero radiation. They discovered it was emitting a lot of radio energy, and clearly had to be very warm. They considered briefly whether Mercury might have an atmosphere but rapidly concluded Mercury was too small and too fiercely irradiated by the Sun to retain one. The only other possibility was that Mercury was rotating, and calculations showed that Mercury could have a 58-day rotation period.
Once Mercury's rotation was discovered, astronomers found it was quite easy to reconcile the published maps of Mercury. The composite map below is based on one published by Clark Chapman of the University of Arizona. Note that it closely resembles the map above.
The animation below shows why Mercury rotates the way it does. The colored bands are zones of increasing tidal force. At perihelion the solar tidal force on Mercury is over three times as strong as at aphelion.
Note that once Mercury is inside the yellow band, the reference line drawn through the planet pretty much tracks the Sun. That is, Mercury rotates at about the same rate as it moves around the Sun. In fact, at perihelion, its rotation is actually a hair slower than its motion around the Sun, meaning the Sun actually moves backward in the sky for a while. If you were standing on Mercury's terminator, you'd see the sun rise, set, then rise again. It would cross the sky normally, set, rise briefly, then set for good. Note also that on the aphelion half of the orbit Mercury rotates just enough so that at alternate perihelia the red and blue ends of the reference line point at the Sun.
The resonance is set by Mercury's eccentricity and by the fact that tidal forces increase very fast with decreasing distance. If Mercury had a circular orbit it probably would have one face locked to the Sun, if the eccentricity were some other value, it might have a different resonance.
In 1973 and 1974, Mariner 10 became the first and only spacecraft for over 30 years to visit Mercury. It used a gravity assist from Venus to achieve orbit, thus becoming the first spacecraft to use gravity assist and the first multi-planet mission. To maximize data, the mission made three flybys of Mercury. Unfortunately, to do so the spacecraft had to be put into an orbit with a period of 176 days, twice the orbital period of Mercury and three times its rotation period. Thus the three flybys mapped the same side of Mercury each time and the other hemisphere was never mapped up close until the MESSENGER mission arrived in 2007.
Radar studies show that, amazingly, Mercury has polar ice caps. Mercury's equator always points exactly at the Sun, and Mercury's poles never receive more than grazing illumination. Polar crater interiors never get sunlight at all. Most of the ice is within craters. It is not yet known whether the ice is on the surface or covered by a thin layer of surface debris.
Bruce C. Murray, 1975, Mercury. Scientific American, vol. 233, no. 3, pp. 58-69.
Created 20 May 1997, Last Update 9 April 1999